AUSTIN MINI SPEEDOMETER CONCERN
QI have replaced the speedometer in my 1964 Austin Mini and the reading now appears to be out. Being concerned, I used an app on my phone to check the reading and this has confirmed that the speedometer is not reading correctly. I was under the impression that they were all the same and now I need to rectify the problem, possibly fitting another speedometer if needed. The old unit was still working, but had a cracked glass cover and was suffering from signs of wear. How do I get the speedo to read correctly – is there any adjustment? Wayne Spencer
A As you have discovered, there were variations of speedometer fitted to the Mini, and it may be the case that you have picked up one that was in the van or another variant. The TPM number (turns per mile) is printed on the front of the display on the Smiths speedometer fitted in the Mini, and this number can be from around 1000 to 2000. The number may be at the top of the display or just below the odometer, and is normally to the right of the display. I have seen Mini speedometers displaying both 1248 and 1378, and there may be even more versions.
If you have the old speedometer then this is easy checked. If you do not, then there is a formula to work out what you need. This is TPM = (1680 x N) ÷ R.
N is the number of turns from the speedo cable and R is the radius of the road wheel and tyre in inches, from the centre of the wheel to the ground. Disconnect the speedo cable from the speedometer and push a piece of card over the protruding inner cable – cutting the card into an arrow shape makes it easier to record the number of turns the cable makes. Next, mark one of the rear tyres with some chalk, get an assistant to sit in the car and watch your cardboard marker, counting its turns while you push the car forwards until the wheel has rotated exactly six full revolutions. The recorded number of turns of the cable can then be inserted into the equation.
So if the radius of the wheel is 8in and the speedometer cable has turned 6 times, the sum would be TPM = (1680 x 6) ÷ 8, which works out at 1260. This would be close enough to the 1248 TPM speedometer to be accurate.