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Old April 4th, 2006, 09:22 AM   #7
Haggar
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Quote:
Originally Posted by FrankNBrew
Here's the formula for using excel to calculate revs per mile:

=20852*((C2/2+((A2/25.4)*B2/100))*2)^(-1.0003)

Copy and paste the above formula into cell D2. Then enter tire width into cell A2, aspect ratio into cell B2, and wheel diameter in cell C2.

In case your real dumb:

Example LT305/70/R16

305 is the tire width in millimeters
70 is the aspect ration (sidewall height is 70% of tire width)
16 is the wheel diameter in inches

Haggar, you might want to add this to your revs per mile tab.
I thought about it, but its better to find the true numbers for your tire. It ends up being dependant on the tire, and the variation from true gets statistically larger as the tire diameter gets larger, and varies brand by brand, which is why I left it that way.

For example, when you look at car tires (I use this for looking at car situations, too, for cruising and racing), they are very very close to accurate, usually about 1%-2% smaller than advertised.

Then smallish truck tires like 31s or 265/75R16s have maybe 3-4% smaller.

Once you get up to big sizes, then its all out the window. A 35" SSR is a full 1.5" taller than a 35" Bogger, so you just need to figure whats best for your tire.

That, plus most people aren't running metric tires.
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