Is it 1020 ,1000 ,980 ,970 ,960 , revolutions per mile ?? I need to know for sure so i can put the right numbers in the calculator to get the right driven gear for the speedo on my `97 . I went from the stock 215/75-15 tires to a 255/65-16 tire from a 01 dakota and my speedo is off to the slow side , the looks and the handleing are way better now with the 16`s but take off is slow with the 3.9 ,,, i definitely need to up the rear gears from 3.55 to the 3.92 when i get money and can find an axel with those gears ready to go !

 

Tire size - http://www.4lo.com/calc/gearmetric.htm

Gearing - http://www.4lo.com/calc/gearratio.htm

Speedo difference - http://www.4lo.com/calc/gearspeedo.htm

When inputting the tire height on the gearing calculation subtract 1" from the calculated tire size.