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WVZR-1 · 09-18-2011

Yours is a very interesting approach to correcting a speedometer error.

It would be much easier to calculate if the "new" tire size/diameter was a "known" value. Then you could use an RPM calculator to get "CLOSE". I mention close because actual speedometer calculations are for "rolling radius" and not based on advertised tire diameter.

You could do it more accurately measuring your present tire "loaded radius" and then using this "speed calculator": http://wahiduddin.net/calc/calc_speed_rpm.htm

I've used this after measuring and it's pretty damn accurate. I don't recall the "loaded radius" dimension from my 315's and I can't measure it presently.

Here's an interesting way to approximate the "rolling radius": http://www.vibratesoftware.com/html_...ded_Radius.htm

To get "CLOSE" use advertised diameters, trans rato 1:1 and this calculator to compare: http://www.csgnetwork.com/multirpmcalc.html

I've used all of these for speedometer, tire or axle ratio comparisons. I always just use a trans ratio of 1:1.

09-18-2011	#3
WVZR-1 Join Date: Oct 2004 Posts: 1,889	Re: Contemplating Rear Gear change Yours is a very interesting approach to correcting a speedometer error. It would be much easier to calculate if the "new" tire size/diameter was a "known" value. Then you could use an RPM calculator to get "CLOSE". I mention close because actual speedometer calculations are for "rolling radius" and not based on advertised tire diameter. You could do it more accurately measuring your present tire "loaded radius" and then using this "speed calculator": http://wahiduddin.net/calc/calc_speed_rpm.htm I've used this after measuring and it's pretty damn accurate. I don't recall the "loaded radius" dimension from my 315's and I can't measure it presently. Here's an interesting way to approximate the "rolling radius": http://www.vibratesoftware.com/html_...ded_Radius.htm To get "CLOSE" use advertised diameters, trans rato 1:1 and this calculator to compare: http://www.csgnetwork.com/multirpmcalc.html I've used all of these for speedometer, tire or axle ratio comparisons. I always just use a trans ratio of 1:1. Last edited by WVZR-1; 09-18-2011 at 08:15 AM.