Speedometer way off

[Dewhite2485]

got the car pretty well done changed an 82 240D to an 82 300DT, changed the engine, trans, rear end, and speedometer everything came out of the same 300D but when driving the speedometer is close to 10 MPH off I didnt check the rear end ratio before I put it in but if I need to Ill get it up in the air and look at the tag, but the speedometer number is 123 542 63 01. I think thats the right speedometer for an 82 300D turbo, the tires are not the correct size but should not make a 10 mph difference if anyone has any ideas as to why its so far off it would help allot