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I looked at this when I changed from M40 to M41 in my Amazon. I used a GPS to calibrate both the speedo and the odometer and found that the odo reads 5% out while the speedo is 10% out. (My speedo reads 110km/hr when travelling 100km/hr and the odometer would indicate 105km for 100 km travelled.)
I took a look at the speedo drive gears from both the M40 and the M41 and they are quite different and not interchangeable. I have seen various speedo gears listed on ebay and on various suppliers websites but I couldn't find the one that I needed. I can't remember what the number of teeth I wanted was but I figured out that it was one tooth different from what was commonly available. I have made a new speedo gear before for a friend, and might eventually get round to making the right one for my car. This link shows how I went about making a new speedo gear: http://amazoniac.wikispaces.com/Re-manufactured+speedo+gear
A new speedo gear is only half the answer for me. The correct speedo gear would allow me to make the odometer accurate. The speedo would then need to be adjusted to make it accurate. I have read before somewhere how to go about adjusting the speedo. It is a bit tricky, you need to get the speedo out of the dash and get a co-pilot to turn an adjustment screw as you drive - calibrating against a GPS.
Personally, I have just got used to the speedo being 10% out and drive accordingly. It impresses my passengers who think it is going faster than it really is.
Not sure if this helps you. I'd suggest starting with calibrating against a GPS , and check both speedo and odo. There will be variation between most cars, a speedo gear that is accurate for someone else with a similar setup might not be accurate for you because it also depends on overall gearing (including rolling diameter of tyres).
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