Morris Minor Owners Club

I have a 1275cc a series engine running through the standard 4 speed gearbox. The car accelerats okay but I think it would be better with a lower geared diff - currently 4.22:1 and with 13" whhels with 175/70 tyres

Any views on what the best ratio to go for is - 3.7 or 3.9?

Without a doubt - the 3.7 ratio!! And - a matching Speedo from a Mini with tpm of 1248 will be v nearly correct for you. It is absolutely correct with my 155 x 14 tyres - but your smaller wheels will throw it off slightly......