you can try to tell by gripping it, if it is firm then it is fully inflated. if it is kind of squishy, then it is under inflated.
With a pressure gauge.
The same as a tire that has not been inflated.
Most likely under-inflated tire
Pressure is equal throughout the tire.
A tire can effect your gas mileage by a lot. If you do not have a inflated tire you can get 2-3 mpg less than use usually get.
PSI is the abbreviation used to denote tire pressure. Tire pressure refers to how much air should be inflated into a car's tires.
Some motorists don't know that their tyres are flat or under inflated others just buy new tyres but others don't have a problem with this because they check their tyres regularly.
It could be over-inflated, under-inflated, have a weak spot in the tyre-wall, have a foreign object sticking into the tyres tread. It could even be the driver is too rough when turning and putting too much wear and stress into the tyres.
There are tires with innertubes and some that don't need them. The tire is filled to recommended pressures of about 30-40 PSI with air or some kind of inert gas. The pressure is great enough to push the sidwalls of the tire against the wheel that keeps air from leaking out.
If your tires are under inflated the resulting "resistance" created will lower your gas mileage. However, if you maintain the correct pressure in your tires it reduces the resistance and improves your car's efficiency. It's also safer.
recommended tire pressures should be on the sidewall of the tire. NO, NO, NO that is not the correct tire pressure. Never use that number!!!! That number is the max the tire can be inflated. The correct pressure amount is on a sticker usually on one of the car door frames or in the owners manual. Or car the dealer and ask them.
Have your tires rotated and balanced every 5000 miles. Check air pressures regularly so exterior or interior wear does not occur depending on if the tires are over inflated or under inflated. Manufacture recommended tire pressures can be found within the door jam or in the owners manual.
The braking of the car and, give the driver heavy steering