tire and ratio change....speedo?
 

I originally has 235/75/R15 and 3.73
By the end of tomorrow, I will have 31s and 4.10

How the heck does this make the speedo change? I have seen the tire change effect calculator on TRS, but nothing about changing gears... What will happen with both being changed? What can I do to correct this on my 1999?
Thanks

For 1998~2000, the speed sensor reads ring gear RPM so axle ratio changes have absolutely no effect on the speedometer.

Tire diameter changes do cause speed errors though. For your 99, the speedometer correction is done in one of 2 modules depending on which ABS you have:

RABS: correction done in the GEM
4WABS: correction done in the 4WABS Module

Ford dealers have the special tool to access these modules.