I read somewhere on this site that 285's can throw off the mileage meter enough to say your getting 12mpg, but with the 245's you get 19...does this hold any truth

I checked my speedo with my aviation GPS and found that wuth speedo showing 70mph, the Gps showed 75mph.

JhnZ71,

I compared my odometer to the mileage post signs on the highway, i noticed my odometer was about .1 off. That wouldnt make a 5mpg difference i dont think, but it might. with my 285's if i compensate and say that its a 1/10th off i get about 20 highway and 16 around town.

The tires make your mileage worse because of resistance and effective gearing change. The difference in the odometer makes the DIC reading be off further in that direction.

Add 7% to your odometer mileage and then divide by gallons used.Your gps confirms the difference of 7%.

With my 285 revos I am off 10% on my odometer and at 75 I am doing 80 mph and that is with using my GPS. so i just add 10% when I do my miles per gallon.

My 315x70-17, with my speedometer correction, has my speedometer/odometer off by 15%. I've tracked my speed against my wife's Tahoe, radar trailer at construction sites and mile markers to verify the 15%. I did loose about .5 MPG going to the larger tires. The good thing, my multiplying by 15 has gotten very good.

WYOMAX, if your speedo is off 10%, at 75 MPH you are actually traveling 82.5 MPH, not 80.

MaxRock