Motorcycle Riders Association of SA

Speedometer Accuracy - Summary

Australian road users should take some time to familiarise themselves with the accuracy of speedometers. The MRASA has created this webpage to help all road users be informed. This webpage is applicable to all road users, not just motorcyclists.

Firstly you need to be aware of the changes in the Australian Design Rules (ADR). The age of your vehicle will determine which ruling you speedometer complies to.

Secondly you need to be aware that regardless of what your speedometer reads, it is the road users responsibility to be within the speed limit. Radar devices are carefully calibrated and have a higher degree of accuracy, and it is that speed you will be compared against. Unless you have had your speedometer tested and know its inaccuracy, you cannot rely on it as an accurate reading.

Speedometer Variances

Under-Read or Over-Read
If a speedometer is not accurate, it could display a lower speed than the vehicle is actually travelling at, thus causing drivers to believe they are not speeding when in fact they are. This effect can occur if a speedometer under-reads true speed. Below is clarification of the terms under-read and over-read.

A speedo over-reads if it displays 100kmh when the vehicle's actual speed is 90kmh.
A speedo under-reads if it displays 100kmh when the vehicle's actual speed is 110kmh.

Other Factors
Apart from design criteria, several factors can affect the accuracy of speedometers. More detail is available from the Traffic Law website (link below).

Worn tyres - your speedometer is calibrated using new tyres.
Tyre pressure - your speedometer is calibrated with normal tyre pressure.
Rim size - changing the diameter of your rim will affect speedometer accuracy.
Differential ratios - changing the gearing of your differential or gearbox will affect speedometer accuracy.
Vehicle load - speedometers are calibrated with no load on your vehicle.
Speedo Needles - the size and shape of needles can make it difficult to tell exactly what speed is displayed.

GPS Readings
Your actual speed can be easily determined these days using the Global Positioning System (GPS). Some GPS units can display speed to the first decimal place. There are numerous apps available for iPhone (or similar) that display speed using the phone's built in GPS (one pictured below). Motorists should check their speedo against a GPS device along a straight stretch of highway (we suggest you have a passenger operating the GPS device).

Australian Design Rules (ADR)

ADRs are the guidelines used by a vehicle manufacturer to gain compliance certification but they are not legally bound to make speedometers accurate enough to be relied upon for real vehicle speed. The variation between pre and post 2006 models also now has no legal bearing anymore. It is up to you to know what speed you are doing and a court does not care how you determine it.

Up until July 2006 the Australian Design Rules required new cars to have speedometers that are accurate to within 10% of actual speed. The current rules disallow under-reading, and permit over-reading by variances shown in the table below.

ADR 18 - Instrumentation
The function of Australian Design Rule 18 is to specify requirements for the provision of speedometers. Here is a table showing the relevant details of each revision of ADR18.

ADR	Accuracy	To Indicate	Motorcycle	Car	Trike
18/00	of +/- 10%	for all speeds above 40km/h	1 July 1988	1 July 1988	1 March 1991
18/01	of +/- 10%	for all speeds above 40km/h	1 March 1993	1 Jan 1993	LEM 1/3/1993 LEP 1/7/1992 LEG 1/7/1992
18/02	of +/- 10%	for all speeds above 40km/h	1 July 1995	1 July 1995	1 July 1995
18/03	The speed indicated shall not be less than the true speed of the vehicle	Markings at specific intervals	1 July 2006 10% of actual plus 8km/h	1 July 2006 10% of actual plus 6km/h	Same as Motorcycle

There are specific formulas mentioned in the ADR which have been removed from the table above for simplicity. Mopeds (UNECE vehicle category L1 and L2) mentioned in ADR18/03 have been excluded from this table. Note that clause 5.3 of the ADR mentions a variance of 10% of actual plus 4km/h, but is further clarified in Annex 3 at the end of the ADR as to which category of vehicle each variance refers to.
If you would like to do your own research, here is a link to the ADR Online Website. Here is a link to ADR 18/03.

The big change with ADR 18/03 means that the speed indicated must always be ABOVE actual speed. Prior to 1 July 2006, a speedo could read over or under the vehicle actual speed - as long as it was within 10% of the actual speed. The driver/rider can only rely on their instrument, their speedo. Owners of older vehicles (made up until July 2006) are disadvantaged. ADR 18/03 requires speedos to overestimate the actual speed of the vehicle.

A Possible Scenario

Lets consider for a minute the end result of different ages of vehicles travelling down the road sign posted as 60km/h. This segment of road will have a varied population:

Those aware of the indicated speedo error on their post 2006 speedos, deliberately driving at the speed limit as set by indicated speed (comforted in the knowledge that they'll never get a speeding fine),
Those aware of the indicated speedo error on their post 2006 speedos, adding a fudge factor to their indicated speed,
Those not aware of the indicated speedo error in their older pre-2006 speedo taking their indicated speed at face value and driving at the speed limit (may be 10% over limit),
and finally those with GPS's who will be driving around at a true actual speed.

If they are all doing an indicated 60km/h, so complying to the prevailing speed limit, their actual vehicle speeds could be anywhere between 49km/h to 66km/h. That's a big differential to manage.

Now add to that mix, those in the 3rd category driving at an indicated 5km/h under the speed limit as encouraged by the likes of the Victoria TAC i.e., the 'wipe off 5' campaign, and the differential in actual speed in a 60km/h zone could be as wide as 44km/h to 66km/h. No wonder there's road rage.

The next time you get agitated by a vehicle travelling just under the limit, have some consideration, because as far as they know, they may think they are doing the speed limit.

Conclusion

"It was pointed out to me by a solicitor that another change made in 2006 was to clarify that the onus for a vehicle's speed is firmly on the operator, regardless of how they determine it. The excuse of having an inaccurate speedometer is now irrelevant in law as it is the responsibility of the vehicle operator to ensure it is accurate or that they at all times are knowing of their velocity be it by GPS or ESP or some God given gift."

Grant Delahoy - VMC

Reference Material and Websites

The MRASA would like to recognise the following sources and contributors of information on the subject of speedometer accuracy.

Traffic Law Website - a Victorian website with a law-centric view of speedometers.
(Authority to quote and reproduce obtained from Sean Hardy, 12 March 2014)
Guy Stanford, Australian Motorcycle Council (AMC) - material from the Australian Design Rules.
Grant Delahoy, Victorian Motorcycle Council (VMC)
Rob Salvatore, Victorian Motorcycle Council (VMC)