Size of the tire - are smaller (narrower) tires really better?

[CapriRacer]

Originally Posted by CargoBoatTails

This is a really annoying argument that defies the laws of physics. Rather than make my own independent claims I'll use an elementary example of a currently mass produced econobox that comes in different trims, ie the Ford Fiesta HB manual. If you get the 205/40/17s the epa rates it at 26/35mpg, if you get the 195/50/16s the epa rates it at 28/36. Now go argue with the epa that the bigger tire should have gotten better mpg and stop telling naive people bigger tires get them better mpg, ridiculous.

And anyone involved in racing knows you need to make less pit stops with a lighter tire.

It's unclear what you are saying defies the laws of physics.

I don't think anyone was objecting to the idea that a lighter tire is better for RR - all other things being equal. But rarely are all other things equal.

In your example, the overall weight of the tire and wheel assembly changes mostly because of the wheel. I tried to see if I could find a tire that comes in both a 195/60R15 and a 205/40R17, and Tire Rack doesn't list any.

But if I do a survey of published tire weights, both those sizes are in the same range - and perhaps the 17" is a bit lighter, which would contradict your theory. But it really is hard to tell because there is very little commonality.

But I am basing my statement on this chart:

http://www.barrystiretech.com/smithersrrcsizemed.jpg

This is about as close to an apples to apples comparison as you will find.

And just like race tires come in different levels of grip - albeit with corresponding levels of wear, Street tires come in different levels of RR with corresponding levels of of wear and traction. These are tradeoffs and weight isn't the big factor here.

Don't get me wrong, less mass in a tire is beneficial, but there are other factors that are more dominant.

[CargoBoatTails]

Now that's an interesting chart you have there. It amazes me that a company would publish that a 215/70/15 has less resistance than a 165/80/13. Here is a perfect example of not believing everything you read, especially when the government is involved. Some things we can agree on, narrower and smaller wheels are better, as far as profile I would have to say LRR is what people should look for, in sets of 4.

[CargoBoatTails]

I really wish your site would just resize attachments to fit the forums limits.

[niky]

It's not a company advertising the tires... it's a study of the tires themselves... not knowing what's in there, I'd say it's likely mixed brands and types.

Also, at a profile of 80, you have to consider that the deformation of the tire itself will play a very big part in increasing rolling resistance.

-

As MetroMPG himself found out, an ultra-narrow, an ultra-light and narrow tire does not always make for less rolling resistance.

http://ecomodder.com/forum/showthrea...e92-19126.html


If all things were equal, that safety spare should perform better. In fact, the VW XL1 uses a very similar type of tire, at 115/80R15 (do note, sidewall height is exactly the same as on a 205/45 tire... so those are pretty low-profile sidewalls, anyway!). But they're not.

[CargoBoatTails]

I'm sorry but I don't see the relevance to cruising speed, really not trying to play devil's advocate here. A more revealing test would simply be measuring the deceleration times from say 65-45 mph when you take your foot off the gas. There's a lot more going on at highway speeds. I know when I had the 225/45/17s on my Versa it would slow a lot faster than the 195/55/15s, both being sticky UHP and Snow tires.

[niky]

Again... would depend on whether all other factors are equal. They usually aren't.

Also, at speed, there are a lot of other factors at play that make such measurements difficult.

[mcrews]

When you are 'cruising' you are at the top gear ratio in the vehicle: 5th gear, overdive, whatever.
the engine is at the lowest rpm.
This is the 'test' for HIGHWAY MPG.
What is so hard to understand???

You remind me of the guys who say they get more HP at WOT. Wide open throttle. So what???? YOur only at wot 5% of your life (max)
Unless your a delivery vehicle, making stop n go intown all day, your coast down is not relevant to everyday driving.

[ijames]

If you look at MetroMPG's results table reposted above, and just compare the 155/80-13's to the 175/70-13's, the nominal height difference is within .4% (0.1" at 22.7" diameter), and both of the 155's beat both of the 175's. By an average of 20' or 11%. Even the junk 155's beat Goodyear 175's (of course, so did the snow tire 175's :-)). At constant height narrower wins for similar design tires. I don't think you can directly compare the compact spares because their construction is substantially different to achieve a similar or greater load rating in a much narrower tire; they may be better or may be worse but you couldn't predict which without testing.

[CapriRacer]

Some thoughts on the chart I provided:

It's from a presentation made by Smithers, an independent tire testing company based in Akron, OH, to the California Energy Commission. Here's the whole presentation:

http://www.energy.ca.gov/transportat...%20Testing.pdf

This was part of the CEC's investigation towards writing a tire regulation, with the idea of mandating that only tires with low rolling resistance were to be sold in California.

It doesn't say in the presentation, but I learned that ALL the tires in the tire size study were Goodyear Integrity's (not sure where I picked that up), but it makes sense in context. By looking at an entire line of virtually identical tires, the CEC could understand what they were faced with in trying to write a regulation.

They also didn't state what test they performed, but in order for the chart to have any meaning, the test had to be the same. Please note, that there are standard RR tests that correlate very well to steady state energy consumption in a tire - and because it is a lab test, the conditions can be closely controlled to eliminate things that a coastdown test would have trouble with - wind conditions, road surface variations, etc.

I think it is obvious from the charts that the task of writing a regulation would be very difficult.

So focusing on that one chart - RRC by tire size - it says that small tires (and here I am using "large" and "small" to mean load carrying capacity) have worse RRC than larger tires.

I ran a regression analysis on the 3 numbers in the tire size and developed a formula that best fits the data in the chart. Basically, the formula says that increasing any of the 3 number in the tire size helps RRC - for short: Bigger is Better.

But the percent change is small - a couple of percentage points for what will fit under a given fenderwell without rubbing.

[gone-ot]

Quoted from the last paragraph on page 39 (red color emphasis, mine):

"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."