probably a dumb tyre pressure question?

[baggabones]

My 924 is running 205/55 tyres on 16" compomotives as opposed to the standard 185/70 on the original 14" alloys. Any advice on tyre pressures? OG is 29 front and 36 back.....do I need to change or keep the same? Any thoughts or first hand experience? Thanks

[ferny]

Stick 32 in the front and back and have a play to see what's best for you. Then after you've decided what you like check the tyres after a few hundred miles for uneven wear from over or under deflation. The book figures are there to give the best MPG and life of the tyres and it's a mixture of the car maker and tyre manufacturers thoughts. That doesn't mean it'll be what's best for you or that changing it by a few PSI will make it blow up.

[waveman1500]

Tyre pressures are mainly based around the axle weights, rather than tyre size, so the original specs will still be fine.
I'm surprised that they recommend 36psi at the rear, that seems a bit high to me.
Essentially, increasing pressure will make your contact patch smaller and hence reduce the available grip. If you make your front pressures too high then you will tend to lose some turn-in and the car will understeer, if the rear pressures are too high then it will be more likely to oversteer. As ferny says, varying the pressures up and down by about 5psi from the recommendations should be fine, just evaluate the ride and handling and see if it feels better or worse.

In terms of first-hand experience, I have had a few very light cars which were sensitive to tyre pressure.
In a Beetle, the front tyre pressure is critical and should be kept much lower than most people think. When you buy tyres, most tyre shops will inflate them to somewhere between 32psi-40psi, regardless of the vehicle. On a Beetle, this will cause severe understeer.

The other experience I had was with my Suzuki Sierra four wheel drive. My mate and I both owned one at the same time. The factory handbook recommends 25psi all round, my pressures were set at about that. For some reason my mate insisted on putting 40psi in all four tyres. This made the ride very harsh, his car was much bouncier and jarred over bumps and potholes. Sierras are very light and have leaf-sprung live axles, so they are bouncy at the best of times! He did get better fuel economy though.

[baggabones]

Yeah, tell me about beetles! My bread and butter and like you say standard 32psi tyre fitter spec when it should 17-19 at the front isn't much fun I'll have a play around and see what feels 'right'