Whenever I inflate my tires to medium/recommended pressure the mileage is better and obviously when it is very low the mileage affected. But I want to know why my mileage affected even if I keep pressure above recommended values.
Recommended values give you the handling and tire wear that the manufacturer intends for the vehicle. If you increase above that, the contact patch decreases, giving better gas mileage, but you start sacrificing handling and causing abnormal tire wear.
When your tyre pressure is lower,there is more rubber in contact with the road, so your engine has to work a little harder to keep you moving forward.
I’ve heard folks claim mileage gains by running their tire pressure above the manufacturer’s recommendations. In theory, that makes sense: With more inflation pressure, the tire bulges in the center of the tread and creates a smaller, skinnier contact patch with the road. This should mean less rolling resistance and increased mileage. we know under-inflated tires have a negative effect on fuel economy.
When you turn your steering wheel, even ever so slightly: as the tyres turn the tread on the tyre 'shuffles'. The more your tread shuffles, the greater the tyre wear. The manufactures recommended pressures will give you the optimum performance and wear from your tyres. Under inflation allows the edges of the tyres to take all the load and shuffle-increasing tyre wear on the outside edges. Over inflation causes the tread to be stiffer in the centre of the tread area, allowing the treads central area to take a greater share of the load and increased shuffle-increasing tyre wear to the central tread area. Use the recommended pressures from your vehicles manufacturer for optimum/maximum tyre life, and fuel economy derived from the correct/optimum rolling resistence for the vehicle.
Cyclists know the answer to this only too well... They use the maximum pressure the tyre will allow, when racing or time-trialing (say 140 psi) But the ride is so bumpy.
When training they let their tyres down to 80-90 psi for the comfort, and better road-holding, at the expense of more effort required. (i.e. a worse mpg.)