Hey guys, I've recently purchased a 2014 mustang GT automatic. The vehicle has had a bit of work done by the previous owner including axle-back exhaust and staggered wheels with my rears being 305/35ZR19. The dealer I purchased from didn't have the tuner with the vehicle and they had the vehicle returned to stock. I've noticed that speedometer is off in varying differences (for example: speedo reads 80, gps reads 60). I've done the tire calculator and converted my rears to 735 revs/mile from stock using the sct x4 tuner that I recently acquired but it didn't really make changes to my speedo (pretty much the same differences in speed). I'm aware that I can take the diff cover off and count the teeth and make the calculations. Just wondering if I could manage a least invasive approach by measuring diff gear by jacking up rear end and marking the driveshaft and the tire and counting revolutions of driveshaft in relation to 1 tire rotation. However, stock wheels are 18 inches and my rear are 19 inches so I'm sure it will cause a difference. Just unsure if it would cause a huge difference or would playing around with the tuner and changing axle ratios to known ratios such as 3.73 and 4.1 be a safe method. Worst case I will remove the diff cover over the weekend. Just curious on any input for this situation

 

Did you call the dealer to discuss the issue. (You say that you recently purchased it.) I definitely would play around with the tuner and change axle ratios to known ratios, before I pulled the differential cover.

 

Why not do the 1 revolution on tire and count driveshaft revs? Tire size doesn’t matter because you want effective axle ratio anyway for the tuner and speedometer, right?

 

Hey sorry I haven't replied to this post but I ended up doing the turns and found out it was 4.10 gearing. When I updated through my tuner the speedometer pretty much matched with google maps. Thanks!