Fuel Economy Logic

[Boz Mon]

I've been thinking about posting this for awhile so here it goes...What is the ideal speed for best fuel economy? Is vehicle speed the deciding factor or engine speed or a combination of both? Why I ask this is, I used to have a Honda Prelude that would consistantly get 26MPG. It was mainly highway driving at 80+MPH. That particular car was geared really low. Going 80 the engine would be turning almost exactly 4000RPM. Now I have a Lexus SC300 and driving the same speed on the highway, the engine is only turning about 3000RPM. I am constantly struggling to get 19MPG (this is usually driving anywhere from 65-80MPH). The other day I took a trip up north which was all highway driving of probably anywhere from 85-90MPH and I easily managed 24MPH. I know vehicle weight has a lot to do with this but is there something else? I would think that driving faster would be better for a car to a certain extent. If you are driving at 65MPH and the engine is turning 2000RPM I would think that this would be less efficient because the engine isn't making much power down at that speed. It would seem better to drive somewhere around 4K RPM because of less strain on the engine. Maybe theres something that I'm missing? Someone please explain

 

[Sage]

Well, you can’t directly compare those two cars, because they have different engines – a Lamborghini Diablo is going to get much worse mileage at any speed compared to a Toyota Corolla, no matter what the speed or “strain” on the engine.

Anyway, you mention that in the Lexus, you got better mileage going 85–90 instead of 65–80. My guess is that when you were doing 85–90, you were being consistent – acceleration is, to my knowledge, the big fuel killer, not necessarily engine speed or car speed. In your day-to-day driving, you’re probably less consistent due to traffic (I range between 60–85 mph every day on the freeway); you lose a lot of energy in the deceleration and acceleration process.

But I nearly didn’t pass the Mechanical Physics AP exam, so I’m probably not the person to ask.

 

[Wolfe]

Well, the optimum speed will be different for different vehicles, because it has a lot to do with gearing, engine performance, and aerodynamics, but the basic concept goes something like this:

At low speeds, an engine will use more fuel than it needs to, because it takes so little effort to maintain a low speed.

At high speeds, an engine will have to work much harder to maintain its speed because of air resistance, and if the speed is too high, you'll be using a lot of fuel just to push the car through the air.

The optimum speed is somewhere inbetween, where you're travelling fast enough to not put any fuel to waste, but slow enough to keep the engine from overworking itself against air drag.

For my car, the optimum speed is somewhere around 40-55mph, in top gear, at around 1750-2500rpm. My car has a MPG indicator, so I can observe where fuel economy is at its highest, and at that speed, I'm doing 40+mpg. Unfortunately, I can't stand to drive at that speed on the highway (and other drivers probably wouldn't like me ), so I'm stuck doing ~27mpg at 65-80mph.

 

[Boz Mon]

At low speeds, an engine will use more fuel than it needs to, because it takes so little effort to maintain a low speed.

I understand this but why are auto manufacturers making cars that turn 1900RPM while traveling at highway speeds? Why don't they gear the cars to make optimum fuel economy while crusing?

 

[Max_DC]

Well, I'm no expert aswell, but you also have to see the size of the engine. A 1,6 litre 4 cylinder needs less than a 5 litre V8 at the same rpm, same psi, with same aerodynamics, with the same body weight, with the same type of tire, with the same kind of transmission, with the same kind of mechanical parts in the car etc.

Simply because the small engine just supplies 1,6 litres gas/air and the big engine 5 litres every time.
But you when you have a look at my "with the same" list you see some of the other factors you have to consider and if they are different you can't really compare two cars anymore. Notice that I'm only talking about cars which use normal/premium gas, diesel is another story aswell as turbo/supercharged cars.

Diesel cars should be accelerated fastly for good fuel consumption, while cars with normal gas should be accelerated moderatly.
The rest ...welll depends on the engine characteristics. You have to consider the diagrams of power and torque aswell as gear ratio etc.

So I would say that in general you can say : Drive constantly while avoiding any form of acceleration ( if you have to, then moderatly ). Avoid high rpm's, avoid rpm's below 1000. Speed ? I don't know if a normal speed is better for fuel economy than low speed - when the "low speed" is performed without acceleration ( which is not the case in cities etc ). Maybe yes, maybe no. High speed ( again different for every car )is always bad .

 

[retsmah]

Figuring out where fuel economy will be the best is kinda complicated. The main things are going to be drag, and engine thermal efficiency. However, the engine thermal efficiency is dependant on loading, speed, and throttle opening, and the engine speed will be dependant on gearing. Thermal efficiency usually increases the harder you push an engine, which could explain why fuel mileage might increase with higher speeds.

Although a lot of it could also be that you are maintaining speed better at higher speeds like Sage said.

I remember reading a study a while back by some manufacturer that found it's best to accelerate very fast, like 80% throttle, for fuel economy. Again most likely because your thermal efficiency increases as the engine gets pushed harder. This is assuming that you are just accelerating up to freeway speed then holding it... doesnt work accelerating fast up towards a red light or something.


The logic of a 5l engine using 5l of gas / air while a 1.6L uses 1.6l doesnt quite work, because you have a throttle, which drops the pressure of the air headed into the cylinders. The air coming into the five liter engine could be at a lower pressure, which means that the actual number of air molecules going into the bigger engine would actually be the same or lower than the number going into the smaller engine. I'm not saying one or the other will get better fuel mileage, but a bigger engine doesn't necessarily have to burn more gas than a small one for one revolution.

 

[YSSMAN]

From what I understand, you are looking at a 60-70 MPH range in most situations to get the best gas mileage possible, atleast here in the US. When the EPA tests the cars, they do a very silly test where they drive the car on a "test track" for a given ammount of time at an assortment of speeds. The speeds they travel at never surpass 59.9MPH, which is how they calculate the Highway MPG on the window sticker.

On could go on and on about how stupid that its, but here are a few tips:
1) Go easy on the acceleration. They say that if everyone accelerated at even 1/8th the pace at which we do, everyone would see a rise in their fuel economy.
2) Try not to run unnecessary equipment when driving the car. I know that is kinda hard in a Lexus, but things like the electronics and the A/C drain fuel economy.
3) When driving down the highway, DONT use the cruise control. For some reason (and I dont know why) crusie control sucks fuel faster than your right foot. I assume it is the abrupt throttle action the computer does, which can also hurt fuel economy... Ask any NASCAR driver, you have to roll the throttle out gently to save gas.

In my Jetta with the 2.0L I4, I've been able to achieve about 27MPG on average running up and down the East Beltline and a few side roads, but on the highway traveling at about 70-85, I've been able to achieve 33MPG back and fourth from Chicago to Grand Rapids. Mind you I'm spinning more than 3500RPM once the speedo gets around 80MPH, so thats surprising...

I understand this but why are auto manufacturers making cars that turn 1900RPM while traveling at highway speeds? Why don't they gear the cars to make optimum fuel economy while crusing?

GM is notorious for doing this, and they do it because the engines use LESS fuel at lower RPMs. Thats why you see Pontiac GTO's and Corvettes spinning about 1600-2000RPM when doing 70-80 on the highway and manage to pull off 30MPG. Its a similar situation in the trucks and SUVs as well. When I went to Chicago with my father last weekend in his Avalanche, we were spinning just about 2000RPM at 80MPH, and we managed about 22MPG there and back. Compared to the usual 16MPG the truck gets, thats pretty damn good.

 

[High-Test]

Well, it could be overdrive, I've heard of many people I know getting 25+ at 80, while getting a little more than half of that in the city. Try using the overdrive on a long highway trip, as when my SC was running I used on long hauls on hwy 36. Doesn't overdrive work by changing the gear ratio and lowering rpms to get better mileage?

 

[B_B_B]

not all cars have overdrives porche has a 1-1 final but yes most cars come with economy gearing in them already because you cant really make OD that much different than the prev gear or breakage could occur


but best bet for best economy is like stated before keep your foot out of the carb/TB

 

[Keef]

Read this:

http://www.caranddriver.com/article.asp?section_id=27&article_id=10499&page_number=1

It's an editorial by Patrick Bedard from the Feb. 2006 issue. He did some gas mileage experiments with Car and Driver's long-term Lexus RX400h and got some interesting results. Seriously, you should read it; you'll like it. It actually changed my ways...a little.

 

[Gil]

I believe that consistency has a great deal to do with it.
My 8,000 lb Ford Excursion gets right at 20 MPG at turnpike speed (Near 80) on the way to and from Colorado.
The mean engine speed is between 1900-2200 RPM. And it will do this all day, as long as I do my part and keep myself from spooling up the turbo.

By the same token. My Ranger gets anywhere from 19-27 MPG. Average is about 24. My daily drive is 36.2 miles most of it at highway (60-75 MPH).
If I'm diligent about running 36 PSI in all four tires I run 25 MPG.
So I guess rolling resistance is also a factor.

 

[NSX-R]

According to Road and Track "our-driving fuel econmy" data:

2005 Chevrolet Corvett C6: 16.6 mpg
2005 Porsche 911 Carrera S (997) :15.7 mpg

This confuses me. I don't know a lot about engine load, gear ratios or anything related to fuel economy for that matter, but how does a 6 liter V8 get better fuel economy than a 3.8 liter flat six?

 

[Gil]

According to Road and Track "our-driving fuel econmy" data:

2005 Chevrolet Corvett C6: 16.6 mpg
2005 Porsche 911 Carrera S (997) :15.7 mpg

This confuses me. I don't know a lot about engine load, gear ratios or anything related to fuel economy for that matter, but how does a 6 liter V8 get better fuel economy than a 3.8 liter flat six?

Which one has the widest tires?
What are the RPM loads at 65 MPH? (Usually included in the R&T fact sheet).
What is the Co-efficient of drag for both cars?
If the Vette has a lower Cd, narrower tires than the Porsche, and is running at less RPM at Cruise speeds it will likely get better mileage.

Also, R&T test loop is thru some hilly country. The Torque of a V-8 means not having to downshift to climb the steeper portions of the loop.

Also, which one of the cars has the most miles on it. If the Porshe has less miles on it, and is still in the "break-in" period it will also get lower mileage.

With every thousand miles I travel in the Excursion, the mileage gets better. I assume that, to a point, the same will be true with cars.

Add to that the fact that the R&T test loop isn't very long (well under 100 miles if my memory is holding up). And if two different people are filling up the cars, one may top off, and the other may stop filling at the point the nozzle shuts off.

The difference in the two mileage figures is less than one MPG. There are plenty of variable that might be capable of causing this.
With two different testers, the results may be different.

 

[YSSMAN]

Corvettes, Camaros/Firebirds, GTOs, CTS-Vs, etc. that are all equipped with the LS1/LS6/LS2 and the Tremec T56 6-speed manual get great gas mileage if youre driving them right. A few friends of mine have been able to get 30MPG in their '00 Corvette quite often, owed mostly to the outrageously tall (.65 I believe) 6th gear. There is also the skip-shift feature built into the Tremec T56 that makes you skip 1-4 and so on when accelerating slowly to "save fuel" and to "lower emissions" but it is more of a pain than anything...

 

[neanderthal]

BMW conducted studies about fuel economy in the early 80s and concluded that it was better to accelerate briskly and get up to cruising speed as quickly as possible, without doing drag race launches, because at cruising speed you use much less fuel than you did accelerating. the results held even when the cruising distance was pretty short. i dont recall the exact distances, but somewhere in the region of two blocks.
this affected the gearing of the fuel efficient eta models they introduced, with aggressive low gearing leading to a nice overdrive ratio. they even mapped the transmission controls to hold on to lower gears when accelerating

so YOU ARE ALL WRONG who tout accelerating slowly. bear in mind, accelerating briskly to cruising speed and drag racing are two different things.

of course if you are doing a lot of stop start driving this may not be correct or applicable to you. but if you are going to do a lot of uninterrupted driving, like on a freeway, then getting to cruising speed quickly is best

*acceleration, especially hard accelration, kills fuel economy
*aero drag kills fuel economy
*weight kills fuel economy
*poor car maintenace (clogged injectors air filters, fuel filters, bad/ old spark plugs, old spark plug wires, poor ignition timing, low air pressure, dragging brakes, slipping clutches
*running ancillaries like air conditioning
*open windows and sunroofs

those are things that you as drivers have control of.

in my car (mercedes 300E, EPA 19 city 23 hwy) i can get close to 30mpg on long trips. more like 28/ 29. this is due to me almost never using my AC, i have taller, marginally wider tires than original, i never drive with my windows and sunroof open, i have a very light foot when so inclined (i ease off the gas and let the cars momentum push it on the slightest of gradients, i accelerate on downhills if the is an uphill following, i dont accelerate uphill as much as possible, i leave a big enough gap between me and the car ahead of me that i can anticipate when i have to slow down and stop, i coast to stops anticipating the traffic lights by watching the pedestrian signals and so on and so on) my car is in a good state of tune and fortunately well designed so i can eke out better milage simply by altering my driving.

when its hell for leather all the time i can get milage in the low teens. ive gone through a whole tank in about 120 miles, and my tank is 17 gallons. of course, that week, i was late everywhere and speeding everywhere.

 

[YSSMAN]

Interesting what BMW found, as it is the EXACT OPPOSITE that we are all tought by not only the Government but also the vehicle brands here in the US. When I accelerate harder to get up to speed faster and then proceed to cruse in my car, that hurts gas mileage more than it does to drive gently, so I doubt the same rule applies anymore.

 

[Keef]

If you guys would have read my Car and Driver article, you all would see how your actual cruising speed has a much more profound effect on your mileage than does rate of acceleration. It was a hybrid he was testing, but even when accelerating at full throttle (the gas engine is on, obviously) there was only 7mpg difference than when he used light acceleration (with only the electric motor running).
Now, his data shows that, for every 10 mph difference in cruising speed there is about 7 mpg difference. He cruised at 70 mph and got an average of 31.7 mpg. He stepped it up only 10 mph, to 80 mph, and averaged 26.1 mpg. That's a whopping 5.6 mpg difference just form an extra 10 mph. When he cruised at 40 mpg he got 49.3 mpg. The main reason for these huge differences is that as you speed up there is more wind resistance, and your engine needs to have a heavier load/higher rpm to counteract this the faster you go. So if you are looking for the best way to get amazing gas mileage, just put her in 6th and go as slow as possible.

 

[neanderthal]

YSSMAN

i thought so too. but since ive started to adhere to the style of driving that BMW endorses (based on their old research of course) ive improved my fuel economy tremendously.

anyway, i liken it to riding a bicycle; its easier (you use less effort) to get to cruising speed quickly and kinda coast along for a longer time than accelerating slowly and coasting for a shorter time. and i rode my bike to work for most of 2001, so i know this one first hand, its not a recollection from an article i read a long time ago. the acceleration is the work, and you shorten it by accelerating relatively quickly.

 

[Wolfe]

I understand this but why are auto manufacturers making cars that turn 1900RPM while traveling at highway speeds? Why don't they gear the cars to make optimum fuel economy while crusing?

Less RPMs = Fewer engine rotations = fewer times the fuel injectors inject fuel into the cylinders = less fuel used

not all cars have overdrives porche has a 1-1 final but yes most cars come with economy gearing in them already because you cant really make OD that much different than the prev gear or breakage could occur

Uh...first of all, all "overdrive" means is that the highest gear (or gears) has a ratio of less-than-1:1. Most, if not all, cars sold today have overdrives. Second, a final ratio is completely different from an overdrive. The final ratio is the ratio of the differential. Finally, you can make the overdrive as different from the previous gear as you want, at least on a manual-transmission-equipped car (I'm not so sure with how auto trannies work). It works for the same reason why you can shift from 2nd to 5th without a problem.

Anyway, neanderthal sorta explained this, but I'm going to reiterate it and emphasize it --

The best way to save fuel is to conserve momentum.

I see so many people violating this simple concept on the roads. It's actually amazing how ignorant people are of it.

When approaching a red light, begin coasting early and anticipate when the light is going to change back to green. If you do it right, you'll still be rolling when the light changes, and you can then take off again. You'll save fuel on acceleration, save on brake pads, and pass the people who were already waiting at the red light, if your lane is open.
The only drawbacks are that other people will angrily drive past you and cut you off (just so that they can speed towards the red light and have to stop) because you're driving "too slow," and that you shouldn't zoom into an intersection exactly when the light turns green, or you risk hitting someone who decides to run a red light.

When making a turn, especially onto a side street (as opposed to a parking lot), take it at a brisk pace. Don't corner so hard that you squeal your tires, but definitely, definitely do not slow to about 10mph just to make a simple 90-degree turn onto a side street. Not only are you pissing off other drivers in doing so (like me ), you're wasting fuel accelerating again after completing the turn. If you're a lead foot, speeding around, slamming on the brakes, turning at the pace of a turtle, and then slamming on the gas again (I've seen people do it), frankly, you're an idiot.

If you have a manual transmission, coast in gear, not in neutral. The momentum of the car keeps the engine rotating, and the fuel injectors barely use any fuel at all, if any (I think I read somewhere that they just turn off). As neanderthal said, accelerate downhill if another hill is coming up. If it is safe to do so, don't bother using your brakes to maintain a steady speed down a hill. Use your gears (use the lower ones if you have to keep it slow), and just ride the hill.

I'm sure there's more, but I can't think of anything else at the moment.

 

[YSSMAN]

I've always had that problem with putting the clutch in while going around a corner. Its like something bad is going to happen if I dont, and yet I still do it...

@ neanderthal: I used to race my bicycle for my high-school cycling team, and although the general concept of getting to crusing speed quickly is a good idea, its not necessiarily the best. Atleast with me and a few of my fellow riders, it was the harsh acceleration that killed us in practices and races, not the overall fast crusing speeds. Its different for different people, and being that I was not a good sprinter, hill-climber, or high-speed racer. I was there to break the wind for my fellow teamates for the begining of the races, and then I would be left to my own ways untill I caught up with other teammates that held similar positions that I had.

 

[Keef]

If you are coasting down a slight hill it's best to coast in nuetral/with the clutch in. I live off a street that has a decline of about 5 degrees, which is pretty shallow. About a half mile down that street is the busy street that takes you to wherever. Anyway, whenever you go down this road from my house, wither direction, you are going down this slight hill (it's like an arch). The speed limit is 25, but I usually get up to about 35 and press the clutch in and coast at idle. The hill is too shallow to leave it in gear; you would slow down too quickly. On steeper hills I leave it in gear and coast so as to not break the speed limit too much. That's inmy dad's truck.
My car is a Cavalier (whooptie doo) and is an auto. I wish it was a stick. It slows down like a biotch, even in fourth, so I have to shift into neutral to coast at all.

 

[neanderthal]

wolfe
you nailed it by condensing it to "preserve momentum." it is comical to observe other drivers racing from stop light to stop light. yet simply timing right gets you there at the same time.
theres a street in burbank, ca with a 35mph limit. the lights are timed for that. on that particualr street (victory blvd BTW) i do 35 and just sail past everyone who then promptly race past me. its hilarious.

-----

id only argue about coasting in gear. if the engine is turning at 3000rpm in gear but no throttle, surely its using more gas than at idle (600-800rpm) i could be wrong.

one thing though, is that the engine is not designed to be driven by the wheels, so i would advise NOT keeping it in gear while you coast. stick it in neutral and use your brakes when you need to stop.

 

[Wolfe]

wolfe
you nailed it by condensing it to "preserve momentum." it is comical to observe other drivers racing from stop light to stop light. yet simply timing right gets you there at the same time.
theres a street in burbank, ca with a 35mph limit. the lights are timed for that. on that particualr street (victory blvd BTW) i do 35 and just sail past everyone who then promptly race past me. its hilarious.

-----

id only argue about coasting in gear. if the engine is turning at 3000rpm in gear but no throttle, surely its using more gas than at idle (600-800rpm) i could be wrong.

one thing though, is that the engine is not designed to be driven by the wheels, so i would advise NOT keeping it in gear while you coast. stick it in neutral and use your brakes when you need to stop.

Well, think about it this way -- an engine uses fuel when it needs to produce power. Which takes more effort, maintaining RPM's at idle, or letting the weight/momentum of the car do the work? For many (if not all) cars, idle consumes more fuel.

Coasting in gear does nothing harmful to an engine. If it did, cruising on the highway would wreak havoc on your engine -- whenever you aren't adding a bit of throttle to maintain your speed, you're coasting in gear.

What you shouldn't do is go around coasting in gear whenever you can, as if it were some magic MPG-booster -- that's just silly. Instead, just leave the transmission in gear whenever you do any coasting.

 

[Keef]

A lot of times I throw it in neutral or push the clutch in to coast down an enourmous hill. I live in Dayton OH, we have a few big'uns. On the road I'm talking about, a big four laner, the limit is 45, but I'll coast down it in neutral and reach about 60. I can make it another half-mile before I slow back down to 45. If I leave it in gear while coasting I'll stick right around 50 the whole way down.
But I agree, timing your lights is way better than racing in between them, though sometimes I get in my acceleration-type moods. It is funny passing people like that, though I can remember a bunch of times when I was the one they were laughing at.

 

[LeadSlead#2]

"If you use your engine for breaking, it will surely break."

Your computer will feed your car enough gas for the RPM's you are doing, on or off throttle, to prevent it from running lean, this is especially true for anything GM. So if your coasting in 3rd gear at 3000rpm's, your using enough gas to keep the engine from running lean at 3000rpm's...no matter what.

Your car will liekely use jack crap at idle, and you should save fuel by coasting, assuming you don't coast when you going back on the throttle. In other words, don't coast down a hill, if you can speed up 5-10mph, and slowly get back down to speed. Do, however, coast to that red light, clutch in or in neutral.
In 3.5 hours, my friend's 89 Accord used 1/8th of a tank of gas, (left it on at work) <---(dumbass) regardless, he used jack crap for 3.5 hours of the engine running.

I can also tell you that a 98 Grand Am with a 2.4L 4-cyl will average about 33mpg at 65-70, 32 at 70-80 and less and less. I got 34(for the whole tank) once when I drove about 50-100 miles at 20-25mph in 4th gear, basicaly idling along (ICE). meaning it gets the best at around 1000rpm, in (likely) 5th gear. don't know how good, but im sure it's at least 40+... if you can keep speed.

My brothers car's DIS readout says it's getting about 50mpg at 35, in 5th, doing about 1100-1200 rpm... 32mpg at 75mph, and 3200rpm...
that is a 2.3 4cyl in a 90 oldsmobile.

Likely, for any car, you'll wanna go as slow as possible, in Top gear, without bogging the engine. Most any car should be fine over 1500rpm, even turbo's, for the most part.

If your getting better fuel milage at 80 than 60, it's simply cause it's so drastically easier to maintain speed at higher rpm, and that's your driving (no offense) not the car.
Remember: it takes more power to go from 60 to 70, than it does from 50-60, and the faster you go, the bigger the difference is.
Air resistance is not proportionate to MPH (or KPH), it will increase faster than speed, much faster.

 

[Wolfe]

"If you use your engine for breaking, it will surely break."

Good thing we're talking about using the engine for braking, then, which doesn't do anything harmful.

Where do you guys get these ideas?

 

[Blake]

It isn&#8217;t good for the drivetrain, though.

 

[Wolfe]

It isn’t good for the drivetrain, though.

That's true if you downshift through the gears to slow down, which isn't what I'm talking about. All I'm talking about is decelerating while keeping the transmission in gear, which is about as harmful as accelerating or maintaining a steady speed. Sure, coasting in neutral puts no stress on any powertrain components, but especially on a large hill or mountain road, you'll be working your brakes pretty hard. Coasting with the clutch disengaged is just a bad idea.

 

[Keef]

Nothing is good for the drivetrain. Putting stress on it to accelerate is no different than stressing it to decelerate; it has been designed to handle both forces equally well. If you're using the compression of the engine to engine brake while going downhill so you don't go over the speed limit, fine. But it's better to just let her glide down the hill as fast as it wants to go, as long as it's safe and you're allowed to do it. Not only will you save gas by leaving it in idle as you're going 40+ mph, but you'll save that much more gas from the time you saved getting to where ever you're going. On the hill I was speaking of, the car won't roll any faster than 60 mph anyway, so I'm not doing anything too extreme. If you'll notice, when going down a big hill on th ehighway while coasting, your car probably won't go any more than 70 or so without help because of the air resistance. Put the clutch pedal in as often as possible. In an automatic you'll wear it out by shifting in between neutral and drive too much, so only do it when it is very beneficial, like on a long, straight hill, or when coasting from high speed to a stop light.

 

[Blake]

Coasting with the clutch disengaged is just a bad idea.

@keef