NEW GEAR RATIO FOR 39.5" TIRE?

[JD_MILLS]

I'm currently a 39.50x13.5 r20 and really need to change gear ratio's still got plenty of power but i know they're killing my mileage. any ideas on what ratio i should use? i don't do any type of towing with it or off-roading. truck is strictly for the street.

[Karls03]

Math 101: your gear ratio should be the same change rate as your tire change to keep things equal. So 39.5/31.5= 1.2539. This rate of change is now multiplied by your rear gear to find your "new" gear, 3.73 x 1.2539= 4.67. While you're speedo won't be 100% correct you'll be closer with a 4.56 gear than a 4.88 gear. In gearhead speak, go 4.56 if you daily drive and care about mpg, 4.88 if those don't matter.

By the numbers, your truck is performing as if it had 31.5s and a 3.00:1 rear gear. YIKES!