Some changes to your early model Holden can result in the speedometer reading being inaccurate - these changes can include changing the tyre and/or wheel size from that originally fitted, replacing the differential with a different ratio, or replacing/upgrading the gearbox.
The speedometer accuracy is maintained by selecting the correct size driven gear fitted to the gearbox end of the speedo cable - each gearbox has a number of different sized (and colour coded) gear combinations available to suit nearly any combination wheel and differential variations. Selecting the correct gear combination can be very difficult, and the calculator below will make this much easier.
To use the calculator, you must first gather some relevant information. You will need the fitted (or proposed) tyre width, profile and wheel diameter - these may be obtained from the markings on the tyre sidewall (for instance 185/75R14). You will also need to know the ratio of the differential - this is usually stamped on the oil seal cover just behind the pinion where the universal joint bolts on at the front of the diff. You will also need to know what gearbox you are using and what coloured gear is fitted to the output shaft in the gearbox - you should be able to see this by removing the speedo cable from the gearbox and looking in the hole with a torch.
Select the relevant options on the calculator according to the information above, and if known, also select what coloured driven gear you currently have on the speedo cable - the "Speedo Details" section will reflect your speedo's accuracy. You can then try different driven gears to improve this accuracy if need be - you should aim for as close 0% as possible, but err slightly on the high side (if your speedo reads slightly high, then there is less chance of being picked up for inadvertantly exceeding the legal speed limit).
Once you have found the gears combination that provides the highest accuracy, the original GM part numbers of the selected gears will be displayed - as most of these are no longer available from Holden, the equivalent Rare Spares part number is also given (if this is blank, then they do not have an equivalent).