Sure, you can buy a hybrid or a car that claims to have great fuel efficiency if you want to save on gas, but what's the point of doing that if you're just going to blatantly ignore small maintenance chores that are directly related to your car's fuel consumption habits?
The U.S. Department of Energy says that for every one PSI drop of tire pressure in all four tires under the recommended number, gas mileage can worsen .3 percent.
If you're not paying attention, that can really add up.
safercar.gov
Not to mention correctly inflated tires live a longer life than tires that aren't properly inflated.
Factory recommended tire pressure numbers can usually be found in the car's owners manual or in the driver's side door jamb.
So if you see that yellow Tire Pressure Monitoring System light on your gauge cluster, pick up a tire pressure gauge, figure out which tire is low, and go fill it up.