I'm bored at work, so trying to figure out the formula to calculate what speed a car is traveling based on the rpm, gear ratios, and just realized I need wheel diameter too.

Anyone know it? I want to check it once I'm done. Need to run out to the car to measure the wheel diameter.

Right now, I'm guessing it is something like:

rpm * (gear ratio / final drive) * circumfrance of the wheel

I know it is wrong... that probably gives the distance travelled per minute. Hrm. Need to think about this some more.

 

(rpm *grear ratio)/differental*tire circumferencein ft/5028 = miles a min *60 = MPH

 

What is the differential for the integra? Don't have my Helms on me.

The one I ended up coming up with, but can't verify because I don't know the primary drive ratio for the Integra (do cars have them? I know motorcycles do)... anyway:

rpm * primary drive ratio * (gear ratio / final drive) * circumference / 63360 * 60 = mph

63360 = inches in a mile, and 60 is minutes in an hour

 

RPM / gear ratio / final drive / 336 x tire OD

7500 / . 909 / 4.25 / 336 x 22.7 = 131.16 in 4th gear, still have 5th left!

FYI this is D series (the slow ones) info.

 

What is the tire OD? Outer diameter?

 

yes, usually 22.7 for 175/70/13 and 195/50/15