In my box of autojumble goodies is a spare, working, needle speedo (See photo) which was easy to fit as I also had a spare, correct cable (remember there are two sorts of ends—see REFURBISHING THE SPEEDO article for photos). But after a couple of runs it was clear that both the odometer and the speed were 66% out. This obviously raises the question about which speedos fit our cars given the variety of rear axle ratios, wheel sizes and, I thought, gearbox take-off gears as I had just swapped over my gearbox to a newer 4-speed synchro ‘box. After a lot of asking questions around the club, and aren’t there a lot of wise old men, I think I have sorted out a few ‘truths’ -
TYPES
OF SPEEDOS: All our 4-speed cars, with the exception of Nippys (and Type 65s?)
have one of only two types—moving dial (also called the Magmo) or moving needle.
But these are completely interchangeable given the correct cable. The different
rear axle ratios and wheel sizes don’t make a hap'orth of difference and all
4-speed gearboxes have the same speedo-gear output. 3-speed, early Chummys have
an old brass-looking speedo called a P.A. Speed and a different speedo-gear
output.
IDENTIFICATION: However, not every Smith’s 2 inch speedo was made for Austin 7s. Speedos were made for different cars based on their TMP (Turns Per Mile) - i.e. the number of turns the back wheel makes per mile. For all A7s (with the exceptions above), the number of TPM is 1040 (for Nippys it is 1000) and this number can be found printed in the middle of the face. As can be seen on my replacement speedo, the number on the dial is “1575” and so the readings will be in the ratio of 1040/1675, i.e. 66%! I had picked up this speedo in error at Beaulieu—possibly from a Morris 8 or an early Land Rover. I hope others may be spared buying a wrong spare next time you’re jumbling—remember “1040” it is.
David Whetton