My 1963 Amazon wagon has recently been upgraded with a M41 box (J-Type) from a 1971 142E. I have finally got round to borrowing a GPS to check the speedo calibration and find that the actual speed is about 10% less than the indicated (reading the top edge of the strip speedo). It means I can safely sit on an indicated 110 km/hr when the local open road speed limit is 100km/hr. It also means that I need to downgrade calculated fuel consumption.

I realise that speedo error is dependant on lots of things - not least of which are the wheels and tyres you are running.

Can anyone tell me what the theoretical difference is between the gearing on the speedo output of a 1971 M41 and a 1963 M40?

I probably don't want to try and fix it, but at least it is handy to know what speed I am going so that I don't hold up other motorists. I now have it in my head that roughly speaking: 55=50, 65=60, 80=70, 90=80 and 110=100.

It's just worth noting that if you do the overdrive upgrade with the idea of improving fuel consumption - don't trust your calculated data.

I wish I had tried the comparison with a GPS before I took the M40 out. Maybe someone who is planning this upgrade could do a "before-and-after" and let us know.