How To calculate speedo drive ratios for given tyre and final drive combination

For determining the speedo drive ratio which is right for a given tyre size and final drive ratio, apply the following formula.

Turns/km (wheel) x no. of teeth (spindle)/no. of teeth (pinion) x final drive ratio = turns/km (speedo)

By measuring the circumference, I calculated the following:

For a 165/70-10" tyre, 655 turns/km

For a 145/70-12" tyre, 625 turns/km

For a 165/60-12" tyre, 630 turns/km

Standard speedo drive ratios :

6/17 for a 10" Mini with 3,44 diff

7/16 for a 12" Mini with 3,105 diff

5/18 for a 12" Mini (12A engine TBi) with 3,2 diff

5/17 for a Metro 1275 with 3,44 diff

The turns/km of the speedo on old type Smiths speedos can be found below the odometer, on the right hand side. Mini's with 10" wheels/3,44 diff (until ca. 1981) have a speedo with 800 turns/km. Mini's with 10" wheels/2,95 diff ('82-'84) have a speedo with 780 turns/km (as do Van's from '80).