Tyres change speedo reading

Testing this week by the NRMA and the Daily Telegraph has shown that 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 car makers 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. Similiar increases were achieved as the car went faster.

"If a vehicle as produced by the manufacturer has a speedometer calibration margin near zero (ie the speedo is very close to accurate) an 8 per cent change due to 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" said NRMA vehicle safety expert and senior policy adviser Jack Haley.

The NRMA and the Daily Telegraph 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 to the speedo reading plus how the speed was effected by altering tyre pressures and changing to a different profile tyre.

The results found that none of the cars travelled at the same speed as 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 120km/h. None of the cars went faster than their speedo was showing but Mr Haley said that could occur depending on the car and the level of modifications.

The tests also found that speeds do not alter much if your tyres lose some pressure.

"Varying tyre pressure does not affect speedo accuracy significantly. The largest difference was 2.8 per cent" Hayley said.

He said that with the introduction of mobile speed cameras motorists should not rely on their speedo reading if they altered their car from standard.

"Motorists should be very careful if fitting optional or accessory wheels and tyres."

Subaru spokesman David Rowley said "any change to non-specified tyres can have an effect. We recommend that owners stick to the tyres specified by the manufacturer."