Wheel Circumference

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Cribbage619
I have been working on a formula to calculate wheel circumference, with little luck. Not sure if it is an AI problem, or my math. Can one of you guys help?

Details:

1st Gear Ratio: 2.673
Final Gear Ratio: 2.651

This results in 7.086 engine rotations for each wheel rotation.

@ 5000 RPM speed is 54 MPH

5000 RPM equates to 705.6 wheel rotations per min.

54 MPH = 285,120 Feet per Hour = 4,752 Feet per Minute

So 4,752 Feet Per Minute and 705.6 rotations per minute equates to 6.73 feet of circumference.

I know this isn't right, but I can't find the mistake.
 
That could very well be right; 6.73 feet of circumference provides a 2.14 diameter. Taking into account the tire sidewall, that... might be right, depending on the car.

I used to have a similar formula back in GT2 since the gear charts disappeared, but I lost it years ago. This sounds roughly right, though 👍
 
It is right, I found my mistake. I was working backwards from the calculated circumference to calculate speed at target RPM for last gear. I had a mistake in that formula, not this one.

Thank you.
 
It's not impossible, no. You'd likely need a bike engine though.

Also, what relevance does idle speed have? You're going ZERO mph at idle. Still, crack on and try to reach anything over 30mph at any point of your tach in your own car in first gear.
 
It's not impossible, no. You'd likely need a bike engine though.

Also, what relevance does idle speed have? You're going ZERO mph at idle. Still, crack on and try to reach anything over 30mph at any point of your tach in your own car in first gear.

That's 30 mp/h. But, my Subaru (which only has 4 gears) could likely reach around 15-16 km/h at 1000 rpm. It's when you lift off of the throttle, that it's actually possible to do that, in the Subaru.

So, given a car with the right circumstances, I believe it is.

I've yet to find one, but, come on... It can't be that hard. :lol:

In fact, people have put motorcycle engines in Smart cars; 💡
 
My 325i idles at about 700 i try not to let it drop any lower or it just cuts off and my fiesta dont have a rev counter :lol:
 
You're going ZERO mph at idle. Still, crack on and try to reach anything over 30mph at any point of your tach in your own car in first gear.
I would go and do that right now if I still had the RX8.

Oh, and Mr M, what does the p stand for in mph? That's why you don't need the slash.
 
I would go and do that right now if I still had the RX8.
:lol:


Oh, and Mr M, what does the p stand for in mph? That's why you don't need the slash.

Don't bug me about the smallest of details at 7:38 AM. Bug me about why I'm still awake, and trying to have a logical conversation. Therefore, even though it's just passed midday, goodnight. :lol:
 
That's 30 mp/h. But, my Subaru (which only has 4 gears) could likely reach around 15-16 km/h at 1000 rpm. It's when you lift off of the throttle, that it's actually possible to do that, in the Subaru.

So, given a car with the right circumstances, I believe it is.
This is all twaddle.

The 4EAT in the Impreza has a 1st gear ratio of 2.785 and a diff at 4.111. On a 16" wheel with 225/45s (don't know what the standard wheels are, don't care - it's a ballpark guess) you'd be doing 6.1mph (9.8km/h) at 1,000rpm.

It has nothing to do with when you lift off the throttle or what your car idles at - when the engine spins at 1,000rpm in 1st gear, the wheels will spin at 6mph in that car. They're mechanically locked.
 
This is all twaddle.

*bandwidth snip*


I see your point. But, that's halfway there, in a car that never was known for being a low-revving, diesel/motorbike type. :lol: My point, in saying that it's likely possible a different car could do it, in the correct circumstances.

If my Subaru can come close, maybe it is possible, for something like a Smart car. Or... :lol: at the thought, a Peel P50.
 
Hence my original question of:
Famine
What car are you driving that has a 1st gear ratio of 11mph/1000rpm?
In GT5 the only stock car I can think of that might come close is the Caterham Fireblade - though it could of course be a heavily modified transmission - but there's almost no road car on Earth that will be geared too much beyond 30mph in 1st gear, much less 54mph, much less 54mph at just 5,000rpm.

Your Subaru not only doesn't come close, it doesn't come close to close - 6mph/1000rpm is normal for road cars in 1st gear and quite some distance away from 11mph/1000rpm in 1st gear, which isn't normal. I don't even know why you brought it up, nor what speed it idles at - which is even less relevant.

The topic didn't need this segue to your Impreza - few topics do.
 
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The speed it idles at tells me that the motor isn't high revving immediately.

But, of course, since you see no purpose... I'll get my coat.
 
The speed it idles at tells me that the motor isn't high revving immediately.
It tells you nothing of the sort - my Accord Type-R idled at 750rpm and revved to over nine thousand* while my 800rpm idling MX-3 V6 goes to 7,200 - and it tells you absolutely nothing about the gear ratios or the wheel speed for a given engine speed because when it's idling the engine and the wheels are not connected to each other. And what was the question about? Calculating wheel size from road speed using gear ratios.

Fourteen posts later, the GT5 question remains and all we've learned is that you drive an automatic Impreza and can make up numbers. This topic has not benefitted from your input. Please learn when a topic will benefit from your input and when it will not.

*Went there
 
What car are you driving that has a 1st gear ratio of 11mph/1000rpm?


*Went there

All I wanted to say is that it doesn't seem horribly illogical for some cars to be able to reach 11 mph, below 1000 rpm in first gear. Yes, it seems like a challenge, but, I'd like to see if any car is capable of it.

Then, everyone took me serious, saying that it's nearly impossible, which is something I'm not denying. I'm not trying to argue, I'm just speculating that, given the scope of the automotive following, there's probably been a car made, somewhere, that can go 11 mph below 1000 rpm.

Whether there's a car like that in GT5, or not, I don't know. I'm just speculating. I found it weird that you implied that it was 100% impossible, when it's likely just improbable and unlikely.

And, I'm saying that, as we both are fully aware, most cars idle below 1000 rpm. Given the correct gear ratio (only relevant to the first forward or reverse gear,) it could possibly be possible (see, speculating) to reach 11 mph with under 1000 rpm.

I'm not questioning your knowledge. I'm more just challenging the great minds of GTP to think outside the box, in the sort of conundrum niky normally finds. :lol:
 
All I wanted to say is that it doesn't seem horribly illogical for some cars to be able to reach 11 mph, below 1000 rpm in first gear. Yes, it seems like a challenge, but, I'd like to see if any car is capable of it.

Then, everyone took me serious, saying that it's nearly impossible, which is something I'm not denying. I'm not trying to argue, I'm just speculating that, given the scope of the automotive following, there's probably been a car made, somewhere, that can go 11 mph below 1000 rpm.

Whether there's a car like that in GT5, or not, I don't know. I'm just speculating. I found it weird that you implied that it was 100% impossible, when it's likely just improbable and unlikely.
I implied no such thing. I asked what car he was driving that did that - on the basis of it being so far out of normalcy to require the question.
And, I'm saying that, as we both are fully aware, most cars idle below 1000 rpm. Given the correct gear ratio (only relevant to the first forward or reverse gear,) it could possibly be possible (see, speculating) to reach 11 mph with under 1000 rpm.
And still idle speed has nothing to do with it because at idle the wheels are not connected to the engine. There is no logical step from "It idles at x so it could do y mph at z rpm" because idle speed has nothing to do with it.
I'm not questioning your knowledge. I'm more just challenging the great minds of GTP to think outside the box, in the sort of conundrum niky normally finds. :lol:
Trying to suggest the idle speed is in any way related to the in-gear road speed isn't outside the box, except in the same way that VV Cephei is outside the state of Idaho. You may as well be suggesting that the car has air conditioning or suede trim, so it's possible.

At idle, the engine and the wheels are separated. When driving along in gear, they are not. What the roadspeed per engine revolution in any given gear is, it is completely unrelated to the warm idle engine speed. I have no idea why you keep on bringing it up, except from the position of having almost zero knowledge of how transmissions work.

Sixteen posts, all of which could be deleted and the thread wouldn't suffer.
 
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There are a few cars in GT5 that have bike engines/transmissions but don't have the primary reduction ratio applied to them because PD didn't remember to set it; resulting in gearing that is around 2/3rds higher than it should be (including the Caterham and a few of the 60's Hondas). Maybe that is what he's trying to figure it out for?
 
I implied no such thing. I asked what car he was driving that did that - on the basis of it being so far out of normalcy to require the question.And still idle speed has nothing to do with it because at idle the wheels are not connected to the engine

That is not entirely correct. A large V8 or diesel motor has enough torque at idle to engage first gear - just let the clutch out slowly.
If it's an auto, it will depend on the stall converter. I have an '84 WB holden V8 with a 3 speed auto. Take your foot off the brake and it idles through first and shifts into second at approx 15 - 20 km/h.

With the accelerator pressed to the floor, that same first gear is good for about 75km/h
 
That is not entirely correct. A large V8 or diesel motor has enough torque at idle to engage first gear - just let the clutch out slowly.

You don't need a V8 or diesel to get a car moving without touching the loud pedal.

On, off topic. This thread shows that the classic statement of "A little knowledge is worse than no knowledge", entirely true.
 
That is not entirely correct. A large V8 or diesel motor has enough torque at idle to engage first gear - just let the clutch out slowly.
Anything has enough torque to engage 1st at idle speeds - my first driving lesson involved putting a 1.4CL Golf Mk3 into 1st at idle and letting it roll down the road. But with the car actually at idle - defined as at rest, in neutral and ticking over - it's not actually going anywhere!

What rpm a car idles at is irrelevant to what rpm a car requires to go at any speed in any gear.
 
[QUOTE="Famine]What rpm a car idles at is irrelevant to what rpm a car requires to go at any speed in any gear.[/QUOTE]

This is true. When the car is at idle, the RPM that is being turned is irrelevant because the engine and the wheels are not actually connected.

Because of that, it's impossible to make any correlation of RPM and speed based solely off of a car's idle RPM.

Now, if you can go 10 MPH at, say, 2500 RPM, then maybe you could possibly calculate the RPM at, say, 20. This is because the clutch is engaged at this speed in any normal car and so the wheels and the engine RPM are in correlation between each other.

But at idle, this just simply isn't true. It doesn't matter if your car idles at 500 or 5000 RPM, it's pretty much impossible to guess the RPM at any speed in any gear if you're going off idle RPM because that's totally irrelevant to the RPM reached when actually moving.

Famine's done a great job of explaining it, better than I could do. I simply don't see what there is to not get. I don't have a broad understanding of the way cars work but even I understand it, it's just listen to what he's saying because he's making good sense and I think if you read what he said closely then you'll understand.

EDIT: And if any car can reach 11 MPH at 1000 RPM, either the designers have no understanding on what good ratios are for a car, or it's an extremely radical car as in not an everyday road car anybody would see often.
 
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