Tests show up speedo pit­falls

Switch­ing tyres can put you over the limit, writes David Fitzsi­mons

Herald Sun - Motoring - - Onroad -

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.

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