I'm about to change gears to 4:10 and go to 37" tires. What is the best route to go for making my speedometer correct? Currently I have 3:54's and 315's and am off by about 5mph. Will a Smarty correct gear ratio and tire size or just tire size? Thanks

 

Gear ratio does not matter because the pick up sensor is on the differential ring gear itself--which is directly counting rear tire revolutions.

Smarty will correct your speedo regardless of the gear ratio. You can change tire size in 1/4" increments.