Given the boom in participants in historic rallies, but also the insanely high fuel prices, changing an end reduction - the differential - becomes an option. A shorter final transmission means brighter acceleration but a lower top speed at the usual maximum speed.
Before you change anything - maybe 'let' change? - it is wise to first find out which differential is mounted and which should be mounted ... After a life of say 35 years you can hardly assume that your classic is still equipped with the final reduction that is noted is stated in the technical data of the cart or the workshop manual. Measuring is knowing, guessing is missing? In many cases, numbers are stamped into the differential. For example with an Austin-Healey Sprite or MG Midget, 9 / 38 which means that the transfer is 4.22 on 1. If that is not stated, then it is a matter of dismantling the differential and counting the teeth of the crown wheel and pinion. If, for example, there is 4.22 on 1 and you want to get out of place faster, it is advisable to look for a differential with an 4,55 on 1 ratio. However, if you see more at a lower speed at the cruising speed you use, you should go for a differential with a ratio of 3,9 to 1 in such a case. Regarding the Austin-Healey Sprite or. MG Midget will be looking, because they are scarce. Pretty scarce.