What have you done to your JK this week? 2.5M Views

[Macca2801]

I'd suggest that running the correct tyre pressure is critically more important for tyre adhesion on the road and at 40 psi your contact on the road is the size of your fist!

[PapaSmurf]

Originally Posted by SeaComms

Ummmm..... rolling circumference of the tyre does not change with tyre pressure. They have steel belts in the tread and they dont stretch or shrink with pressure.

That's not exactly the case. The steel belts don't stretch when you increase the pressure, but they bend when you lower it. That's the principle we use offroad to increase the traction by lowering tyre pressure. The footprint increases not only across the tyre, but more importantly along - at the cost of the tyre radius.

[SeaComms]

Agreed, but they should not change the rolling circumference of the tyre though?

[Gravel]

I run my 33's at 30 psi and they are wearing really well... Far more comfortable and responsive.

Tire life is more important than 100% accuracy on the gauge (having said that my GPS and Speedo are spot on at 100 kmh).

[oldmacdonald]

Originally Posted by brian unlimited

Installed my outback draws and fridge slide today fits well looks nice will hopefully do the job for the Kimberleys trip in august.

As in 4wd Interiors Outback Drawers? Would love to see some photos.

[oldmacdonald]

Originally Posted by SeaComms

Agreed, but they should not change the rolling circumference of the tyre though?

Won't change the circumference necessarily, just "shift it" away from the part of the tyre in contact with the road. Lower pressure will reduce the height of the hub from the road. Speed is calculated, ultimately, in proportion to wheel RPM. A smaller effective radius of the trye between the hub and the road will increase RPM for a given speed and if the difference in measured speed is actually appreciable the speedo will read higher with lower pressures.

[PapaSmurf]

Originally Posted by SeaComms

Agreed, but they should not change the rolling circumference of the tyre though?

The circumference of the tyre doesn't change, but the radius does. If you let enough air out of a 33" tyre to lower the axle by 1/2", the 1km distance requires 391 revolutions rather than 380 - and the speedo is based on the revolution count.
You can think of the circumference of the tyre as a caterpillar tract rather than a perfect circle. Inside it there's a drive wheel that changes the size as you let the air in or out.