Speedometer Error and Tire Size
Date: 07/13/99 at 09:48:29 From: Michael Sugarman Subject: Speedometer error vs tire size I have a car that is used in competitive events like rallies where average speed and exact speed are the most critical factors. As I change tire size, speedometer error comes into play. I need to know my exact speed at a given rpm based on tire size. I know the complete dimensions of the tire, the drive ratio of the car, and how far the tire goes in a straight line per tire revolution. How do I figure my speed in miles per hour? Any help would be appreciated. Michael
Date: 07/13/99 at 10:54:19 From: Doctor Rob Subject: Re: Speedometer error vs tire size I assume that "the drive ratio of the car" is the number of axle revolutions per crankshaft revolution, and "rpm" is crankshaft revolutions per minute. Multiply the rpm times the drive ratio of the car times the distance the tire goes in a straight line per revolution (in feet) divided by 88. The factor of 88 is to convert from feet per minute to miles per hour. (88 = 5280 feet per mile/60 minutes per hour). Of course, this is assuming that you have perfect traction, the distance the tire goes per revolution is constant with speed, and all the turns average out, and a lot of other factors are negligible. This will fail if you have an automatic transmission, since the rotations of the crankshaft cannot be directly tied to axle turns, but I suppose you knew that. - Doctor Rob, The Math Forum http://mathforum.org/dr.math/
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