RPM equals MPH?

Ronald Wendt

Registered User
Is there a formula to figure out how fast you are going using the RPM of the engine in a given gear? I know my speedo is off some after changing the rear end gears and taller tires. Just want to know how much I am off. When I did the speedo gear calculation I am somewhere around 19.5. So I put a 19 gear in which reads slower than I am going.

Yesterday on the Indiana and Ohio pike's I showed 85MPH at 2400 RPM. I know I was not doing 85 but it would be good to know how fast I really was going. I think I am off 10 MPH but not entirely sure, and I think the faster you go the bigger the gap between what the speedo shows and actual speed?

Thanks

Ron
 
I read those threads but unless I am missing the obvious once again, I do not see anything where I could calculate my speed on the highway at a certain RPM.

Bang me upside the head with a frying pan if need be to show me where that info is

Ron
 
Prestage

www.prestage.com, go to the car math section there is a speedo calculator to figure speed vs RPM and stuff . . . you will need to know the diameter of your tires, if they are the stock size, 225 60 R16 then the diameter is 26.6, if you have another tire wheel combo use the tires size calulator to find what the tire diameter will be. You will also need to know your transmission gear ratios and your rear end ratio . . .

Dave
 
way cool, exactly what I was looking for.

The formula is: MPH=rpm x tire diameter divided by rear gear ratio x 336

so for me in fifth gear I had to multiply the 3:27 gear by .75 fifth gear overdrive.

So, with my 27.1" tall tires when I was showing 84MPH at 2400 RPM I was actually going 78 MPH or so

thanks again
 
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