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#1
08-12-2019, 09:48 PM
 Charter Member Join Date: Apr 1999 Location: 742 Evergreen Terrace Posts: 6,264

## Car gear ratio and final drive ratio calculation

I really botched title, sorry. I was typing in the title when I didn't know it for a moment. Would a moderator be nice enough to change it to something reasonable?

I think have this understood from reading web pages, but could someone proofread my workings?

If the gear ratio is .79:1, then the engine is turning 79% of the rpm the driveshaft is. If it's doing 7,000 rpm, then the driveshaft is doing 8,861 rpm.

If that's true, and the final gear ratio is 3.5:1, then the wheel is doing 1/3.5 of the rpm of the driveshaft, so the wheel is doing 2,532 rpm.

We can then multiply by the circumference of the tire, and multiply by 1hr/60min to find the final mph at that engine rpm.
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Last edited by Cardinal; 08-12-2019 at 09:50 PM. Reason: I typed my number into the title while being a pillock.
#2
08-12-2019, 11:17 PM
 Robot Mod in Beta Testing Moderator Join Date: Mar 2001 Location: Pennsylvania Posts: 24,492
Moderator Note

Quote:
 Originally Posted by Cardinal I really botched title, sorry. I was typing in the title when I didn't know it for a moment. Would a moderator be nice enough to change it to something reasonable?
Fixed.
#3
08-12-2019, 11:59 PM
 Guest Join Date: May 2015 Location: 123 Fake Street Posts: 7,849
You reasoning seems correct, but your units might be confusing. Are you measuring the tire circumference in miles?
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#4
08-13-2019, 05:31 AM
 Guest Join Date: Aug 2010 Location: Under Oveur & over Unger Posts: 12,036
Quote:
 Originally Posted by Cardinal I really botched title, sorry. I was typing in the title when I didn't know it for a moment. Would a moderator be nice enough to change it to something reasonable? I think have this understood from reading web pages, but could someone proofread my workings? If the gear ratio is .79:1, then the engine is turning 79% of the rpm the driveshaft is. If it's doing 7,000 rpm, then the driveshaft is doing 8,861 rpm. If that's true, and the final gear ratio is 3.5:1, then the wheel is doing 1/3.5 of the rpm of the driveshaft, so the wheel is doing 2,532 rpm. We can then multiply by the circumference of the tire, and multiply by 1hr/60min to find the final mph at that engine rpm.
You’re going ~180-195 mph depending on the specific tire size.
#5
08-13-2019, 05:38 AM
 Guest Join Date: Feb 2007 Location: Challenger Deep Posts: 12,057
Quote:
 Originally Posted by Cardinal I really botched title, sorry. I was typing in the title when I didn't know it for a moment. Would a moderator be nice enough to change it to something reasonable? I think have this understood from reading web pages, but could someone proofread my workings? If the gear ratio is .79:1, then the engine is turning 79% of the rpm the driveshaft is. If it's doing 7,000 rpm, then the driveshaft is doing 8,861 rpm. If that's true, and the final gear ratio is 3.5:1, then the wheel is doing 1/3.5 of the rpm of the driveshaft, so the wheel is doing 2,532 rpm. We can then multiply by the circumference of the tire, and multiply by 1hr/60min to find the final mph at that engine rpm.

You're going to want to multiply by 60min/1hr; your answer needs hours on the bottom, and it needs minutes to be gone.
Also, you need to get the circumference of your tire in miles.

So for example, let's say your car has a 225/55/17 tire. The rim is 17 inches in diameter, and the height of the tire is 225mm * .55 = 124mm = 4.87 inches.

Your tread diameter is 17 + 4.87 + 4.87 = 26.74 inches.

Your tread circumference is 3.14 * 26.74 / 12 / 5280 = 0.001326 miles.

At the quoted rotation rate, your road speed is 2532 * 0.001326 * 60 = 201 MPH.

Unless I've totally hosed the math (quite possible), this is a very unusual car. You've quoted basically the top gear ratio and maximum engine RPM, which is why we're going so bloody fast. are we in a supercar?
#6
08-17-2019, 01:46 PM
 Guest Join Date: Jun 2016 Location: Maine Posts: 148
Quote:
 Originally Posted by Machine Elf Unless I've totally hosed the math (quite possible), this is a very unusual car. You've quoted basically the top gear ratio and maximum engine RPM, which is why we're going so bloody fast. are we in a supercar?
Not necessarily a supercar. I once entered the gear ratios for my 25 year old, manual transmission, Chevy pickup truck into an online calculator It told me my maximum speed in 5th gear was something like 190 miles an hour, if it had the horsepower to pull all the way to the redline. With 195,000 miles on the 180 horsepower engine, that seems unlikely. At 55mph in 5th gear, it is a little over 1100 rpm on the tachometer. The low gear ratios are also for fuel economy. I can't recall what the final drive ratio is, but it was the lowest one available.
#7
Today, 05:59 AM
 Guest Join Date: Feb 2007 Location: Challenger Deep Posts: 12,057
Quote:
 Originally Posted by KMS94 It told me my maximum speed in 5th gear was something like 190 miles an hour, if it had the horsepower to pull all the way to the redline.
That was kinda my point: if we're cruising at 200+ MPH, we're going to need something like 600-700 horsepower.
#8
Today, 11:05 AM
 Guest Join Date: Mar 2015 Location: Portland, OR USA Posts: 507
Quote:
 Originally Posted by KMS94 The low gear ratios are also for fuel economy. I can't recall what the final drive ratio is, but it was the lowest one available.
And I think KMS’ point was that, since the same gear ratio that provides a high theoretical top speed at redline also lowers fuel consumption at legal speeds (compared to numerically higher gearing), the tall top gear on his truck is there for fuel economy, not top speed.

As an aside, describing gear ratios as “low” or “high” is ambiguous. A “higher gear” or “top gear” is one that’s numerically lower than most of the others, so things can get confusing.

For clarity, enthusiasts often say “numerically higher” or, more commonly, “taller.” A final drive ratio that results in high RPM at highway speeds is a relatively “short” ratio.

Many in this thread already know this, but the terminology can be confusing to those not already in the know.

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