Tyre size.

[Doosan]

The tyre size on my XT3 is 225/65R17 What I would like to know is can I use 225/70R17 ? The reason I ask is I have been offered 4 of these new tyres for £50 each.

[FROSTYBALLS]

The higher profile will make the speedometer under-read by 2.9% - http://www.autopartuk.com/tyre-calculator/ - so will probably need to calibrate the speedo.

[IanML]

Speedos typically read in the region of 3% high, so that will probably put it close to accurate.  However, be aware that your safety margin with speed cameras, etc, will be gone.

[FROSTYBALLS]

25 minutes ago, IanML said:

Speedos typically read in the region of 3% high, so that will probably put it close to accurate. 

It is actually illegal for a speedometer to show a lower speed than is being travelled, and speedometers are designed to be optimistic to comply with current legislation, which states:

Legislation in the UK requires that the speedometer must never show an indicated speed less than the actual speed. For all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower than this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph.

For example, if the vehicle is actually travelling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph.

[IanML]

That explains why mine reads 34 when I am doing 30.  Surprising that it could read 39.25 and still be acceptable.

[Heidfirst]

3 hours ago, IanML said:

Speedos typically read in the region of 3% high, 

I would have said nearer 7% on my 2 Avensis.

[IanML]

8 minutes ago, Heidfirst said:

I would have said nearer 7% on my 2 Avensis.

At what speed?  I have compared my RAV4 at 30, 40 and 50, and it seemed to be a constant 4mph high.  That is 13% high at 30, but only 8% at 50.

My original 3% was from experience with previous cars, and I thought my RAV was an outlier.  Looks like that may not be the case.

[Doosan]

Thanks for the replies guys, very interesting.

[Heidfirst]

16 hours ago, IanML said:

At what speed?  I have compared my RAV4 at 30, 40 and 50, and it seemed to be a constant 4mph high.  That is 13% high at 30, but only 8% at 50.

My original 3% was from experience with previous cars, and I thought my RAV was an outlier.  Looks like that may not be the case.

pretty much all (this is using gps v speedo reading).