My son in law was out of town and being the so called mechanic he is put a complete rear end in my truck the old one tore up he said have not looked but the ratio was a 3.92 an to begin with he put a 3.55 in it does ok but wont shift right and of course mph is off can i reprogram to fix that or should i try to fix or find a a 3.92 Thanks
Is it a 4wd truck? If it is you have to change the gears out to match one or the other. You’ll blow up your transfer case. Gear changes shouldn’t effect the speedometer, the dodges use a speed sensor in the differential or in the front abs hub so the gear don’t change the speedo, only tire size changing.
VSS is read off of the differential tone ring and has nothing to do with gear ratio. It simple meases differential rotation. Speedo adjustment is only needed if tire height is changed and then it is adusted in the tune.
My speedometer was off by 4 mph, I used GPS to determine speed of truck, then adjusted with my SuperChips CORTEX tuner. Reverified speed with GPS. I am now about 1/2 mph off, reading high, works for me.