Speedo in op on 4 speed OD trans.

Ok, I have a different problem know on the 74 Duster. I just got the car runnig after a 8.75 rear end upgrade from the styock 7.25. That went ok and I have very little problem with the conversion. But here is the rub. During the test drive I noticed that my speedo was ready way high, like on the order of 30 40 miles high. When I installed the manual trans during the resto, I notice about 10 miles differnet from when the 904 came out and had not realy bothered with worring about it. But know I have to do some thing. I have a 833 OD trans driving a 8.75 rearend with 15 wheels. What speedo gear do I need. I try to adjust mine that is currently installed and it will not work the speedo when set in eithe rof the 2 positions marked. I have to rotate it a whole nother 980 degrees just to get the gears to mesh and then It reads way high. Can one one give me some pointers.