Equation to determine final ratio?
Thread Starter
Racer
Joined: Jun 2005
Posts: 397
Likes: 1
From: Erie PA
Equation to determine final ratio?
Is there a math equation to determine the final ratio using transmission ratio, tire size and rear differntial? I am interested in seeing how they relate the trans ratio to the diff. I know there are calculators to determine that stuff but I would like to crunch the numbers myself.
Thanks in advance.
Dave
Thanks in advance.
Dave
Pro
Joined: May 2003
Posts: 601
Likes: 0
From: Nottingham
Yeah, you just multiply! 
If your diff ratio/final drive is 3.55:1, and your gearbox ratio is, say 1.5:1 (top gear is normally 1:1, but I'm just using a number other than 1 to demonstrate), then your overall gear is 3.55 x 1.5 = 5.325:1
So when your engine's spinning at 3000rpm, the back wheels will be spinning 5.325 times SLOWER, ie 563rpm.
Or, say you have an overdrive box where top is 0.65:1, then with the same rear end the overall ratio is 0.65x 3.55 = 2.31:1 In this case at 3000 fwrpm, you have 1300rwrpm.
The circumference of a tyre is pi x Diameter, then you have all the information you need to work out road speed.
If your diff ratio/final drive is 3.55:1, and your gearbox ratio is, say 1.5:1 (top gear is normally 1:1, but I'm just using a number other than 1 to demonstrate), then your overall gear is 3.55 x 1.5 = 5.325:1
So when your engine's spinning at 3000rpm, the back wheels will be spinning 5.325 times SLOWER, ie 563rpm.
Or, say you have an overdrive box where top is 0.65:1, then with the same rear end the overall ratio is 0.65x 3.55 = 2.31:1 In this case at 3000 fwrpm, you have 1300rwrpm.
The circumference of a tyre is pi x Diameter, then you have all the information you need to work out road speed.
Last edited by theoUK; Feb 23, 2007 at 11:26 AM. Reason: typos!
