[CapriRacer]

Originally Posted by Frank Lee

It's a coefficient; just like drag coefficient, the load (area for aero) must be accounted for. It's pretty well shown on pg 38 with the 15" and 17" examples: while the 17" shows a LOWER COEFFICIENT in the best case scenario vs the 15", clearly the actual rolling FORCES for the 17" are worse than the 15"; in fact the worst 15" value is better than the best 17" value!........

Yup, but don't forget that RRF (Rolling Resistance Force) = RRC (Rolling Resistance Coefficient) X Load.

It's easy to get confused in this report because Bruce goes back and forth between RRF and RRC.

The situation we are discussing is changing a tire on a vehicle so the load is basically the same. So looking at page 38, there are some 17" tires with lower RRC's than 15" tires, so it is possible to get better RRF by making a switch from 15" to 17". (Discounting the fact that the 2 tires mentioned in the report are just not interchangeable.)

Originally Posted by Frank Lee

.....
Pg 24 shows that the heavier the tire, the higher the r.r. force; shown even more graphically on pgs 30 and 32........

Again, don't get confused between RRF and RRC.

Certainly, pg 24 (where the tires are the same size), a heavier tire would generally have a lower RRF and lower RRC (same load), but notice how much scatter there is to the data. It's only a 20% R squared value - That's TERRIBLE!!!

This makes sense if we consider that the rubber compound has a HUGE effect. And different rubber compounds are pretty close to the same density.

But pg 30 and pg 32 are about RRF and not RRC.

Originally Posted by Frank Lee

......

Pg 31 shows coefficients with the big tires outperforming the small. Yup, big ones do better at high loads. The empty light end of my car isn't subject to high loads........

But it is subjected to a load and RRC X Load = RRF.

So if we keep the same load, a larger tire with out perform a smaller tire.

Originally Posted by Frank Lee

........

I did not expect to see r.r. increase with increased diameter! ..........

Sure you did! Larger diameter = larger capacity = higher RRF.

But again don't confuse RRF with RRC.

While page 38 says the correlation between diameter and RRF is 76%, RRC is only correlated 56%. That's the best value, but it is not very good!

Originally Posted by Frank Lee

.......

Unfortunately I did not see much useful info relating to width except for what may or may not can be read into pg 30 results.

If you do the math, RRF goes up with width (More width = more capacity), and RRC goes down with width. But there is a lot of scatter to the data.

Trying to apply this report to the situation under discussion (changing tires on a given vehicle), the gist is that larger capacity is better. Trying to compare other parameters (such as width or diameter) gets confounded with load capacity. But there is a lot of variability - meaning a particular combination might not result in an improvment.

So while the data indicates that a larger tire will be directionally better for fuel economy, don't expect it to work every time.