I don’t know of any law that says speedometers have to over-read, but there are a few that prohibit speedometers from reflecting a lower speed than the vehicle is actually travelling. In the United Kingdom, for instance, speedometers may legally be up to 10% optimistic, but there’s no leeway for under-reading.
Apart from removing from motorists the defence that they were speeding without realising it, this also means that people who think they are breaking the law very often aren’t.
The downside of an optimistically errant speedometer is that the old codger trundling at 88kph while his 1971 Austin Marina’s speedo tells him he’s doing 120kph truly believes he is travelling at the speed limit, and won’t budge from the fast lane.
Taking a sample of 20 vehicles from the dozens tested by Car magazine last year, using equipment accurate to within 0,2kph, we established that actual speeds, when the cars’ instruments reflected 120kph, were as much as 10kph lower.
The Chrysler 300C Hemi was the only car in our sample to be spot on at 80, 100 and 120kph, while its speedo under-read by just 1kph at a genuine 60kph. Worst of the bunch was the Toyota Tazz, which indicated 120kph while plodding along at a 110kph.
On average, most cars were doing between 113 to 116kph when their speedometers indicated they were travelling at the national speed limit. In most cases, the error increased as speed did.
Car recorded the Toyota Tazz as having a true top speed of 159kph, at which time the speedo needle was pointing at the 175kph mark. At an indicated 250kph, the Chrysler 300C Hemi’s instrumentation was still truthful. Many of the other cars managed to get to within 10kph of their indicated top speeds.
There are advantages to knowing just how optimistic your car’s speedo is. Speed cameras are usually set to catch motorists travelling at least 10kph over the limit, meaning that those who like to push the boundaries can cruise at a genuine 126kph or so on the freeway without fear of being fined.
We assumed that because modern speedometers are usually about 5% optimistic the cars’ odometers would over-read by the same percentage. If this were the case, the long-term implications would be enormous when you consider the overall picture: a car that needed servicing every 20 000km would, in fact, be taken in every 19 000km, and 100 000km warranties would expire after a genuine 95 000km. Factor this into the amount of money spent on repairs and servicing in South Africa every year and the total leached from motorists by the industry would be enormous.
Other questions too could be raised: tourists who hired cars for their grand adventure would pay for kilometres they hadn’t actually driven, and the claims of people on car allowances would depend to a large extent on the vagaries of their odometers.
Much to our surprise, the 20 cars’ odometers were much more accurate than their speedometers, with half actually recording marginally less than the actual distance covered.
The least accurate of the lot, the Smart ForFour 1,3 Pulse, over-read by just 2,67%, while the Hyundai Atos under-read by 0,5%‚ — that’s just 5m per kilometre. Our conspiracy theories about manufacturers diddling motorists out of millions in dishonoured warranty claims and unnecessary services were blown out of the water.
The manufacturers may not be supplying cars that provide you with erroneous information, but beware, you may be doing it to yourself. Speedometers and odometers calculate how fast you’re going and how far you have gone by taking into account the rate at which the wheels are turning, and then factoring in the rolling circumference of the tyre — the distance the car travels for each wheel revolution.
If you fit tyres with a lower profile, or sidewall depth, than those that came with your car, the rolling circumference will be less and the resulting speedo and odo error can be enormous. Not only will you think you’re travelling faster than you are, but your service plan and warranty will expire long before you have reached the actual distance for which you were covered.
Your car’s overall gearing will also be lower, resulting in higher engine revolutions at any given speed, and — most probably — higher fuel consumption. Lower profile tyres will usually require larger diameter rims to maintain the same rolling circumference as standard rims and rubber.
Tyre wear and incorrect tyre pressure can also affect rolling circumference, and thus speedometer and odometer error. Although at most one or two percent on their own, these, along with the other factors mentioned, can lead to very significant issues.
So, the next time you’re toddling along in your ’71 Austin Marina and some yuppie in a BMW flashes at you to move over to the slow lane, please do so. He may be speeding, but probably not by as much as you think.