I used to stick 30 in my Astra all round, but that was before I read the manual, which said 34!
Shredded the inside of 4 tyres that way within about 20k before I thought to check. Now on Michelin's with 36 and doing much, much better.
I go with manufacturers recommendation + 2 as a general rule, the reasons being:
- Pressure gauges aren't exactly a certified item (anyone had there's calibrated recently?).
- Tyres may loose a bit of pressure between checks anyway.
- The temperature/weather changes too much to worry about how deviating slightly from the manufacturers settings will affect performance.
- From Corsa's to Focus's, I've heard of shoulder wear on all cars caused by keeping to the exact manufacturer's suggestion.
As for the rears, don't get me started. How many of us adjust them when going out? Mine's a difference of 8psi between a 2 man crew and a full load! How should I know how much my crap in the boot weighs? Is that another person? Should I adjust the pressures to be lop-sided when only I'm in there? I just put the same in as the front: eases the tension in the queue at Sainsbury's!