Speedometer Wrong

I recently purchased an '89 GMC 1500 with the 350/700r4 combo. I got a good deal because the trans was on the way out and I swapped in a rebuilt unit myself. The guy I bought the truck from told me that he had swapped the rear end out for a "posi unit". I don't know what year it came out of or anything else about the rear. My problem is the speedo reads 10-15 mph high at freeway speeds (65-70mph). The tires are the stock size 235-75-15. When I bought the truck, this was the least of my concerns, 'cause I figured there were plastic gears in the tailhousing, and I would just need to change these to compensate for the numerically higher rear end (i'm guessing at that). When I swapped the trans, I found out there is a metal gear on the tailshaft and the speed sensor is basically magnetic driven. Do they make different size metal gears for the tailshaft, or do you change the speedo reading electronically? If they make different number of teeth gears, how do you figure out which one you need? Sorry so long, just wanted to list everything I could think of that might help.