Your tires play an active part in fuel economy.  Being the only part in contact with the road and being a major part of the car’s forward propulsion design, it has a large impact on fuel economy and getting the best mileage.

There are three factors in tires that affect your cars best mpg:
1. Pressure
2. Cleanliness
3. Maintenance

Pressure. The higher the pressure, the lower the rolling friction - resulting in better mileage. The suggested tire pressure can be seen in the owner’s manual or on a placard on the end of the driver’s side door (sometimes inside the glove compartment) — this is the minimum pressure you should set your tires to.

Wayne Gerdes actually recommends going for the max sidewall pressure listed on your tirewall, although he cautions that you shouldn’t go higher than 25% of that value as there are legal limits to these things.

On the plus side, you get better mileage, lower thread wear, more even threadwear across thread widths, better handling (sometimes) and shorter braking distances in some situations. However, your tires become noisy, vibrate more and you get a harsher ride.

Cleanliness. Yes, one should clean their tires. Mud, small rocks, gravel or whathaveyou stuck in between the threads will aversely affect your tire’s grip. Slipping tires translate to loss of kinetic energy that should have been translated into your car’s forward motion and directly translates to loss of fuel economy.

Maintenance. One should also maintain their tires, as poorly maintained tires lower your fuel economy by a significant amount (personal experience is along 30%).

Tire maintenance made simple

1. Maintain proper air pressure. Check air pressure once a month, because normal air loss is 1 psi per month
   Vehicle manufacturers list recommended tire pressures for original vehicle tires in the owner’s manual or on a placard on the end of the driver’s side door or in the glove box. An underinflated tire has more friction with the ground - this translates to more heat, damaging the tires and more fuel consumption. A grossly overinflated tire will have uneven threadwear, which will also damage the tires. So be sure to fill them up just right.Use the recommended tire pressure and vary only slightly depending on temperature and load conditions (colder temps and/or bigger loads = bigger pressure). As a rule of thumb, each 10°F (12°C) change in outside temperature causes a 1 psi change internally in the tire.
2. Rotate every 8,000 km (~5,000 mi)A. If driving a 4WD or rear wheel drive vehicle, rotation should go:
   a. Left front wheel to right rear position.
   b. Right front wheel to left rear position.
   c. Left rear wheel to left front position.
   d. Right rear wheel to right front position.B. If Driving a front wheel drive vehicle, rotation should be:
   a. Left front wheel to left rear position.
   b. Right front wheel to right rear position.
   c. Left rear wheel to right front position.
   d. Right rear wheel to left front position.  
3. Evaluate your tires everyday for threadwear and sidewall damage.
   • Expose thread bars are for replacement
   • Irregular wear are for camber and/or toe-in toe-out inspection
   • Tires with less than 2.1 mm (from a top of a penny to the top of Lincoln’s head if you want to measure it that way, just insert a penny into the thread - if the top of Lincoln’s head shows then it’s time to change tires)

For readers residing in the good ole’ USA you can get great tire discounts on quality branded tires here.