Top Speed

[DDAAVVIIDD82]

How far off is the posted top speed in the transmission interface? Is it a set amount or is it on a case-by-case basis?

 

[NjLowrider]

Its is the top MECHANICAL speed for all cars. it does not factor in air resistance.

So a slow car might be able to come close to the number because air doesnt really play a huge part. But as the car gets faster the air slows you down exponentially more. And superfast cars especially with max downforce will not come that close to the trans max speed.

Basically dyno numbers with no drag.

 

[Negrumir]

Its is the top MECHANICAL speed for all cars. it does not factor in air resistance.

So a slow car might be able to come close to the number because air doesnt really play a huge part. But as the car gets faster the air slows you down exponentially more. And superfast cars especially with max downforce will not come that close to the trans max speed.

Basically dyno numbers with no drag.

Yes, this is true.

But I think he means like when you have a high power car and top it out. You're bouncing of the rev-limiter, but the car never achieves as high a speed as the transmission says it can. Happens with some cars, others it doesn't seem to.

You'd think that in top gear with the engine turning at its highest possible speed, that the car would be traveling as fast as it says, but it doesn't.

 

[MasterFocus]

It is case by case. The top speed you see is limited by the red line across the graph, which represent the engine cutoff.

 

[Morgoth_666]

It does vary a little bit by car, but in the vast majority of cars it seems you will hit the rev limiter about 8-10 miles per hour lower than what the transmission menu calculates to be the max.

That is assuming you have the throttle floored. If you're balancing the throttle to keep the limiter from kicking in you can get a bit more speed, and even more so if you do so while slipstreaming.

 

[NjLowrider]

Yes, this is true.

But I think he means like when you have a high power car and top it out. You're bouncing of the rev-limiter, but the car never achieves as high a speed as the transmission says it can. Happens with some cars, others it doesn't seem to.

You'd think that in top gear with the engine turning at its highest possible speed, that the car would be traveling as fast as it says, but it doesn't.

But if it says top speed 205 and you hit limiter at 195 it is the drag of the air making it "only" 195. If there was no air it would sing at 205. Mechanically it is going 205.

They say you save almost 1/4 fuel if you drive 55 instead of 75.

Air is dense!

 

[DDAAVVIIDD82]

Sorry to bring this one back, but is the posted top speed in the transmission interface accurate enough (at the rpm shown) to determine tire diameter using an existing formula that calculates top speed at a given rpm with a given gear ratio?

Happy Holidays everyone!

 

[Smuttysy]

I'd say it closer represents the theoretical top speed of the gearbox, rather than anything to do with the car. I say this as it goes beyond the redline of the engine (as already stated), so if you could run the car all the way to the end of the redline in the final gear it could, potentially, hit the figure quoted.
With this in mind it will vary from car to car because of the different redline power cutoff points. I know that my Charger has a gearbox rated top speed of 250mph or so, but thanks to the low redline and cutoff it will max out at around 170mph (ish)

 

[Jubby]

I'd say it closer represents the theoretical top speed of the gearbox, rather than anything to do with the car. I say this as it goes beyond the redline of the engine (as already stated), so if you could run the car all the way to the end of the redline in the final gear it could, potentially, hit the figure quoted.
With this in mind it will vary from car to car because of the different redline power cutoff points.

This. 👍

 

[DDAAVVIIDD82]

So, theoretical top speed when tire diameter is factored in among the rpm as listed and the gears as equipped? I'd like to take my transmission tuning to an outside source and not every car has a known tire diameter (at least known to all) such as the Group Cs and various concepts.

 

[DDAAVVIIDD82]

Anybody at all?

 

[ConRollins]

Anybody at all?

Well, I would say anytime you are trying to calculate top speed in a mechanical system, you need to take into account all parts of the system.
Knowing the engine RPM, the gear ratio, and the final drive ratio will not tell you the top speed of a car. All you would get is the RPM of the axle shaft! Throw in the tire diameter, and viola, you can determine theoretical top speed.

So I would say yes, go ahead and calculate the tire diameter based on the data given. Don't be surprised if you see some strange tire diameters!

 

[aadil717]

I notice on 5 Speed Gearboxes (SL600) you can achieve a more maximum speed than the same speed set on a 6 Speed Gearbox.
This maybe due to gear ratios too..that said, 5 Speeds have a noticeable decrease in acceleration compared to 6 Speeds.

 

[DDAAVVIIDD82]

Don't be surprised if you see some strange tire diameters!

That was my primary concern. I calculated the MK1 GTi and got a tire diameter substantially taller than what would have come stock. Have yet to plug it into my graph though.

 

[EH3B18C93]

if your graph is based on just mechanical speed without drag or traction then the top speed shown at the rpm shown in the transmission menu should be able to tell you the diameter of the driven tires

 

[DDAAVVIIDD82]

Yeah, I've since checked a few dozen cars, comparing their OEM tire sizes to the diameters determined through calculation and this seems to be consistent.