You be the judge
Okay, the question is--
Does it pay to buy efficient tires if you intend to run them at high pressure?
I ran some cases through my fuel economy program. Standard tires probably have Crr = 0.015 at their normal pressure. Good energy tires are probably at Crr = 0.009 at normal pressure.
According to one hypothication, the rolling resistence varies inversely with tire pressure. Let's assume this is true. For comparison, let's say normal pressure is 30 psi and high pressure is 50 psi. Changing from normal to high pressure changes rolling resistance like this--
Standard tire: 0.015 x 30/50 = 0.009
Energy tire: 0.009 x 30/50 = 0.0054
Here is the effect on mileage at 70 mph, for a Jetta, my calculation:
Crr = 0.015: 15.1 hp for aero + 8.6 hp for tires = 23.7 hp, 43.4 mpg
Crr = 0.009: 15.1 hp for aero + 5.1 hp for tires = 20.2 hp, 48.2 mpg
Crr = 0.0054: 15.1 hp for aero + 3.1 hp for tires = 18.2 hp, 51.6 mpg
Remember that for lower highway speeds and around town, the power for aerodynamic drag will be much lower, but the horsepower needed for rolling resistance will still be the same, making the effects even larger.
Okay, you be the judge, if you put 50 psi in the tires, are the energy tires better (Crr = 0.0054 at 50 psi) than regular tires (Crr = 0.009 at 50 psi)?
Ernie Rogers
watercop said:
Odo reading will change (be multiplied by) a factor equivalent to new tire revoulutions per mile divided by old tire revolutions per mile. Tire revs per mile are posted at Tirerack for most of what they sell.
I have a question for all y'all tire experts: Does the advantage conferred by low rolling resistance tread compound (silica) decrease as tire pressure is raised? In real world terms, folks that like to run 30 or so psig for reasons of ride and handling may well gain a lot more than those of us who max out tire pressure.
My pickup uses load range E tires with a max pressure of 80 psig. A big boat trailer I owned for awhile had tires rated at 100 psig, and I have read that commercial truck tires are rated as high as 120. I haven't come across much info about low rolling resistance tires for trucks, and I gotta figure the commercial fleets would adopt any proven technology to eke out another 0.1 mpg or whatever.
In other other words, if I run tires at 51 psig am I wasting money buying low rolling resistance tires? Are they "pearls before swine"