Yeah, the wheel circumference is different but the gearing ratio (worm gear) in the axle mounted sending unit maybe different as well. I wouldn't use it. You should be able to find a good used one.
Mechanical speedos typically use a non-contact magnetic mechanism to cause the spring loaded needle to turn, the spring returns it to 0 when you come to a stop. The ODO, trip and total mileage, is directly gear driven so will still be accurate and functional. Issues with the speedo needle could be drag or binding of the needle pivot shaft, or perhaps something that increased the distance between the rotating magnets and the pickup that would decrease the magnetic field that turns the needle.
