speedometer off

The former owner of my 1990 ranger xlt changed the 3 speed for a 4 speed with overdrive transmission. Can the speedometer be adjusted to compensate for a 10 mph error or do I have to change the cable or gears with in the transmission it self. Thanks for help

I ended up trading in my '03 for a similar '10, but I didn't care for the P235/75R15s on the '10. So, after being told by dealer 'no problem', I had them swap the LT245/75R16s onto the '10. Mechanic is telling me he calibrated it as much as he could, but my speedometer is off by 4-5 MPH. Instead...