Quote:
Originally posted by smithbling:
This method is FAAAARRRRRRRR more accurate than the calculators.
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I can't agree with this statement at all. Your method of calculation through experimentation makes the assumption that the speedometer is correct. If it's not, the amount of error will also double or triple along with the velocity you're multiplying.
Using a calculator is the only way to do it accurately. If exgmguy really wants to know, he can take a tape measure and measure the diameter of his tire. I say this since measuring the height of the tire would be more difficult to do accurately and the size numbers of the tire are probably not correct anymore because they are worn.
Assuming that the tachometer is correct (which I do since it is an expensive one that is designed to be accurate), use this formula (It's essentially the same as yours):
Achieved RPM X Tire Diameter
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Trans Gear Ratio X Axle Gear Ratio X 1056
The
346 in your equation should be a 336 (1056/PI). The 1056 is the number of inches in a mile divided by 60.
Also, just for anyone who cares, if you must go by the size on the tire, you can figure tire/rim height in smithbling's equation with this formula:
(Aspect Ratio * Width)/1270 + Rim Height
Like I said, though, that assumes a tire with no wear. Oh, aspect ratio should not be converted to a percent for this equation to work. That's handled by the 1270 which is just the conversion factor for milimeters to inches multiplied by 50. It's 50 instead of 100 so that you don't have to multiply by two for the top and bottom of tire.
I would like to know why you said to decrease the result by 7.4%, smithbling.
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351W 89 Mustang GT Convertible