How to check your speedo

Who wants to be able to drive at the posted speed limit and know they won't be booked for speeding?

But the reality is that speedos are inaccurate, that's something we have to accept, and they are allowed to be within a specified tolerance by the relevant Australian Design Rule, ADR 18.

Before July 2006 the design rule specified an accuracy of plus or minus 10 percent of the vehicle's true speed when traveling above 40 km/h.

That is, at a true speed of 100 km/h, the speedo could legally indicate anywhere between 90 km/h and 110 km/h. On top of that there was also a requirement for the odometer to read within plus or minus four percent of the real distance travelled.

Owners of cars built before July 2006 should note the variation in speed they could see on their speedo, which could indicate a speed that is lower than the real speed at which they are driving. On those cars it is well worth checking your speedo reading against a known speed measurement, such as an overhead speed indicator that have been erected on our highways.

While they too have their tolerances and can't be relied on 100 percent they can be taken as a guide to the accuracy of our speedos. After July 2006 the rule was changed and the speedo on vehicles built after that date must not read lower than the true speed of the vehicle, but is allowed to show a higher speed than the true speed by 10 percent, plus 4 km/h.

Under this revision to the rule a vehicle's speedo must read between 100 km/h and 114 km/h when the true speed of the vehicle is 100 km/h, it cannot read lower than 100 km/h. Alternatively, at an indicated speed of 100 km/h the vehicle can be traveling at between 86 km/h and 100 km/h.

The odometer accuracy was also changed and now has to read within plus two percent of the real distance travelled; it cannot read lower than the real distance travelled.

While automotive instrument specialists can measure the accuracy of the speedo in your car there is little they can do to make it more accurate. For buyers of used cars it's worthwhile checking for modifications that might have an effect on the speedo accuracy.

Things like the incorrect wheel or tyre size can have a marked effect on the reading, so to can a different differential ratio or a gearbox with different ratios.