Does the size of tyre make a difference? I'm running 225/45/17's.

It certainly does - look inside your fuel cap lid and it'll tell you what you need to know

I used to work for a company that was involved in the testing of tyres.

These days the test work is largely done by the tyre companies under contract (and sometimes in conjunction with) the vehicle manufacturers.

Like everything else to do with cars, tyre pressures are a compromise between various parameters such as ride, handling and wear.

In general the recommended pressures are those which give the greatest and most evenly-loaded contact patch on the road. This also gives the most uniform wear pattern and greatest level of adhesion. During tyre tests we used to take high speed phtotgraphs through a special glass 'window' which was laid into the test track and the tyre driven over it. The test track could be flooded with water so that we could also phtotograph the contact patch on wet surfaces.

As tyre pressure is increased the tyre sidewall becomes more rigid and so the steering response becomes more immediate and the steering feels sharper. It will also feel a little lighter, however there will be some reduction in grip.

With higher loads and for sustained high-speed cruising the manufacturers generally recommend raising pressures by 10-20%. This is to stiffen up the tyre so that there is less sidewall flexing as the tyre rotates - this means that less heat is generated by the tyre and it runs cooler.

There are particular circumstances where you might want to run higher pressures (such as track driving where there is continual high-speed cornering and a stiff sidewall is desirable to avoid scrubbing off the shoulders) however for normal road driving it is best to stick to the recommended pressures. This gives you the greatest grip under all normal conditions and the most uniform wear.

I used to work for a company that was involved in the testing of tyres.

These days the test work is largely done by the tyre companies under contract (and sometimes in conjunction with) the vehicle manufacturers.

Like everything else to do with cars, tyre pressures are a compromise between various parameters such as ride, handling and wear.

In general the recommended pressures are those which give the greatest and most evenly-loaded contact patch on the road. This also gives the most uniform wear pattern and greatest level of adhesion. During tyre tests we used to take high speed phtotgraphs through a special glass 'window' which was laid into the test track and the tyre driven over it. The test track could be flooded with water so that we could also phtotograph the contact patch on wet surfaces.

As tyre pressure is increased the tyre sidewall becomes more rigid and so the steering response becomes more immediate and the steering feels sharper. It will also feel a little lighter, however there will be some reduction in grip.

With higher loads and for sustained high-speed cruising the manufacturers generally recommend raising pressures by 10-20%. This is to stiffen up the tyre so that there is less sidewall flexing as the tyre rotates - this means that less heat is generated by the tyre and it runs cooler.

There are particular circumstances where you might want to run higher pressures (such as track driving where there is continual high-speed cornering and a stiff sidewall is desirable to avoid scrubbing off the shoulders) however for normal road driving it is best to stick to the recommended pressures. This gives you the greatest grip under all normal conditions and the most uniform wear.

Good info!

It certainly does - look inside your fuel cap lid and it'll tell you what you need to know

Erm, in relation to this thread I mean.

This morning I'd a 0.2bar higher presure on the 'sunny' side.

Waited until tonight, brrrr, to set them all to 2.2bar.

In the big freeze (December) I checked the tyres on my works car, the recommended pressure was 38 front and 41 rear - they were all down by about 10psi due to the cold. One poster noted the TPM was playing up too - they did on the works cars as soon I we got upto any kind of speed the TPM came on but when checked the tyres were fine.

Rich