I haven't done it myself but I understand that the speedo is adjustable if you can be bothered going to all the hassle of extracting it from the dash.

It would pay to figure out exactly how far out the speedometer and odometer are before you look at how to fix it. Get hold of a GPS and check the speedo at a range of speeds. You can check the odometer against a GPS too.

My wagon is 10% out on the speedo and 5% out on the Odo. (speedo indicates 110km/hr when I am traveling 100km/hr) To correct the Odo would require a different gear on the speedo drive at the back of the M41. The gear I need is un-obtainable but I have a plan to make one to suit. The odometer is a straight gear driven affair with no slippage or adjustment in the workings. If I ever wanted to fix the speedo, I'd start by getting the Odo as close to correct as I could by replacing the drive gear.

With the speedo assembly free of the dash, there is apparently an adjustment screw somewhere in the workings that can be used to adjust the speedo. This is a 2 person job - one to drive and the other to read the GPS and tweak the speedo adjustment. From what I have read - the hardest part is getting the speedo out of and back into the dash.

The easiest fix is to just get used to how your indicated speed compares with actual speed. In my case, my passengers are impressed that I appear to drive everywhere at 110 km/hr without worrying about speed cameras or cops.