Because you should check your tire pressure regularly! Underinflated tires produce more drag and heat – making it more difficult to ride smoothly. This increases fuel consumption and tire wear. You have alot riding on your tires – like your safety – so you should properly take care of them! Measure your tire pressure every other time you fill up with fuel and you should be ok. Usually you can find the correct tire psi listed on a label on the driver’s door jamp and in your owner’s manual.
I know for a fact that my wife’s Land Rover LR3 gets about 18 miles a gallon on the highway, but if the tire psi gets low the fuel economy drops to about 14 miles a gallon. That is a huge waste of fuel. I have a customer with a Jaguar V8 who runs 34psi in all four tires. The ride is a bit stiffer, but he claims to be getting 25 mpg on average.