Need some help. Changed my wheels from the stock 16" to 15". My speed reads about 30 mph when I am actually doing about 48-50 mph. I have the old SM 420, 4-speed. Is it possible to change out the speedo gear to get the speed odometer lined up with the actual speed? Any help would be greatly appreciated. Also, I am looking at changing gears in the rear end to the 3:55's in the near future. Will that have an impact? Thanks Slinger
Tire diameter and rear differential gearing are the 2 factors that affect the mechanical speedometer reading. So yes changing the rear gear ratio will change the speedometer reading. What diameter rear tires are you running now? A bigger diameter will help tame highway engine rpm's.
I wouldn't change speedo gears till after the rear change as going from the factory 4.11 to 3.55 rear gear will put your reading much closer to right with the present speedo gears. Your speedo is off by about 35% now and the 3.55's are about 30% shallower than the 4.11's so it will be close enough to avoid a ticket. I've got so used to my GPS that I don't even look at a speedometer anymore. Tires and gears are a non-factor to it.
