do you know you can have nitrogen inflated in to your tyres,free of charge at COSTCO
With nitrogen being a inert gas it does not respond to temperature rises or falls and will keep your tyres at the required psi for the life of the tyre, assuming there is no blow outs,leaks etc,this means your tyres don't loose shape,will wear out more evenly and you will probably get better mileage.
also the AA do it but charge about £7.00 for a full set of tyres
bobplum