Tire sidewall height vs rolling resistance?

[lasitter]

Originally Posted by CapriRacer

Second, does your vehicle NEED a 235/75R15 XL? I ask that because tire shops are not likely to apply a tire with a lower load index. Put a different way, what is your vehicle - year, make, model?

1996 Ford F-150 4.9L (Straight Six) with many modifications ...

The current tires are 108T, but going down to 106 would represent the loss of 89 pounds of capacity per tire. I'd much rather go the other way though.

Originally Posted by CapriRacer

Third, I hope you realize that wheels are going to be expensive and you are not likely to recoup the cost. So are you willing to spend the money in spite of knowing you won't recover the cost in fuel economy?

Oh yes. If I can improve my fuel economy via this experiment, it will entirely worth it in terms of entertainment value.

The major benefit with having winter tires on other wheels is that swapping is easy. RWD trucks typically have really lousy traction on slippery surfaces, so I'm even considering studded winter tires.

I also have a 2007 Passat that will have non-studded winter tires when there not snow out here in western Massachusetts. Just this past winter (my first here) we set a record with over 110", so I'm kind of spooked about it right now.

Originally Posted by CapriRacer

And lastly, the actual amount of improvement in fuel economy you will receive is very much dependent on your actual driving conditions. For example, someone doing stop and go driving is not going to experience much of an improvement in FE compared to someone who has a long commute on an Interstate highway.

Exactly. I have a specific driving test / goal in mind, which involves a big stretch of cross country driving.

[CapriRacer]

Originally Posted by lasitter

1996 Ford F-150 4.9L (Straight Six) with many modifications ...

The current tires are 108T, but going down to 106 would represent the loss of 89 pounds of capacity per tire. I'd much rather go the other way though. .....

OK so that eliminates everything lower than a 108 Load Index.

Originally Posted by lasitter

........ Oh yes. If I can improve my fuel economy via this experiment, it will entirely worth it in terms of entertainment value. ....

This was the response to the cost of wheel question. That gets us down to this list:

General Tire Grabber HTS LRR 235/75 R15 109T XL 2271 lbs./51psi 12.0/32" 32lbs 7.2"
Michelin Latitude Tour HP LRR 255/65 R16 109H SL 2271 lbs./51psi 10.5/32" 33lbs 8.5" (owners hated it)
General Tire Grabber UHP --- 255/65 R16 109H SL 2271 lbs./51psi 12.0/32" 34lbs 7.7"
General Tire Grabber UHP --- 255/55ZR18 109W XL 2271 lbs./51psi 11.0/32" 32lbs 8.5"
Pirelli Scorpion Verde AS LRR 255/55 R18 109V XL 2271 lbs./50psi 11.0/12" 36lbs 9.0"
Continental CrossContact LX20 LRR 255/55 R18 109H XL 2271 lbs./51psi 12.0/32" 35lbs 8.6"

I would eliminate the anything with a higher than T speed rating - extra cap plies means higher RR.

That leaves only the 235/75R15 General Grabber HTS.

Now your current Kumho Solus KR21's are so very similar to the General Grabber HTS's, I don't think you will experience any improvement in fuel economy. In fact, if you make this change, you will start off WORSE, and it will gradually get back to where you are now - worn tires.

The only thing I can suggest is to go larger - P265/75R15. There MIGHT be fitment issues, and the improvement is small - 1% or 2%. Personally, I don't think this is worth the risk.

[Focus-Ak]

[ The chart below has been corrected. The two green dots were in the wrong place; both should have been for a tire profile of 60. Related text below (for 16" rims is also corrected. ]

I got curious about the data in Lambillotte's slide #30 (in a powerpoint found through Barry's web page on tires). Here is a parameterized chart of the data for the 28 tires, with tire width as a parameter, and colors indicating rim diameter. The chart gives rolling force vs. tire profile.

Some of the data points for the 14" rims seem questionable from this view of the data (i.e., it is odd that the 185 curve is flat and the 195 curve has a negative slope -- both in comparison to the others.) Also, the two 16" rim data (green dots) are anomalies, since these are the only tires with a profile of 60. This low profile seems to have made up for the efficiency loss that the high width of these tires would cause.

Nonetheless, I think this analysis provides a handle on the complicated relationships involved, which the slideshow chases around but seems to miss. (See URL embedded in the chart.)



My apology for the poor legibility of the tire width labels -- I had to downsize for the size limits of EcoModder photo albums.

[lasitter]

Originally Posted by Focus-Ak

My apology for the poor legibility of the tire width labels -- I had to downsize for the size limits of EcoModder photo albums.

I only got one green dot when looking on the graph. Is that all that is for 16" wheels?

[CapriRacer]

Originally Posted by Focus-Ak

I got curious about the data in Lambillotte's slide #30 (in a powerpoint found through Barry's web page on tires). Here is a parameterized chart of the data for the 28 tires, with tire width as a parameter, and colors indicating rim diameter. The chart gives rolling force vs. tire profile. ........

Which is why I did a regression analysis based on the 3 "size" numbers for RRC. What was interesting was the r squared value was pretty low - indicating there was something else going on.

I actually did quite a few before I settled on that one.

Nevertheless, in every case, the regression said that bigger was better - and I think I understand why that is so. There must be some amount of variability EVEN though the tires are supposed to be "the same". "The Same" is kind of a misnomer in that the construction for each size MUST be different - but they ought to be proportional - and it appears they aren't.

I wish we had more data to work with.

[MkVer]

Originally Posted by lasitter

1996 Ford F-150 4.9L (Straight Six) with many modifications ...

The major benefit with having winter tires on other wheels is that swapping is easy. RWD trucks typically have really lousy traction on slippery surfaces, so I'm even considering studded winter tires.

I'd be careful with studded tires. Besides being illegal in some states, they barely outperform stud-less tires and sometimes not at all. I'd post a link to TireRack doing an ice rink test showing stud-less tires outperforming studs but I'm too new to post links.

[lasitter]

Originally Posted by MkVer

I'd be careful with studded tires. Besides being illegal in some states, they barely outperform stud-less tires and sometimes not at all.

In Massachusetts they're legal from November to April. I had them once on a 1990 Ford Aerostar and loved them.

I think that with the studs, about 500 pounds in my 8' bed, and the Eaton limited slip differential, I should be OK ...

[lasitter]

Originally Posted by CapriRacer

That leaves only the 235/75R15 General Grabber HTS.

Originally Posted by CapriRacer

Now your current Kumho Solus KR21's are so very similar to the General Grabber HTS's, I don't think you will experience any improvement in fuel economy. In fact, if you make this change, you will start off WORSE, and it will gradually get back to where you are now - worn tires.

It all boils down to a couple of things:

Do LRR tires by General roll any easier than non-LRR tires from Kumho? Factor: The new tires would weigh 3-4 pounds more.

And: This would be offset by the new wheels being up to eight pounds lighter than the existing ones.

More research / other possibilities ...

Firestone Destination LE2 HAS --- P235/75 R15 108T XL 2183 lbs./50psi 12.0/34" 34lbs 7.4"

Two pounds heavier, but a much more highly rated tire than the Grabber HTS. Probable difference from the Grabber in fuel economy?

I don't have revolutions per mile for some of the tires in the list, and I'll work on that today.

[Focus-Ak]

Originally Posted by lasitter

I only got one green dot when looking on the graph. Is that all that is for 16" wheels?

The are two, one above each end of the same red line.

[Focus-Ak]

Originally Posted by CapriRacer

Which is why I did a regression analysis based on the 3 "size" numbers for RRC. What was interesting was the r squared value was pretty low - indicating there was something else going on. I actually did quite a few before I settled on that one.

Nevertheless, in every case, the regression said that bigger was better - and I think I understand why that is so. There must be some amount of variability EVEN though the tires are supposed to be "the same". "The Same" is kind of a misnomer in that the construction for each size MUST be different - but they ought to be proportional - and it appears they aren't.

I wish we had more data to work with.

Here is my assessment and some conclusions, based on my chart.

1. I think the problem is too complex for simple linear regression, there being five variables involved. Four are shown on my chart (rim diameter, tire width, tire profile, and the dependent variable - rolling force). The fifth variable is the unknown effect of any differing tire construction. A parametric chart lays out the relationships quite clearly.

2. I suspect that two data points were misrecorded during the test or in subsequent data handling. I think the two data points for the blue (14" rim) 195 width line got reversed. If so, correcting that makes that line agree with the others. The blue point for 185 width is also suspiciously low compared to the other two points on that line and the other lines on the chart. But these are all just suspicions. I haven't seen actual data, and took numbers for my chart off of Lambillotte's bar chart (his slide 30).

3. As you said, more data would be better. Nonetheless, there is clear consistency in my chart of the data in Lambillotte's bar chart (when disregarding the above questionable data):

** Rolling force increases with tire width (i.e. within each rim diameter category).

** Rolling force increases with tire profile(i.e. within each rim diameter category).

** With the exception of the two 16" rim tire sizes (different tire construction for these?), rolling force increases with rim diameter.

** Those 155 and 175 tires on 13" rims seem like the gold standard, for applications where they will work. 175-65-14 and 175-70-14 also quite good.

These conclusions are mostly opposite of information/assumptions I have seen posted on this forum and elsewhere. (So maybe this should be a separate topic somewhere in this forum.)