Quote:
Originally Posted by Mpower
Anybody have the specs on stock Prius and Civic/Insight Hyb tires? You would think those are super LRR.
Last year, I replaced GY Weatherhandlers on my wifes's Camry with Michelins from Walmart. Picked up grip but lost 2-3 mpg right away  . They have super high threadwear rating (700+). I wish I knew what I know now.
You measured it yourself? How do you do that? |
Hello, MPower,
Yes, you can measure your own rolling resistance, but if you do it my way it takes some work. First, the easy way-- rolling resistance is supposed to be independent of speed, so IF you find a section of sloping road where the car rolls very slowly at a constant speed, then the rolling resistance coefficient is equal to the slope (grade) at that point.
Now, the hard way-- Here is the formula for the retarding force on the car:
F = Cd A 1/2 rho V^2 + Crr Mg
and also F = M a
In words, the retarding force is equal to the sum of the aerodynamic drag and the rolling resistance. The aerodynamic drag is the product of Cd, the drag coefficient; A, the frontal area of the car; rho, the density of air = 1.2 kg/cu.m.; and the air speed squared, divided by two. The rolling resistance is the product of Crr, the rolling resistance coefficient, and Mg, the weight of the car. Remember that M is mass and Mg is weight. Use M in kilograms and to get Mg, multiply by g = 9.81
And when you are in neutral, the acceleration of the car (rate of change of speed) times the mass of the car equals the retarding force.
Okay, the test procedure is to find a lonely stretch of level road, and no wind. Speed up to about 75 mph, then put the car in neutral and roll. As you slow down, click the time with a stopwatch at say 70 mph, 60, 50, 40, 30, 20, and 10 mph. (Do this several times in both directions so you can get a good average.) Draw a graph of speed versus time. The slope of the curve is the acceleration. Now, plug all of the numbers into the formula above until you find a consistent result for both high and low speeds. That gives you both the Cd, the drag coefficient, and Crr, the rolling resistance.
Rolling resistance depends a lot on tire temperature and tire pressure. So, control the test conditions accordingly. And, remember that this rolling resistance value (as with the easy method) includes all of the friction in the drive train past the transmission.
As for your bad luck with the Michelin tires, a word of advice-- if you want a good price and long wear, you are doomed to failure. The low rolling resistance tires are made with soft rubber that won't last very long, no matter what brand. Find the tires with the low or no mileage warranty that are probably expensive-- they are sure to give good mileage.
By the way, I noticed that the Michelin Energy MXV4 S8 tires are being supplied at the factory for the new 2009 Jetta TDIs, showing the factory has agreed with me on the best tire for fuel economy, at least for VWs.
The next time you buy tires and they give bad mileage, TAKE THEM BACK. I did! Most reputable tire makers provide a 1000 mile satisfaction guarantee.