Tyre Pressure Measurement??

I'd trust the gauge more than the pump - but neither will be true unless calibrated.

luckily I have access to calibrated gauges at work, so cross check my own against these.

Sainsburys fuel station tyre pressure gauge shows the same as my pen type, so maybe they are calibrated and trustworthy. DaveK.

Or both wrong by the same amount! But does it matter? - as long as your guage is working reliably, so you can keep the pressure constant. The handbook figure for pressure is only really valid for the original fit tyres anyway. Once you have found a pressure that works for your particular car/tyre combination, I would stick to it even if it varies from the handbook figure.

I'm glad that this works for you, Bob, but how do you work out what's "right" for your car? Could most people tell? What is there to be gained by running on pressures specified by the manufacturers of the car or tyres.

Most of the tyre makers publish pressures for each vehicle they will fit. Stick with those or those of the car maker, they've done more R and D than "a bloke down the pub who runs his tyres at 10 PSI because it makes it grip better" etc

'arry

Well agreed I would trust Michelin and Dunlop rather than some bloke down the pub, but if you think that they spend a lot of time doing tyre pressure testing, you're mistaken. It's just maths - the principle is easy. The weight of the car is distributed (unevenly) across the four wheels, and you want a certain size of contact patch for each tyre, so once you know the weight distribution you can work out what the front and rear tyre pressures need to be to give the contact patch required (and this contact patch, I agree, is researched extensively by the makers.) If you change tyre width or profile, the ideal contact patch and hence inflation pressure changes. If you load the car up, the inflation pressure should be adjusted - but in the real world it's impractical to constantly inflate and deflate the tyres. As I frequently carry heavy loads, I keep my tyre pressure closer to the 'max load' figure given in the handbook, as a compromise - so I have slightly less contact patch than ideal when not loaded up, but I don't have excessive sidewall flex for the majority of the time when I have cargo. My tyres wear evenly across the width of the tread, which is a good indication that the tyre pressure is close to optimal. The figures given by the manufacturers have to be simplified, lets face it most drivers don't even bother checking a single figure between services (or blowouts.)

My point is just this - if you have an accurate, reliable guage, and always set the pressure to the makers recommendation, you may still have underinflated or overinflated tyres. And if you think the manufacturers are always correct, have a read about the Firestone/Ford Explorer fiasco, one of the many problems was that Ford specified a very low inflation pressure for the tyres which caused them to overheat.

If the tyres feel the same temperature across the width after a run, and they wear evenly then they're OK.