View Full Version : formula for calculating MPH

05-03-2005, 07:04 AM
I ran across a formula that allows one to calculate MPH @ different rpm's with different transmission and rear end ratio's.
This formula would be helpful in selecting the correct rear end ratio
for your car.

MPH = (rpm X tire diameter) divided by ( *gear ratio X 336)
*gear ratio meaning: (rear end ratio X transmission ratio)

Does the math work? Is this a valid formula?
( I realize that top end is limited by h.p. and aerodynamics)

05-03-2005, 09:41 AM
www.randysringandpinion.com is what calculator I use. Google search for engine speed calculators and you should find lots.

05-03-2005, 10:37 AM
I use this calculator all the time. Very Very accurate. Really fun to mess with to by pluggin in different gears like 3.42 or 4.11. Or check your speed in different gears of your 6 speed tranny. :3gears:
Richmond Gears Calculator (http://www.richmondgear.com/101032.html)

05-03-2005, 11:03 AM
Is the formula I posted valid??

05-03-2005, 11:07 AM
Is the formula I posted valid??
Yes your formula works perfectly. :headbang: Just takes longer and most people dont want to crunch numbers in case they make a mistake. But yeah it does work.
I used my car as an example. 4.11 gears, .50 6th gear tranny ratio
Approx 2,000rpm with 26" tires. Means im doing approx 69mph.

EDIT2 to help clearify which really isnt necessary but im bored at work right now.

69.136MPH = ( 52,000 ) divided by ( 752.13 )
*gear ratio meaning: (4.11 rear end ratio X .50 transmission ratio)

05-03-2005, 11:17 AM
I would also like to be able to check mph in 1st gear @ 6,000 rpm.
And rpm's @ 75mph in overdrive (.59)
I not sure that this can be done @ the "Randy's" site.

05-03-2005, 11:18 AM
Then you should click on the link I placed above. Its easy to do there. Gear Calculator (http://www.richmondgear.com/101032.html)
EDIT: Whats your rear end ratio in your car? Also need your tire diameter. Also whats your first gear ratio in your tranny?
I could answer almost any question you have with that info.

05-03-2005, 11:56 AM
Yes you can input for whatever tranny gear you want at randys site. It has tire size calculators as well so easy to convert.

05-03-2005, 04:59 PM
Try setting the formula into an Excell spreadsheet. This way multiple columns can be made using differing gear ratios, tire heights, overdrives, etc.

05-03-2005, 07:34 PM
Thanks for all the reply's.
I just wanted to make sure that the math was not flawed with the formula that I was using.

In regards to specifics of "the car". This is a car that is currently being built and I hope to close the deal and finalize the details in the next week or two.. If I purchase the car I will be able to select the transmission and the rear end ratio.