I'm preparing to bump up tire sizes to ether 31x10.5r15 or 33x10.5r15. This is going to change my speedometer drastically. It's a 2002 automatic. I've tried to do research and the only thing I've found is to change speed gears in the transmission but I don't have those or can't find them. The other thing is to program them into the truck but I don't have and can't afford to buy a programmer. Any help?

 

Stock tire diameter is probably 27.5"
If you went to a 33" diameter, when you are showing 30mph you would really be going 36mph
Showing 50mph, real speed 60mph
Showing 60mph, real speed 72mph

2002 uses electronic speed sensor(VSS) so no gear to change, I think you still have the GEM unit behind the Radio, that is the unit that is programmed for trans, axle, and tire size to set speedometer, dealer can program that for you but they do charge for that, worth a call to see how much.

Other than that there is not much you can do, the speed sensor is also part of the ABS brake system I believe so adding a Dakota Digital interface may or may not work, but they are not cheap at $85
Universal Speedometer Signal Interface

 

dealer adjustment or drive with a gps and place the needles where they need to be by hand

 

The speedometer is part of the ABS system one other reason for me to fix it.