[roflwaffle]

Originally Posted by CapriRacer

You are right. It would be nice if someone would duplicate the study in another brand/model. That would verify the effect.

But there are laws of physics involved here, and even though I have reservations about the exact tires that were selected to be tested, I do not think there was any attempt to by-pass those laws. In other words, I think the effect demonstrated is real and reproducible. Until such time as we have additional data that contradicts the Smithers report, I don't think there is any reason to doubt the validity of the effect for every tire.

-- An Insider's Insight --

Since the study was presented to a government entity in public view, if there were any dissenting opinions, they could have been expressed at subsequent meetings. To date, there haven't been.

Further, tire manufacturers would have access to a lot of data. They may not have enough to data to confirm the results, but at the very least they would have data to contradict the results. They would not be afraid to publish the results if there was some advantage to do so - and so far they haven't.

There are 2 possibilities:

1) That the effect demonstrated is real
- OR -
2) That the results would have some positive affect on selling tires

I also know that NHTSA has been very active in testing tires for RR. If there was any reason to doubt the data, they would have published their data. In conversations with one of their test engineers, he acts like the effect is accurately described, but disagrees with the tire industry's position about what regulations should be put in place as a result. (THAT is a whole 'nother discussion)

So not only do I think the effect is as described, others must think so as well - and some of those are sure to have access to data that would contradict.

Someone would need more than just another brand/model. Many brands/models for many years would be needed. Even then, no where in the paper is the relationship bigger = better mentioned, you were the only one who mentioned it.

In the end the author states...

The lack of quality linear correlations between rolling resistances and the basic parameters investigated suggested that if the researcher is investigating manufacturer/tire design differences within a tire size, other more complex aspects of the tire will need to be considered.

So what they are stating is entirely reasonable given the data, and there were no objections because of that. You were the one who stated bigger = better, and that is where the debate is.

Originally Posted by CapriRacer

Doesn't this depend on what your definition of "bigger" is?

If 16" tires are larger than 15", then there is a problem - there are only 2 16" data points: 215/60R16 and 225/60R16. To be consistent, then these should be compared to 215/60R15 and 225/60R15 - but that data doesn't exist!

By contrast, the 15" data has 8 data points - ranging in size from 195/65R15 to 235/75R15. There are a number of ways to look at "bigger", but every way I know about would say a 235/75R15 is "bigger" than a 225/60R16 (with the exception of rim diameter).

That's why I did a regression analysis. That would factor out the effects the other parameters would have on the data and allow one to look purely at the effect one parameter has.

And the regression analysis says that going up in width has a positive affect, going up in aspect ratio has a positive effect, and going up in rim diameter has a positive effect. By any stretch of the imagination, all 3 of these results in "bigger".

I gotcha! I didn't bother to check the outside radius, so that's my bad.

In terms of a regression analysis, that's not exactly kosher. One of the assumptions of a regression analysis is that...

Originally Posted by Wikipedia

The sample is representative of the population for the inference prediction

So w/o data for larger sizes you still can't reasonably state that bigger = better even for goodyear integritys, and certainly not for tires in general.