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Old 03-28-2011, 04:34 PM   #1 (permalink)
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Default New tires, better MPG?

Recently had one of my RE92 rear tires have tread separation, although the inner liner held. Other than some vibration at highway speed, all seemed OK until I got to look at the tire a couple of days later, when it wouldn't hold air in the driveway. Drove great, tough little tire.

These were time worn, being manufactured in 2004, so possibly original (with about 70k miles). Not quite to the wear bars, but not a lot of life left.

So I replaced with a pair of brand new RE92's on the rears. Same inflation (55 psi). All of the sudden, a noticable increase in MPG of about 5 mpg. WTF? After have used a little over half tank, the next quarter has seen the tank average up almost 1.5 mpg.

I am very happy, but I would have thought a tire with more tread will have a little greater rolling resisitance and a little worse mileage, not the opposite.

Just wonderin'..............

Regards,
Jerry
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Old 03-28-2011, 04:54 PM   #2 (permalink)
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What one must realize is that there are way, way, WAYY too many environmental variables to pinpoint the MPG change on one thing.

It could be as simple as the wind is blowing in a different direction. Since our car's MPG scale is so high, +-5MPG once is meaningless.

Anyway, as far as I know, worn tires offer less rolling resistance - at least, that is what I have heard here. The more I think about it though, the less sense that makes. For example, my tires are almost bald, like I'm running slicks. Wouldn't that increase rolling resistance? More rubber in contact with the road at any given point, since there are no tread breaks?
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Old 03-28-2011, 11:27 PM   #3 (permalink)
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Originally Posted by Eli View Post
What one must realize is that there are way, way, WAYY too many environmental variables to pinpoint the MPG change on one thing.

It could be as simple as the wind is blowing in a different direction. Since our car's MPG scale is so high, +-5MPG once is meaningless.

Anyway, as far as I know, worn tires offer less rolling resistance - at least, that is what I have heard here. The more I think about it though, the less sense that makes. For example, my tires are almost bald, like I'm running slicks. Wouldn't that increase rolling resistance? More rubber in contact with the road at any given point, since there are no tread breaks?
I had the same thoughts about you as to tire tread. I thought more tread allowed more squirming, or flexing, and therefore reduced efficiency. I didn't consider "slicks" would have a higher contact patch.

But since the only change made was 2 new tires and the change is dramatic enough to be noticed, the only conclusion is that the new tires are making the difference.

Regards,
Jerry
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Old 03-29-2011, 01:24 AM   #4 (permalink)
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Like I said, that isn't the only change, though.

Was it warmer today? The wind could have been blowing a different direction. You could have unknowingly averaged a slower speed. You could have been focusing more on the FCD than you normally do. Your old tires could have lost air since the last time you checked them..etc, etc.

Just trying to get context. My MPG to and from work varies quite a bit on a day to day basis, and I take the same route 4x/day. I've made a habit of resetting the FCD display each day.
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Old 03-29-2011, 09:25 AM   #5 (permalink)
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Like I said, that isn't the only change, though.

Was it warmer today? The wind could have been blowing a different direction. You could have unknowingly averaged a slower speed. You could have been focusing more on the FCD than you normally do. Your old tires could have lost air since the last time you checked them..etc, etc.

Just trying to get context. My MPG to and from work varies quite a bit on a day to day basis, and I take the same route 4x/day. I've made a habit of resetting the FCD display each day.
Oh, I see. No, this was after about 3-4 days of driving, adding about 80 miles. I drive pretty much the same route every day. And no real hypermiling tricks, other then getting into LB on the highway as often as possible. The tires may have been down a pound or two, I hadn't checked in about 10 days, but even that is minor.

By resetting one of my trip odos, I was able to see a huge increase, but averaged on the tank with slightly less than half left, the effect was less dramatic, although still noticeable. And it was noticable immediately.

I am certain the minor varibles have negligible effect, I am pretty used to the car. I can check LB on the SC II, but I also notice my FCD instant display is consistenly at least one block higher than before when cruising.

It all points to tires, but still not sure why. Could it be the elasticity of the rubber, since the old tires were so old (2004 manufacture) and these are so much newer?

Thanks.

Regards,
Jerry
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Old 03-29-2011, 11:28 AM   #6 (permalink)
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Batch-to-batch differences in the tires? RE92's made 7 years ago might not be identical to RE92's made today, even if Bridgestone is trying not to change anything. Raw materials may change, for example?
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Old 03-29-2011, 09:19 PM   #7 (permalink)
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Batch-to-batch differences in the tires? RE92's made 7 years ago might not be identical to RE92's made today, even if Bridgestone is trying not to change anything. Raw materials may change, for example?
I've thought about this, and that is a nice thought. If they were essentially continually upgrading our tire? My concern is that it will go the other way. I can just imagine some smug executive looking over a cash flow spreadsheet, seeing our poor, low volume, high expense tire and signing off on a change that will ultimately result in the erasure of the tire's original intended purpose.

But maybe that's just my overactive imagination talking. I certainly hope so. How long can we realistically expect Bridgestone to manufacture this tire? 10 years? 20 years? 30 years? You can best be sure there will still be Insights on the road in 30 years.

That's a long time, though. What we really need is for another manufacturer(or Bridgestone) to make a tire that is better than the P165/65R14 Bridgestone RE92. That's what we need.
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