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It's obvious that the only contact between your vehicle and the road make a difference in fuel economy. There are so many factors that go into tires, generally the same applies to any category of the automotive world. It wasn't until the 1980's that people discovered the laws of proper inflation, due to rising fuel costs. 32 pounds per square inch became the magic number. Even now with all our advances, we're putting air pressure monitoring systems inside the tires. 2008 models are required to have some type of sensing unit to relay tire pressure to the computer inside your car. Yes, we're adding one more idiot light to the dashboard. Some cars may even display tire pressure in individual tires (throw those tire checking guages in the trash). Corvettes have already done this in previous years but mainly for safety. In the event of under-inflation the car would not exceed 55 miles per hour to keep the driver from harming himself or his vehicle. Safety has a large portion to do with the recent changes, fuel economy is a secondary thought. You may be outraged to find the extra 80 dollar fee tacked on to an individual tire when you head to the shop to swap or change tires. They havn't developed a way to avoid breaking the sensor inside your wheel. Why does it make a difference? It takes more effort to get an underinflated tire moving. On top of that, friction increases as more surface of the tire contacts the road. Weight is also a key factor. Larger tires, or higher performance, stiffer riding tires for cornering and handling produce a heavier mass for the vehicle to move. A heavier mass equals a greater force to move the mass; which translates to lesser fuel economy. Smaller tires can decrease economy. They cause you to run at a higher rpm at cruising speed due to the smaller radius. When you change the size of your tire you change the gear ratio. Don't think putting a smaller tire on your vehicle(height) is going to increase economy. It lessens weight but increases the rev's per minute when your driving 55mph down the highway. Larger tires may increase gas mileage but their heavier weight causes you to expend more energy taking off. Not only the take off energy but the force it takes to keep you moving at with the taller gear ratio may mean holding the gas pedal 2/3 down instead of 1/3. When choosing tires, it's best to look at factory specifications, they've already worked the equation out for you. Stick with stock tire sizes. Leave the tire selection up to the mechanic. Don't let him sell you some beefed up tire, just simply say, i want your factory recommended tire for my vehicle.

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