how to calculate tire rotations per minute

[Ear Rak]

was wondering how to calculate tire rotations per minute so i can correct my speed for the 315's (yes they threw it off, the speed is 3 mph too fast. ex: i'm doin 37, my speedo says 40) and was gonna do it through the predator tuner

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[fireball168]

Rotation per minute isn't what you want...you want rotations per mile.

Here are the two ways I do it, pick one - the only difference is the way you are measuring the rolling diameter of your tire.
Measure the outside diameter(how tall it is) of your tire, loaded normally. Lets say it is 26". 26X3.14(PI)=81.64" There are 63360 inches in a mile. 63360/81.84 = 774.19 rotations per mile. This assumes your tire is perfectly round going down the road...it isn't.

Then.... Make a mark on the bottom of your tire and on the ground where the mark meets the asphalt. Have a friend drive the until the mark is pointing straight down at the ground again. Measure the distance between the two marks, and use that number 63360/X for your rotations per mile. This will get you closer, in my opinion, but it still isn't perfect as the tire will "grow" more round at speed.

Pick a number in between your two calculations and you've got the best of both worlds. Or pick either one and you'll be close enough.

 

[gnxs]

was wondering how to calculate tire rotations per minute so i can correct my speed for the 315's (yes they threw it off, the speed is 3 mph too fast. ex: i'm doin 37, my speedo says 40) and was gonna do it through the predator tuner

Check the manufacturers' website. Most of them list revolutions per mile for their tires.

 

[Ear Rak]

Check the manufacturers' website. Most of them list revolutions per mile for their tires.

well thats the problem, i'm running wider tires but a lower profile, so my speedo is a couple MPH off

 

[gnxs]

well thats the problem, i'm running wider tires but a lower profile, so my speedo is a couple MPH off

I know that. Didn't you say you needed to find out how many revolutions per mile these new tires were?

What kind/size tires did you take off and what are you putting on?

 

[Randy@SuttonHp]

If you PM me I have an excel spreadsheet that will show rev per mile, this is an example below. Hope this helps


Stock:
Section Width Aspect Ratio Wheel Diameter
255 75 16

Revs Per Mile: 671.3018127


Mod:
Section Width Aspect Ratio Wheel Diameter
285 75 16

Revs Per Mile: 635.0761482

 

[ledfoot85]

http://www.discounttiredirect.com/direct/brochure/info/tmpInfoTireMath.jsp

try that site