Speedo Help

Discussion in 'Fox 5.0 Mustang Tech' started by My25thAnnivStang, Jul 13, 2013.

  1. I picked up a 90 GT and noticed my speedo is off by 10 mph. I have paced another car and checked it by GPS and I am going 10mph slower than what my speedo is showing. Not sure if the rear grades have been swapped. It was built with the stock 273 gears. Does anybody know what gear ratio would produce a consistent 10mph speed difference, or could this just be a speedo issue? Thanks
  2. First thing is to check is what rear end gears you have. A quick way is to jack up the rear and rotate the tires one revolution and count the number of times the driveshaft turns. If it is 3:1 you have either 2.73 or 3.08. Look to see if the original tag is on the rear end cover that will tell you what gear ratio you have. If the cover looks like it was never off, then chances are you have stock gears.

    Next I would check to see what speedo gear you have in the transmission. They are color coded to tell you how many teeth they have. There are different teeth speedo gears to match what rear end gear ratio you have. Also, if you have a substantially different tire size than stock that will impact your speedometer reading.
    striker911411 likes this.
  3. The tire size will make a difference but it would have to be a major difference for 10 mph. Like a 4 inch difference in tire size. The difference between a 28" and a 32" tire isn't even 10 mph
  4. I bet you have the 3.08 gear and a 2.73 rear end. If you think about it....