Mine was originally automatic with 3.73's also. It depends on where you got the W58 from. Mine is from an SC300. Mine reads 83.3% of actual road speed. I verified road speed with the tachometer and a gps to read my speed. after that, I just had a general idea of what speed is where. 20 = ~25 30 = ~35 39 = ~45 44 = ~50 50 = ~60 and 83.3 = 100mph lol I'm still trying to figure out how i can adjust the signal going into the speedometer. I just know everything is reading 83.3%.