Les Felix, a South Australian metrology expert (metrology being the science of measurement), has come up with an interesting alibi for speeding motorists. “Blame the tyres” he says. His treatise is discussed
in this article from The Tire Review .
Les measured 30 tyres on two cars, and showed that variations in speedometer readings of up to 3 kilometres per hour, with similar variation with tyre pressures on
certain types of tyres (my italics). which when combined could lead to 6 km/h variation in speedo readings. Other variances which could affect accuracy were the thickness of the speedo needle, and the height of the driver, which might lead to parallax error.
Adding all these up, which would be uncommon, it can be as high an error as 8 km/h at 60 km/h, and 13-15 km/h at 110 km/h according to Felix.
“Good story” says the officer as he writes out the ticket. “Now prove it in a court of law”
Several other factors need to be identified.
Every tyre design has a set of “nominal specifications”, which in most cases are first laid down by the Tyre and Rim Association of the U.S.A., since they are still the largest automobile manufacturers in the world. But they do not act alone. An interlocking system of Standards between the European Tyre and Rim governing body (E.T.R.T.O), Japan Tyre and Rim Association, South African, Australian ensures that all “automotive oriented” countries work to the same Standards. Then the fun starts. Around each “Standard” dimension specified for a new size of tyre, there are permitted tolerances in manufacture. These tolerances, only available to the tyre manufacturers, are written into the design manual used by the tyre design engineers of any one particular tyre company. Some companies stick religiously to try to hitting the standard dimensions in mid range, others tend to the larger dimensions because it might help the tyres perform better, others to the smaller to save material. All dimensions are taken after the tyre has been inflated for 24 hours at a specified temperature because they stretch a little bit when first inflated. After that, zilch.
So it is not uncommon for tyres to vary between makes by the full extent of these tolerances, which cumulatively could add up to as high as 5%. A study of promotional leaflets published by the tyre companies will show this to be the case.
Radial tyres roll around the diameter of the (steel) belts built into the tyre, and their rolling circumference does not differ markedly with pressure. The belts restrict the tyre growth, which is what they are supposed to do. Early experiments with steel belted radial tyres showed that in excess of 200 p.s.i. pressure only caused a growth of less that .060 inches in diameter. Bias ply tyres do however, change rolling circumference with pressure and with speed. Indeed, speedway cars rely on this to give them their “fifth gear”. However, their use on passenger cars is obsolete.
The most common cause of speedo error is fitting tyres of different section width, which carries with it an increase in diameter at the same profile, or fitting plus 1 or plus 2 fitments without matching tyre rolling circumference (or diameter) within the tolerance permitted by State legislation, which varies from State to State currently, though Queensland may fall into line soon.
If you’re worried about it, most major highways out of the capital cities have a measured 5 kilometres to check your odometer, or on the Hume highway 25 km out of Melbourne, radar guns which flash your speed up as you approach. However, if the southbound lane gives a different reading to the northbound, I know which one I would accept- unless I could find a sympathetic beak to believe my story! The officer writing out the ticket won’t, for sure!
