Axle ratio
You can go to mymopar and look at the manual for your car, in the transmission section there should be charts like the one below. I just changed my gear ratio which required speedometer pinion gear change as well. Using a GPS Speedometer ( i used speedbox, free app) agrees with my speedometer on the speed of the car. (So the chart below was accurate for me) If you have changed tire size - you can find several websites for that
You can go to mymopar and look at the manual for your car, in the transmission section there should be charts like the one below. I just changed my gear ratio which required speedometer pinion gear change as well. Using a GPS Speedometer ( i used speedbox, free app) agrees with my speedometer on the speed of the car. (So the chart below was accurate for me) If you have changed tire size - you can find several websites for that