Tests show up speedo pitfalls
Switching tyres can put you over the limit, writes David Fitzsimons
DRIVERS who change their tyres or wheels from standard risk getting a speeding ticket because their speedo may show the wrong speed.
Testing by the NRMA and carsGuide has shown just changing to a different profile tyre can increase your speed by up to 8 per cent without you being aware.
By law all speedos have to be set to read higher than your actual car speed to allow for margins of error. The actual margin varies between carmakers and models and is not disclosed.
The testing found that one car’s speedo read 50km/h — but its actual speed increased from 44km/ h to 48km/h just by changing the tyres.
The NRMA says the tests show that if you alter your car from standard you could be unknowingly travelling over the speed limit.
NRMA vehicle safety expert and senior policy adviser Jack Haley says: ‘‘If a vehicle as produced by the manufacturer has a speedometer cali- bration margin near zero (the speedo is very close to accurate) an 8 per cent change caused by larger wheels and/ or tyres could easily mean the driver would be travelling at a higher speed than indicated by their speedometer.’’
‘‘If they are very close and you change your wheels and tyres you’d be well over.’’
The NRMA and CarsGuide undertook testing over two days at Sydney’s Eastern Creek raceway this week using four popular cars; a commuter sedan (Toyota Corolla), a sports car (Honda Civic Type R), a family car (Subaru Outback) and a large 4WD (Toyota HiLux).
The tests compared the cars’ actual speed with the speedo reading plus how the speed was affected by altering tyre pressures and changing to a different profile tyre.
None of the cars travelled at the same speed their speedos were showing under any circumstance.
On average they were about 3km/h slower than the speedo reading and that increased up to 6km/ h slower at the top speed checked of 120km/h.
None of the cars went faster than their speedo showed, but Haley says that can occur depending on the car and the level of modifications.
The tests also found speeds do not alter much if tyres lose some pressure.
Haley says: ‘‘Varying tyre pressure does not affect speedo accuracy significantly. The largest difference was 2.8 per cent.’’
Margin for error: your car’s speedo could be giving you a false impression.