Quote:
Originally Posted by TexasCotton
Hello all
I have not posted in this forum long long time.However have follow some of the regular and impressive posters.
I have the following question ,concern , issue, etc
I have a 2004 Toyota Prius (Gen 2) OEM tire size is 185/65R15
so I have 15 inch alum oem rims.
I have witness others go with michelin energy X, brigdestone ecopia LRR tires and kept the oem tire size rating and above 44 psi inflation with minor mpg gains of 1-2 at best. The issue question is would a "skinny" tire make any improvement.
Specifically would a tire size of 175/65R15 make any postive mpg improvement
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You have to be really careful here. "Skinny" can mean different things.
If you start with a 185/65R15 and go to a 175/65R15 (which is your question), you are not only getting a smaller diameter, you are also getting a lower load carrying capacity, which has safety implications (that is, a more intense loading is directionally towards more frequent tire failures.)
PLUS, that is the wrong direction for RR. Tires don't like being more intensely loaded and one of the ways they exhibit that is in the amount energy they consume - ie, Rolling Resistance.
But "skinny" could also mean "narrower, but taller" - like going from 185/65R15 to 175/70R15. The problem here is that load carrying capacity plays more of a role in RR than width.
If you want to improve RR by changing tire size, get the largest tire that will fit under the fenders - without rubbing, of course. There is a couple of percentage points to be gained here.
But the best way to improve RR is by careful selection of the tire. There can be up to a 60% difference in rolling resistance between tires. But that difference is tied up with treadwear and traction and gains in RR are offset by losses in treadwear and traction. You have to find a compromise that works for you.
And be careful: The term "LRR" is NOT an absolute one. It means "compared to other tires with similar treadwear and traction, this tire has a lower RR". It's a relative term.
Here's what I am basing my statements on:
Barry's Tire Tech