awkward subject... according to my manual I need about 32psi on the front and 29 on the rear for sub-100mph and 3 persons or fewer. however I do most of the driving in my friendship circle and as a result one day It might just be me and then the next day I might be fully loaded. I was thinking If I put around 32psi in all round it should be about right if I'm never having the same load twice and don't want to be getting the gauge out twice a day?

sum where on the Tyre in small raised lettering it should have a max psi written on it mine says 51 psi cold tyre. so i inflate to ford recommended for full load of 35 psi and 46 psi regardless of load as it is below the Tyre max of 51 psi and not under inflated. someone may show me the error of my ways but i have never had an issue to date.

Ford would having tested the car with differnt tyre pressures and have given the pressure for ride and handling, but if you are happy with 46psi in the rear tyre then that's fine. My ST had 40 psi in the rears when it should be 26 the ride was terrible.

Overinflated tyres wreck grip.

Higher pressures do give a more 'sporty' feel as it is under the max tyre pressure don't notice a reduction in grip as the tyre is designed for 51 psi. Ford recommend over inflating by 0.1 to 0.3 bar from ford recommended figures in there fuel efficiency tips page

The higher the pressure the less rolling resistance (less grip), & you should go by the manufacturers recommended pressures, not the max tyre pressure on the tyre.

Just depends what you are looking for, less pressure generally means greater rolling resistance as E5GDM says. An extreme example would be having two flat front tyres and trying to drive at 50 or 70mph as you usually would. In short it would be much harder for the car to do so, using more fuel, making more noise etc etc.

Less pressure does improve grip (slightly) but also increases fuel use (slightly) and both over and under pressurising the tyres alters the wear on tyres.

I like to try and over inflate to combat the harsh bumps and potholes my local roads have so that i get as much effective air between the alloys and road as possible. I am not to bothered about grip as the fiesta is a great handling car anyway and the differences in pressure wont make a worthwhile difference for public road driving, the only time it does make a noticeable difference you wont be driving on public roads in a particularly police friendly fashion. This is subject to the quality of rubber you are driving on however!

Over inflating your tyres could make things worse on a bad road, & it doesn't matter how good your chassis is if you haven't any grip.

http://www.kwik-fit.com/tyre-pressure.asp

My fiesta mk6 handbook says front 29 rear 26 but they were dragging along all the time, so I put pressure up to 35 front and 30 rear, sounds a little too much but apparently when ford printed these handbooks, they didn't anticipate the different types of tyres, e.g mine are rain sport 3 as well it's raining most of the time, whereas my other cheap tyres were happy at 29 psi

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