You may have a speedo that needs repair. You can verify what's at fault by doign a mile marker check on a highway. If the odometer reads correct then the speedo itself is at fault. If it checks out OK, There are places that will put together an inline speed corrector that is sometimes easier than playing swap the gears.
Yes and these speedometers were problematic. Hey did you also lube the speedometer cable? I installed a NOS speedometer in my '79 and it still had some slight wobble, so I installed a NOS cable but lubed it real well first since it was old NOS, and that smoothed it out completely. I found a NOS one on feebay (cable) for pretty cheap, but I think if you just pull the cable from the housing like I did and regrease it probably okay. If none of this works I think I'd just get another speedometer gear with more teeth, kind of guessing at it. IIRC used ones still available pretty inexpensively and that's probably the easiest thing to swap.
