This is fairly common on 1st gen cars. That why I take those "My top speed is 170mph" boasts with a grain of salt. If they have the original gear, their speedo could read more than 10 mph high at those speeds.

Anyway, the accuracy can be improved by replacing the speedometer gear. The '91-'93 VR4 TSB # is 93-22-002. The Mitsu part # is MR111502. Dodge should have their own TSB & part #, but the part should be identical. This should be the 28 tooth gear. I assume that MR111501 is the original 27 tooth gear. Ask your favorite Service Manager to print out the TSB for you. He should hand you a nice 4 page TSB with drawings -- looks service manual quality. It's very easy to replace though even without instructions. Just remove the battery & windshield washer bottle. Then you can access the gear housing. Just remove the electrical connector on top, and one small bolt and it should lift out. Then use a nail & hammer to punch out the pin. Put a little oil on the shaft of the new gear when you swap the gears. Reinsert the pin with the split facing outward. Should take the do-it-yourselfer less than an hour to replace.