speedometer is off by 10 mph

[oasat]

Hello everyone I will tell you what my problem is. I have a 1992 Grand Prix LE and my speedometer is off in the car(everything else works fine starts fine shifts fine) I took it to a shop and they said the
vehicle ground speed and computor matches up but the speedometer is off by 10 mph.
they said it may be someone changed transmission in the car before I got it and it was the wrong prom chip in the computor and I needed gm part number prom #16190190.
it has 3.1 engine with a 3 speed transmission- transmission code CHC. the VIN# is 1G2WH54T6NF308349 is there anyone out there with an idea on this as this is driveing me crazy.