My recent experience with some tyres was that 205 width AD08s on a 6.5J rim had loads of grip at 24 psi (cold, so when hot probably closer to 30). But a 185 width AD08 on the same rim didn't grip nearly as well, and improved lots the more I reduced pressure (down to 19 hot = 14 cold).

So is there a general rule of thumb that, since the tyre that is stretched more over the rim effectively has a stiffer sidewall (due to stretching it more), does the more stretched tyre benefit from lower pressure? Or are there other factors at play here?

I'd just like a simple tyre pressure calculator tbh that you put the details in and it tells you what pressure you need

Had my low profiles under pressure and wore the outside down

generally as width goes up required pressure goes down as the mass is spread over a greater area.

Consider (in very simple terms) that the tyre pressure is pounds per square inch. If you increase the number of square inches (tyre contact with the floor), but the pounds stay the same (the weight of the car), then the PSI has to drop down accordingly. I don't think the size of the rim makes much difference.

I've seen video's on Youtube where people have determined car corner weights by measuring the tread contact patch and tyre pressures. Not sure how accurate it is though...

OK, but say I keep tyre size the same and increase the rim width. Is there a general rule for changing tyre pressure to suit?

I doubt if it is possible to make a general rule but I would think that for an overly narrow rim the tyre might tend to bulge in the middle which means low pressure to get even tread pattern but then again the sidewalls will tend to be under-supportive causing the tyre to roll when cornering so you need more pressure - two conflicting requirements for the same situation..

For overly wide rims the exact opposite happens - tyre tread becomes concave so you need more pressure but tyre walls effectively become stiffer so you need less pressure.

The only way to check pressure is to measure tyre temperature profile across the tread after heavy cornering.

Also check impact of pressure changes on braking distances.

Years ago when trying to decide on tyre pressure for my Cobra I spoke to one of the Dunlop tyre engineers and he felt that given the tyre and rim size and weight on each corner 10psi would most probably be fine but 18 psi would be erring on the side of safety. Experience has shown that 16psi works well on public roads and 24psi on the track.

In fact one day the valve was kicked loose from the rim by a vandal and I only noticed because the tyre looked a little flatter than normal but the car was very driveable with absolutely no air in the tyre and I drove it (with care) a few kilometers to the nearest tyre shop with zero damage to the tyre to get a new valve fitted.

[Edited on 29/8/13 by Ivan]