Tire Calculation, a little help

[Graphite 6]

I just put on a set of TOYO 285's for winter, and am trying to calculate milage between fillups. I thought someone had posted this a while ago but couldn't find it. Not the rpm's, but the actual difference in miles traveled. Since I'll run the stockers in spring,( Michelin 265's) I'll wait to do the pinion set for a later date. Or is it the difference in % of the two rpm's?= miles travelled? Thanks.

 

[klenger]

According to the worksheet that I have at http://www.klenger.net/dodge/general-reference/gear-speed-calculator.xls, the 285's will travel 1. 035 times (3. 5% more) as far for the same RPM as the 265's.

 

[Hoefler]

You need to figure the rolling diameter. You mark the tire where it contacts the ground and mark the ground at that point. Roll the truck forward until the mark on the tire is at the point of contact with the ground again. Measure this distance to the mark made before on the ground. This is the rolling circumference. Do this with both tire sizes. With these figures, divide the larger diameter tire number into the smaller one. This is the percent of difference.



Example: 285s rolled 85" 265s rolled 78"

78" / 85" = . 92



The 265s roll 92% the distance as the 285s, so if the milage on the odometer reads 100 miles with the 285s, you actually went 108 miles.



If you are doing 60 mph by the speedo, you will really be doing 64. 8 mph



To get your final figures when you calculate your milage or speed, multiply by 108% to give you the correct number.



You should have the same load and tire pressure for both tire sizes when doing your calculations. I would suggest you use the front tire for you measurements since the load variance is minimal compared to the rear.