Tire Rack Test LRR % Claims?

[Blue Angel]

I was reading through a thread on the Cruzetalk forum and someone linked to a Tire Rack test from 2009, comparing LRR tires on a fleet of Priuses. Here's the article:

Tire Test Results : When Round and Black Becomes Lean and Green

In this article, the following claim is made:

"The Prius hybrids used for this test returned an average of about 52 mpg... [show FE% improvement chart] ...you can also apply similar percentages of improvement in fuel economy to other vehicles. The fuel saving dollar value of eco-friendly, low rolling resistance tires essentially doubles or quadruples when applied to typical cars that deliver 25-30 mpg or light trucks and utility vehicles delivering 12-15 mpg."

Is it just me, or is this statement completely false? Im my mind, taking a theoretical car that weighs the same as the Prius, uses the same size tires and has the same suspension alignment, but gets 30 mpg on the highway instead of 50 would benefit proportionally LESS than the Prius, not proportionally MORE as the article states...

If the LRR tire upgrade on the 50 mpg Prius netted a 5% increase in FE (+2.5 mpg), then the same tire upgrade on the 30 mpg car would only see a 3% increase in FE (+0.9 mpg).

To be clear, since the cars would both lose the same amount of energy due to rolling resistance, both cars would save the same amount of fuel per mile driven with the LRR tire upgrade, but the percentage of FE improvement would be less for the less efficient car since it uses more fuel to begin with. The Tire Rack article claims the opposite.

This is the way my mind approaches this... am I on the right track here, or am I out in left field? I wanted to put this in front of the "experts" before commenting on the other forum. Thanks!

[aardvarcus]

In broad generalizations I believe you are on the right track, assuming the less efficient cars are less efficient because they have higher aero drag due to the body not the tire, more non tire related drivetrain drag, etcetera and also assuming you are going from a "normal" non LRR street tire to a "normal" LRR street tire.

However, there are many instances which defy these rules. For example, compare a truck on 33" 285/75/16 mud tires, versus 31" 245/75/16 LRR tires. Most people report 1-2 MPG difference when making this kind of switch. (All too often reporting it going the wrong direction...) In a pickup truck that only gets 12 to start with, you are talking in the neighborhood of a 8-16% change.

Edit: The statement about the savings in the article is “The fuel saving dollar value” not the percentage mpg difference. Also don't forget to take into account fuel useage is inversely proportional (gallons/mile) to miles per gallon, thus the actual fuel cost impacts for making any improvement will be a lot more substantial on a less efficient car. The extra mpgs help the fuel bill a lot more on a lower MPG car. (Assuming the same distance is driven.)

[Blue Angel]

Originally Posted by aardvarcus

In broad generalizations I believe you are on the right track, assuming the less efficient cars are less efficient because they have higher aero drag due to the body not the tire, more non tire related drivetrain drag, etcetera and also assuming you are going from a "normal" non LRR street tire to a "normal" LRR street tire.

Your assumptions are the same as mine.

[SentraSE-R]

You folks are missing the point. A 50 mpg Prius uses 240 gallons to drive 12,000 miles in a year, at an annual cost of $960 (@ $4/gallon).

By comparison, a 20 mpg car, minivan, or truck uses 600 gallons to drive its 12,000 miles, at an annual cost of $2400.

If LRR tires give a 4% improvement, the Prius only benefits by saving 9.6 gallons/year, or $38.40. The 20 mpg car saves 24 gallons. or $96. That's in the double to quadruple dollar value savings range cited by Tire Rack. 25 mpg and 12.5 mpg examples would show double and quadruple the dollar value savings exactly, but the math figures are a bit less intuitive.

[CapriRacer]

I think Tire Rack has this right. It's about energy consumption.

If car A consumes 50 mpg and only 2 mpg are for tire rolling resistance, then the car changes tires (and let's pick an extreme example) now gets 4 mpg for RR, then the net is now 52 mpg (52/50 = 4% improvement)

Car B consumes 25 mpg of which 2 mpg is tire RR then a change to 4 mpg for RR results in a 27 mpg (27/25 = 8% improvement)

[ksa8907]

I think the confusion is in the percent. Percent is calculated, an actual measured difference is the important part. No modification makes the same percent difference on every car.

[LeanBurn]

Why not:
1) use 1 car in neutral with the steering locked,
2) have it in a perfectly flat enclosed area,
3) roll it down a ramp.
4) the only variable would be the tires.
5) the furthest rolling tire would be the winner.

Any other test to me is tainted.