Low-Resistance Tires
Published April 3, 2006
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.
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In defense of those who like increasing tire pressure, there are a few things to consider. First, the prius is a fairly heavy car, weighing in at nearly 3000 pounds. Second. the 185 mm tire is not a very wide tire for distributing this weight. I have seen pressure recommendations similar on cars of the same weight, but riding on the larger 205 mm tires. to maintain the aspect ratio of the contact patch, a narrower tire placed on the same car needs considerably more inflation pressure to maintain the same tread surface deformation.
Another thing is that the prius is front heavy. front wheel drive cars usually are. the 23 lb chassis battery and the 119 Lb
nickel hydride battery only amount to the weight of a small person sitting in the back seat. up front, you have mg1, mg2, the power splitting device, and the combustion engine, along with the differential assembly, and finally, the driver, and perhaps a passenger putting their weight mostly on the front tires. so there is no problem upping the inflation pressure on the front tires, but be careful not to over-inflate the rear tires. The rear axle of the prius is solid. it's a pseudo-trailing suspension. this is the type of suspension that has a tendancy to lift the inside rear tire right off the ground durring a hard corner. you will notice that the rear tires require less inflation pressure then the front. Look at this as a percentage, not an offset. if you plan to increase the front tires to 42 psi, stay between 38 and 39 psi on the rear. if you get too much pressure in the rear, the back end could skitter on a rough road. the tires will be too round, and will not keep enough rubber on the road for good handling or good tire life. this also puts undue stress on the axle and suspension components. It also flags the vehicle stability control when braking. the higher the pressure, the more noticeable the electronic traction control issue will be on rough road surfaces.
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