Quote:
Originally Posted by joedls
So how many tire revs per mile does your sheet calculate? Because I calculated the revs per mile using the exact circumference of what a 275-60-15 tire should be. I don't allow for any variances and frankly I don't know how your spreadsheet could do that accurately, considering different variances between tire, tranny, and rear-end manufacturers.
Here is my calculation.
(((275/25.4)*.6)*2)+15 = 27.992125 (Tire diameter)
27.992125 * 3.1416 = 87.940059 (Tire circumference in inches)
87.940059/12 = 7.3283382 (Tire circumference in feet)
5280/7.3283382 = 720.49076 (tire revolutions per mile)
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Here's the Excel Fomula for Revs Per Mile I'm using:
=(63360/(((B9/25.4*(D9/100)*2)+F9)*3.141597)) *0.99
B9 = Section Width
D9 = Aspect Ratio
F9 = Rim Diamter
It's basically the same as yours, EXCEPT for the last multiplication. This actually reduces the number of revs per mile and therefore increase the speed for a given RPM. I believe it was the original desginers intention to account for growth in the tire as a function of heat/pressure and rotational inertia.
However, my sheet also inlucdes a big reduction when calculating the speed for a given tire/axle/gear @ RPM x.
=((D18/(B21*B28*H9))*60)*0.97
D18= RPM (shift or redline, or whatever you want to put in, I'm assuming 6k)
B21=Gear ratio
B28=Axle ratio
H9=tire revs per mile
The final multiplication is an additional reduction I forgot about. If I had to guess it's for nonlockup auto trans, but that's just a guess. Thanks for the catch.
I'll take out these constants for future calculations so we're comparing apples to apples.