Replacement Adjustable Speedo??

Guys,

This is an open question to the brotherhood. My speedo has always been off since I bought the truck,(speedo & odometer) it has changed a bit, with tire changes, between 9%-10.5%. My speedo was under reporting speed & odometer under reporting mileage, with new tires it changed again. So I got to thinking ESPECIALLY with an E40D, if the VSS (vehicle speed sensor)on driver's side of tranny tail shaft))then the transmission will NOT be shifting at the appropriate time, if the speedo is not correct. I swapped the little gears, the one in there since I bought the truck is khaki colored & 19tooth, I tried orange 20tooth, purple 21tooth, & finally dark green 18tooth. Well according to the borrowed GPS, the closest I could get was about -.2mi on 30 mile trip, but, speedo under reporting anywhere from 3-6mph depending on speed.

SO...does anyone know of a replacement mechanically driven speedo, that can be electronically calibrated/adjusted, so that the next time I get new tires, if I can't find the exact same physical size, I won't need to start this mess over again? /ubbthreads/images/graemlins/crazy.gif

Only speedo shop(which is where all the Ford dealers send you when you say you're having speedo problems) in town wants $250 to rebuild speedo plus $100 to calibrate it after rebuild, according to them if speedo & odometer are not off by the same amount/percentage then it needs to be rebuilt. /ubbthreads/images/graemlins/shrug.gif

jerry