l have a Citreon C3 1.4 sensodrive HDI
what should the tire pressure be? On the inside of the passenger door it says 2.1 bar . what does that mean in PSI?
l have the tire pressures at 29 PSl at the moment

PS l means Pounds per Square Inch 2.1bar is 30psi so 29psi would do but up them slightly when you check

1 bar = 15 psi approx

That sounds about right - l was thinking 28 psi - l have a peugeot 206 which is much the same car and l run mine at 28

28psi Front 26psi Rear under normal load

14.7 PS l = 1 bar

So 2.1 bar is 29.4 PSI. So the pressure is slightly low but not by enough to worry. l would recommend putting between 32 & 34 PS l in ur tyres as these cars will wear the tyres quite quickly even if they r only a few PS l low. Running the tyres at 28PS l for their life will reduce their lifespan by about 2000 miles. If u r not running Michelin tyres then l would recommend putting 34 - 36 PS l in ur tyres as the recommended pressure is for Michelin tyres & they r a bit harder than most other brands.
This advice is based on what l have seen from hundreds of C3s.

Pressure gauges are normally marked in PS l & BAR.

on the side of the tire it will tell you how much air pressure it should have

l am convinced that both tire manufacturers & auto makers deceive owners/drivers concerning tire pressures.
The auto manufacturers generally would like the owner/driver to think that the automobile rides more smoothly & the tire manufacturer would like to sell more tires, hence they both recommend that tires be somewhat under-inflated. Lower pressure makes an auto ride ''softer''and also would tend to make tires wear out more quickly on the outer edges. l always exceed the recommended pressures by 4-6 PSI.
Even if my assumption is incorrect, l like the feel of a firmer ride & l get good mileage from my tires.
l have to agree that more tire pressure tends to offer less resistance on the contact between tire & pavement, so better fuel mileage should be the result.