This may have been covered before but I have done many searches and have not found the formula for correcting mileage now that I have added 285 tires and have not touched the stock speedometer.


I know that for 265 there is a 3.9% diferenee and I think, by my calculations the difference with 285 is 5.1 but i am no math wiz.


Somebody check my math please

just add one MPH for every 15 shown on the dial.


15 = 16


30 = 32


45 = 48


60 = 64


75 = 80 After that it really doesn't matter.


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245/75R16 = 30.47 inches tall
245mm/25.4mm = 9.65 * (2 * .75) = 14.47 aspect ratio + wheel size 16 inch = 30.47 inches
285/75R16 = 32.83 inches tall
285mm/25.4mm = 11.22 * (2 * .75) = 16.83 aspect ratio + wheel size 16 inch = 32.83 inches
32.83/30.47 = 1.0775 or 7.75 % difference
the bigger wheel turns 7.75 % less at the same speed, your speedo will be off (shows too little) by that amount, also your odo will count 7.75 % too little
This is only the math but tires differ in size from brand to brand a little bit. Hope that helps

Frank

Thanks guys, I should be filling the tank for the first time since the new tires tomorrow. Now I should get an accurate mileage calculation.

Question , if you use one of the tuners that changes the speedo does that also change your odometer to match so that it is correct?

Yes, I beieve so.