Gas mileage is affected by more than just the size and weight of the tire. Rolling resistance of the tire has a larger effect. Here’s the basic rule:
Harder tires = lower rolling resistance
Beware. If the tire is too hard or too round, then the tire-patch (which is the area of tire that is in contact with the ground) will be too small to grip the road surface effectively. This in turn will allow the wheel to skid easier when trying to stop or allow the wheel to slip and spin when trying to roll out from a light.
The same holds true for tread designs and patterns. The more detailed the tread design, the more likely it will grip the road better—but also create the opportunity to trap air in each of the pockets it generates when touching the road surface. When trapping the air/water/dirt/snow in these pockets, the tire is compressing the material that is in the pocket. Therefore, the engine has to do more work to compress these pockets. This makes the tire less efficient to turn, and thereby reduces the mileage of the vehicle.
The tire companies work to balance the rubber compounds and tread patterns to find the most efficient combinations. Along the way, new compounds are discovered as well as more efficient tread patterns.
Hybrids and Resistance
Most of the hybrid vehicles use some sort of low rolling resistance tire. Sometimes these tires are called “low mu.” “Mu” is the Greek letter (used by engineers) as the symbol for “tire friction to the road surface.”
While the low resistance tires will help with mileage, there have been some complaints from hybrid owners that they did not like the way the tires handle on the road. As a result, Toyota has backed off from using the lower resistance tires and has settled with a better handling tire.
It boils down to personal preference. If you drive aggressively, you might want to use a softer tire with better road adhesion. If you’re a fanatic for mileage and a very mellow driver, then you probably should get the lowest resistance tire you can find on the market.
Choice of Tire and/or Tire Pressure
You can minimize resistance, and maximizing mpg by making sure your car uses a Low Rolling Resistance (LRR) rated tire. If you change your vehicle’s tires, by increasing or decreasing the resistance by 20% for example, you may raise or drop mileage by 3 to 5%. While this is a measurable difference, it won’t make as big a difference as making certain that your tires are properly inflated. A vehicle that requires its tires to be inflated to 35 psi will have an increase in rolling resistance of 12% or more if the tires are allowed to become underinflated to just 28 psi.
Many hybrid car enthusiasts recommend an absolute minimum psi of 35 psi—and many try to push the envelope by increasing pressure to 40+ psi. You should read the manufacturer’s recommendations, and decide for yourself how comfortable you are with pumping up your tires for a gain in mpg. There could be safety issues related to over-inflating your tires.
Bear in mind that temperature makes a difference. The rule of thumb is for every 10° Fahrenheit change in air temperature, your tire’s inflation pressure will change by about 1 psi (up with higher temperatures and down with lower). When it gets cold, check your pressure to make sure it’s not too low.