ALERT! A tire problem?

First, let me say that I am no expert on the Toyota Prius. You owners should know much more. (I own a VW TDI.) I discovered the following information that I thought everyone would want to know--it's possible I don't have the right information, and then I would expect to be corrected.

When I did a Google search the other day for information on the Prius, I found that Toyota is not supplying the car with low-rolling-resistance tires. Apparently, they were offering LRR tires earlier. But after receiving many complaints about those tires, they changed to "performance" tires having normal rolling resistance. The primary complaint may have been about poor tire life. (Duh! They didn't design for the right wheel /tire size.)

I just ran my fuel economy model for the Prius, using two tire choices, and got the following results for fuel economy at 60 mph highway:

Normal tires: Crr = 0.012 --> 49.3 mpg
Good LRR tires: Crr = 0.0065 --> 60.4 mpg

This shows a potential mileage improvement from changing tires of over 20%. I am guessing on the rolling resistance coefficient for the OEM tires because I was unable to find any data. 0.012 is my best guess. The LRR tire Crr is the one I measured for the tires on my own car. (My tires are Michelin Energy MXV4 S8.)

In my opinion, this is a very serious matter. If I'm right, millions of gallons of gasoline are being wasted because of a bad choice at the factory. Well, actually, I hope I'm wrong.

Ernie Rogers
-----------------------------------------------------
Driving for Efficiency
May 28, 2006
989 miles /13.3 gallons ---> 74 mpg

2003 Beetle TDI + wing
Michelin Energy tires
Motul specific 506.01 motor oil
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Ron,

I sent you a private message on how to get my fuel economy model. It's an Excel file-- I'll send it to anybody that asks.

60 mpg is pretty easy, I do it all the time on the highway. That's right, good tires are very important to good mileage. My car routinely gets about 30% better mileage than it did when stock, about one-third of the improvement is from better tires. VW TDIs always come with LRR tires, but mine are better than the standard Michelin Energies.

I'm glad to see you have tried your own measurement of rolling resistance coefficient. Here's the routine I have used --

Find some flat, smooth highway with minimal traffic. Speed up to about 70 and put the car into neutral. As the car slows down, start a stopwatch as it rolls through 60 mph, and click the time at 50, 40, 30, 20, and 10 if you can; or stop the test when it starts looking dangerous to be so slow on the highway. Try to pick a time with little or no wind, but I would try to get three readings in each direction.

Average all the 50s, 40s, etc. Then, make a plot of speed versus time. The tangent to the curve is the car's deceleration. Now, you can calculate the rolling resistance from--

F = Ma = Crr Mg + Cd A 1/2 rho V^2

Crr is the rolling resistance you are after. Mg is the weight of the car, pounds /2.2 x 9.81. Cd is the drag coefficient for your car. For a Prius, this should be Cd = 0.26. The frontal area is 2.16 sq.m. Rho is air density, 1.22 kg/cu.m. at sea level. V is best handled in meters per second. Multiply mph x 0.447 --> m/sec.

I would read the slope of the tangent at very low speed and calculate Crr. Then, I would read the tangent of the curve at high speed, and use that value with Crr back into the equation to calculate the drag coefficient, Cd. The calculated number should be close to the expected value of Cd = 0.26. If it doesn't check, then you have a problem somewhere.

Tire or car engineers might complain that my routine is approximate-- it ignores some details. But, until the tire makers are willing to give us the numbers, we will have to figure our own.

Remember that the rolling resistance depends on a lot of factors, as does the drag coefficient. You will get your best (lowest) Crr when the tires are warmed up, and with good air pressure. I run my tires 10% below the sidewall rating (cold).

When I changed out my factory tires, I bought tires that were cheap but were reputed to get good mileage. I asked the salesman if the tires included a customer satisfaction guarantee. He checked with the wholesaler and assured me I had a 30-day return priviledge. Before the 30 days were up, I returned the tires due to poor mileage. I replaced them with Michelin's most expensive "energy" tires, one size larger diameter than factory spec. Since then, I have been smiling all the way to the gas station.

Send me a message if you have questions.

Ernie Rogers

 

Oh, okay, John,

I would say that confirms my suspicion about Priuses having questionable tires. By the way, my energy tires now have 55,000 miles on them and only half the good tread is gone-- I will probably replace them this next winter to assure best performance.

I haven't had any problems with performance on the tires. But, I am a "cool" driver, part of the secret to getting great mileage.

I find that slowing down just 5 mph improves fuel economy by 10%. On my last tank of around-town driving, I got a personal best of 55 mpg. My highway mileage is much better, but I don't have the benefits in stop-and-go that the hybrid has.

Ernie Rogers

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