Speeding and tyre size

[seatrout]

Can the size of a cars tyres affect the accuracy of the speedo i.e. if you have bigger tyres than those reccomended could this cause a car to go fstaer than the speedo shows.

I think its a load of codswallop but a colleague reckons there could be something in it

[tom hopkins]

the profile of a tyre can definitley make a difference.
i.e:-if you put low profile instead of standard tyres on acceleration would be quicker but you would loose top end speed
however to make a car go faster would also require it to have the power.

[homer75]

Speedos are calibrated assuming factory fit wheels. After fitting of lower profile tyres will cause the circumference of the wheel to decrease whereas higher profile tyres will increase the circumference. These factors will affect the accuracy but by a very small amount. You can put 'bigger' ( wider ) tyres on but as long as the profile ( height ) of the tyre remains the same then the accuracy will not be affected. Theoretically higher profile tyres will give a higher speed then is being registered and vice versa.

[jake-the-peg]

Not codwallop perfectly logical.

The speedometer is based on how quickly the gearbox is turning the axle.

A trivial example.

the axle turns at 60 revolutions a minute ( 1 a second ), the circumferance of the tire is 1 meter

Therefore you are going at 1 meter a second and your speedometer says so.

Now replace the tyres with some with a 2 meter circumference.

The axle still turns a 60 revolutions a minute so the speedometer says the same.

Now though every turn of the axle takes you not 1 but 2 meters.

The speedometer says 1 meter a second but you are actually travelling twice as fast.

In practice the difference in the tire size is not going to double but it will be proportionate to the size of the tyre.

20% bigger and 30mph on the speedo is really 36 and 70 is 84

[__LEE__]

some posts are slightly wrong..

If you go for a wider tyre .. you will need to go a lower profile, to keep the same circumference.

[Gef]

I agree with jake. The others are talking rubbish.

[kramer81]

The only thing that matters is the overall circumferance of the wheel (inc tyre). If you want to fit alloys bigger than your standard wheels then in order to keep the circumferance of your wheel the same you must fit lower profile tyres. Otherwise the speedo will not be accurate.

[homer75]

If you fit massive wheels ( perhaps need to remove parts of the vehicle body to accomodate this ) you will get an incredible top speed although the speedo will read low - the sky's the limit !

[seatrout]

Thank you all particularly Jake. Sorry if the question wasnt clear but I was referring to tyre circumference. I shall eating humble pie at work!

[__LEE__]

Tyre size calcalator

http://www.miata.net/garage/tirecalc.html