How Accurate is Your Speedo?

To minimise the risk of road accidents or speeding fines it is important to know we are complying with the placarded speed limits in each zone.

A reader has asked “what are the laws on speedo accuracy and are GPS speedos better?”

Australian Design Rules (ADRs) specify vehicles manufactured after July 2006, must not show a speed lower than your actual speed, but allow speedos to show over your actual speed by up to 10 percent plus 4kph.

Vehicle manufacturers understandably wish to avoid penalty for under indication of actual speed, so they tend to over-estimate speedometers as standard.

Our experience suggests most vehicle speedometers “over-estimate” the actual speed by about 5kph. The least we have found indicates 2kph over actual speed, and the most up to 10kph over, when travelling at 100kph indicated.

This might explain why some slow drivers in the right hand lane think they are doing 100kph on the highway, when in fact they are travelling at about 90kph.

Changing the size of your wheels can also effect accuracy of your speedo, as most speedos are determined by wheel rotation, so check with the dealer before making changes.

We suggest everyone confirms the accuracy of your speedo so you can be confident of compliance with speed limits. For example, compare in-car indicated speed with roadside speed checks or use a GPS unit.

Are GPS Speedos Better?

Global Positioning System (GPS) indicated speed is more accurate when you are travelling steadily at highway speed. However, GPS speedos rely on clear view of the sky for signals from satellites so they will not work or be accurate; in tunnels, under heavy tree foliage, or when shielded between tall buildings.