NarrowerTires?

[Frank Lee]

Skinny 165/80R13 on the back (185/75R14 on front) :



Got a decent pair off a Metro and put 'em on Tempo rims. No testing or even impressions yet.

[Swiftbow]

Originally Posted by CapriRacer

Doug,

First, a 155/80R15 83S is 4 load indices lower in load carrying capacity. That means that all other things being equal, you will need to use 4 psi more than what is on the vehicle tire placard for comparison purposes.

According to Tire Guides, a book that summarizes vehicle tire placards, a 2009 Ford Focus with P195/60R15 87T's should have a tire placard that says to use 32 psi. Please check the vehicle placard to see if that is correct.

What this means is that even if you inflate the tires to the maximum, the tire has less load carrying capacity than the original tires. Not only is that going to hurt the fuel economy, but it's a safety issue, too. Less load carrying capacity increases the risk of a load related tire failure, which sometimes has tragic results.

PLUS, going down in speed rating (T to S) is also the wrong way with regard to safety.

While the amount of tread rubber is important, there are 2 other considerations for a tire's contribution to fuel economy: The rubber, and the amount of deflection.

As I pointed out above, the lower load carrying capacity means more deflection and worse fuel economy - all other things being equal.

The type of rubber is very important - and probably the most important thing. Tires designed for good fuel economy (like what came OE on your Focus), are going to use types of rubber compounds that give good fuel economy. That is generally at the expensive of traction (especially wet traction) and tread wear (or both!)

Going narrower also means a smaller footprint, and that has implications for traction.

So while going narrower may seem like a good idea, there's a lot of down sides to this. It has to be done carefully - which why you asked the question, isn't it!

Interesting Capri. How is a load indices calculation performed? The focus is a fairly light car so I was surprised that this tire wouldn't have the load capacity. I'm also surprised that even with the tire filled to max it would give deformity degrading FE. 195 to 155 width is a substantial amount where deformation of that magnitude is surprising. BUT I really don't want to compromise safety, especially in a situation where I could harm someone else in what in the scheme of things ... is just a simple test.

As to the earlier comment on rims, yes, I would have to purchase narrower rims to go down to this width. I was looking at some cheap steel rims with some possible moon covers.

Doug

[Frank Lee]

I rarely have back seat passengers or even a really heavy load in the trunk. The rear of fwd cars typically carries 1/3 the weight. If it doesn't see max loads I don't think safety is compromised.

[CapriRacer]

Originally Posted by Swiftbow

Interesting Capri. How is a load indices calculation performed? ........

Load Index is a quick, and easy way to express load carrying capacity.

There are several formulae for calculating the load carrying of a tire, but they are complex - and of course, it varies accoprding to inflation pressure. Usually load carrying capacity is published in the form of a table.

I explain that in more detail here:

Barry's Tire Tech

Originally Posted by Swiftbow

......The focus is a fairly light car so I was surprised that this tire wouldn't have the load capacity.........

I explain that also in the link, but the short version is "Over-design / Under-utilize" - meaning you should use tires (and other products) at less than their maximum capacity - and the further away from the maximum you are, the safer it is.

Originally Posted by Swiftbow

.....I'm also surprised that even with the tire filled to max it would give deformity degrading FE. 195 to 155 width is a substantial amount where deformation of that magnitude is surprising. BUT I really don't want to compromise safety, especially in a situation where I could harm someone else in what in the scheme of things ... is just a simple test.

...........

Doug

The rolling resistance of a tire is directly proportional to its deflection, so ANY deflection has a consequence on fuel economy. And changing tire sizes changes the deflection. When it comes to fuel economy, going a smaller tire is the wrong direction. Larger tires (meaning larger load carrying capacity) have lower RR for a given load. However, this effect is pretty small and huge improvements should not be expected.

[Frank Lee]

IF loads are light enough to safely use a smaller tire the gain is in the thinner rubber and fewer belts. And as noted if you can air them up enough so that they aren't suffering increased deflection vs original. I doubt going bigger/stronger than stock results in lower r.r.

[CapriRacer]

Originally Posted by Frank Lee

IF loads are light enough to safely use a smaller tire the gain is in the thinner rubber and fewer belts. And as noted if you can air them up enough so that they aren't suffering increased deflection vs original. I doubt going bigger/stronger than stock results in lower r.r.

If you sort through all the data here:

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

you'll find that RRC is slightly better for larger capacity tires. (pg 31)

So going larger than stock is directionally better for fuel economy, and using a tire that can barely carry the load is not the best move to make.

[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!

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

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.

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

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.

[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.

[aerohead]

I caught this thread but am away from home and library.I'll throw out some things,hope they're correct,verify once back home,and be back.
Many of the low-drag high mpg concept cars incorporated narrow,large diameter,elliptical,low aspect ratio,high-pressure radials,with tread designs which had all individual tread blocks,designed for low squirm,and total absence of "quiet-bands" which,while reducing tire noise,do so at the expense of higher drag.These tires were compounded for high-hysteresis and may have traded low-R-R for both dry and wet traction.Some of these tires were also designed for run-flat capability which would allow the automaker to delete the spare tire as a weight-saving technique for better urban mpg.
The FORD Probe-IV had 155 size tires specifically to reduce aerodynamic drag by virtue of their 10% reduced frontal area,compared to conventional radials of that period.
Many reports on the development of particular vehicles will sight the variance in Cd as a function of tire width.There is a definite correlation,there has to be,as drag is an arithmetic function of CdA.
And while I believe modders are more attuned to the safety ramifications of diverging from O.E.M.spec equipment and will attempt to compensate with driving style and technique,the automaker must design for those compelled to have their Darwinian moment.
I consider my own driving to be very conservative and I don't consciously desire to pose a threat to myself or innocents around me,and strive not to be "blinded" by mpg "euphoria"and inadvertantly do something really stupid and dangerous in the name of better mpg.
Always think safety.