I would get a rear tire valve stem at 6 0'clock in a parking lot & roll the car straight forward till the stem is back at 6 0'clock (~8 ft) & tape measure that "rollout" distance then divide Pi (3.1416) into that # to get your exact tire diameter. I'm sure you know if those gears are 2.76's or not. you might also go out on the interstate & go past 4 or 5 mile markers & see if your ODO is correct cuz if the axle gears/speedo pinion teeth are correct then the odo must read correct & the prob is in the magnetic speedo head gismo & it needs to be sent to a specialty shop to be recalibrated but my gut tells me something else is wonky here. post how it goes. GPS would solve this or if you see a portable lighted digital roadside sign or if you see a cop parkedon the side of the road ask him if he will clock you! have it a steady easy to read MPH when you pass him such as 40 or 50 etc. post how it goes/what it ends up being.