Okay… I have 35 inch tires on my truck and just swapped my 3.73 gear ratio for 4.10. What tire size would I dial into the ECU so my Speedometer is correct?

 

Check out the link for a chart and formulas for tire sizes and gearing.

http://www.4lo.com/calc/geartable.php

 

Since the 3rd Gen trucks use an sensor ring on the rear differential ring for ABS and speedometer sensor, changing the ring and pinion ratio does not affect the speedometer and odometer.

Start with 35" in EFI Live and then check it with a GPS or a GPS app in a Smart Phone. Or find a place on the highway with the "mile marker" signs.

 

Just a FYI and good reference to have ------>

Here is an excellent gear/tranny/tire speed calculator that was developed by Klenger.

The link is here:
http://klenger.net/dodge-ram/

Scroll down under the Dodge Ram section and find the "Gear Speed Tables" Excel link and download it for future reference. You can pick any available transmission, rear gear, tire size and it will calculate your speed. You can click on either the PDF link for online calcs or the Excel link to download if my memory serves me well.
(Thanks to Klenger for going to the trouble to develop it.)

 

Cool, thanks!

I figured out that the truck Speedometer is reading 6 km/hr too fast. Maybe it has never been corrected from the last tune I did and this is the difference between 33 inch tires and 35 inch tires.