i need some ideas for correcting my speedometer. I just installed a gps system and its a little off.

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98 f-150 extended cab 4x2
fabtech 6.5
35x12.5 super swampers ssr
16x10 eagle alloys
geniune steel pro steps
billet grills
toneau cover
white face guage with carbon fiber overlay
12" jl audio's
6'5 component polk audio
3300 xtant amp
Clarion autopc
Alpine GPS
hypertech power programmer III

 

Did you swap rear gears? Those 35" tall swampers will really change things.

My experience was not with such a tall tire, but still the same rules should apply.

I have 3.55 gears. Went from a 235/70 to a 265/75. About a 2.5" difference..... My stock speedo gear was a 20 tooth. Changed to a 19 tooth, now it's all synced up again.

Here's a site that will help you determine all the different things that's going on.

http://www.bncoffroad.com/tire_gear_cal.html

You will need to know your current/past gears, and tire sizes, but should get you close.