Am I correct in thinking the rear tyres on a volvo 340 should be 30psi and the front ones 27psi? A friend told me that tyre inflators in garages are very rarely accurate. I usually just put 30psi in all four tyres at a garage...

Yeah that's what is recommended. I always found it made the steering too heavy for me so I used to stick in 32 all round.

Depends on what tyres you have on! It say's what the recommended PSI (air pressure) is on the sidewall of every tyre i have ever seen!
13" steelies will mostly typically be somewhere between 30 - 36 i reckon, 15" and above alloys could be 50 odd psi,

checkout number 4 on this handy attached diagram!



Max Pressure ...?psi

under inflated tyre's = higher fuel consumption

Max load is only the highest pressure you should put in though - for road comfort and spread tyre wear, 30 is about right. Low profiles can go up to about 50 sometimes but I wouldn't take it up to this unless I was on a track. And even then I wouldn't happy that the whole tyre was in contact with the road.

The info on the tyre only shows the max pressure, not the recommended.

Im gonna stick with 30psi all round I think...

You should use the recommended pressures (30+27). And yes gauges vary widely, get a good quality/good brand gauge of your own with a small full scale reading as possible (i just got an inflator for my compressed air system and it reads up to 220psi..ridiculously inaccurate). Any gauge is only as good as its calibration, and garage ones get a hard life and tend to be neglected. When you have you own you can see the variation and get an idea of how accurate yours is.

Tyre pressure is a compromise for load and temperature variation to get the right contact patch on the road. The wrong pressures will cause the tyre to not contact to road fully and will reduce grip and increase wear. So it may be reasonable to increase pressures if you only do short journeys, or if you carry higher than average loads.

I have experimented with different pressures and the recommended ones give the most grip, higher ones the car feels a little faster and tighter but definitely less grip.