Speedometer Reading Low On Stock 03 Gt

Probably due to the car having a different rear end gear ratio and never having that corrected in the computer.

Sounds like the replacement rear axle came out of a car with a taller rear gear than the factory 3.27 that came in all '03's. Based on your first post, I'd guess you have 2.73's.

You could try disconnecting the battery for half an hour, but I can't see any scenario where that will solve your problem.

I'm sure you get better mileage on the freeway and around town, but those tall gears are really hurting performance. I don't know how long ago the work was done, but if insurance paid for the work you should let them know the job isn't done, or wasn't done properly. Your car should be as good as before the accident or better. Right now it is worse, and you could have dealt with those tickets by demonstrating your speedo was improperly calibrated due to the second-rate work done by the shop.

silvergtcoupe03 said:

I have a problem with my speedometer where it reads that i'm going slower than i really am. If the speedometer reads that im going 40, it will really be like 47, or if im going 45 i'll be going 52 or 53. I've already gotten 2 tickets from this and i need to find a solution for this. I was rear ended by some kid about a few months ago and needed a new (used) rear axle, i have no idea if that could be any cause. Any ideas?

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You need to tell us where the replacement rear end came from.

I agree with 40oz: The math seems to indicate that the rear axle under your car right now has a 2.73:1 ratio. This might suggest that the rear came from a mid 90s Mustang. Just out of curiosity do the rear wheels seem to sit further "under the car" than before the replacement? Earlier rears were narrower than New Edge rears were...

I wouldn't rely on a differential tag being 100% right. They may have only looked at that and not the internals. You could always do the old school method of jacking up the rear end, putting the car in neutral, and spinning the tires by hand to see how many revolutions the driveshaft makes in relation to one tire rotation. (If your car has a limited slip, both tires should spin the same direction, and you only need to spin them one full rotation. If you have an open differential, one tire will spin forward and the other backward. If it's an open diff, leave one rear tire on the ground. Turn the tire that's off of the ground two full rotations, not one.) Mark a line on the tire you are spinning, and one on the driveshaft.

So, Limited slip: Both wheels off the ground, turn one complete rotation and count driveshaft rotations.

Open Diff: Lift one wheel off the ground and turn 2 complete rotations, then count driveshaft rotations.

It might be a bit hard to count the rotations. I usually make chalk marks to line things up with. If your driveshaft turns roughly 3 and a quarter turns, that should be 3.27s. If it only spins 2 and a quarter turns, sounds like 2.73s.