Sounds like you have 4.10's which can be corrected with a tuner or speed cal. First find out what gears you actually have and then go from there. 3.73's put mine off by 10 mph and every ten miles I was adding an extra mile to the odometer. Easy way to check your gears is to jack the rear of the car and put it on jackstands. Then mark the driveshaft and one tire with a piece of tape or a marker that you can easily see. Then rotate the tire one revolution while watching the drive shaft. Count the drive shaft rotations. If it turns a little more than 4 times to the tires one, then you probably have 4.10's. If it turns a little less than 4 times, then you probably have 3.73's.
You can look at the speed cals American Muscle has. I would call them to make sure you get the right one. A dyno tuner can correct it, but of all the options that would be the most expensive. A handheld tuner from AM will do it, too, and they will put a tune in it for whatever mods you have at the time. Make sure you mention the gears.
Edit: When you add lower (higher in number) gears it makes the rear wheels turn slower per rpm's.