Club 80-90 Forums

Afternoon,

I have just fitted some porsche wheels to my van, I now have:

Fronts: 225/50/17
Rears 245/45/17

Now what tyre pressures do people run with tyres this size? I am assuming that the pressures will need to be lower with the increased surface area.

Cheers

John

jc_geddes wrote:I am assuming that the pressures will need to be lower with the increased surface area.

The weight on each tyre is pretty much the same, so the contact area is the same, if the tyre pressures are the same. It's just a different shape - short and wide, rather than long and narrow. The tyres will change shape (the sidewalls bulge) until the pressure exerted on the ground is the same as the pressure inside the tyre.

OK, it's not _quite_ that simple, because the lower profile tyres will have stiffer sidewalls, but that's the basic theory.

I'd start off with the same pressures as normal, and adjust them up or down until it feels right - then keep a close eye on wear. If the centre of the tread wears faster than the shoulders, the pressure's too high, if the shoulders wear more, then the pressure's too low. If you've got an IR thermometer, you can check the temperatures are even across the width after a run.

Speaking of IR thermometers, I'm gobsmacked to see 'em for less than a tenner on fleaBay - anybody know if they're any cop? Obviously, I'm not expecting F1-team accuracy, but...

jc_geddes wrote: Now what tyre pressures do people run with tyres this size? I am assuming that the pressures will need to be lower with the increased surface area.
John

Afraid it doesn't work like that. Lower profiles on larger rims decreases the contact patch only compensated by having wider tyres so you will still have around the same contact patch but with higher rolling resistance or in other words increased MPG.

Just make sure the tyre can take the pressure before you start lamping it in
The Max pressure is written on the side wall