4psi rule

4psi just right
More than 4psi then under inflated
Less than 4psi then over inflated

To add to the succint explanation above:

The change in pressure is an indication of the amount of heat generated by the tyre as the carcass flexes under load.

If a tyre is underinflated the flexing of the sidewall will be excessive and the tyre will generate a lot of heat which will cause a larger increase in pressure that would occur if it was correctly inflated to start with.

Conversly, if the tyre is overinflated, there will be very little flexing of the tyre carcass and a correspondingly smaller increase in the inflation pressure...

Cheers,
Pete

Tyre pressure should be checked when cold, although sometimes this is not possible. Try your high and low pressures when cold then go for a decent drive. Then recheck them and see what they read when hot. Remember your hot pressures for those times when you have to alter the them while hot.

As for the 4psi rule, I sometimes vary my pressure by more than this depending on weight etc.

Topend

I read somewhere a while back that it was 4 psi for car tyres and 6 psi for 4 x 4 tyres. Does anyone know what is right?

Eric

Check out this link to the NRMA http://www.mynrma.com.au/travel/do/cc4/cara3.shtml

Cheers

Dave n Jude