Prius 15's are better than aftermarket 17's! (Wrong wheels/tires can wreck your MPG)

[TexasCotton]

Originally Posted by CapriRacer

I think the "wider is better" works regardless. Look at it this way,

If RR is energy loss - and the energy is lost in form of heat by the internal friction of the tire and the friction with the road surface plays a minimal role - then most of the heat is generated within the tread rubber, because it is both the largest mass and because it is deflected the most. Sidewalls have a much smaller mass and depending on where on the sidewall you look, they are defected as much as the tread.

That means the ply materials, such as polyester and steel don't have a very big impact on RR so replacing them doesn't result in much difference. But changing the tread rubber does.

Also, the amount of deflection is controlled by both the inflation pressure and the load. So going to higher inflation pressures gives you better RR - and going to a lighter load gives you better RR.

In fact, RRC (Rolling Resistance Coefficient), where RRC times the load on the tire = RRF (Rolling Resistance Force), is fairly linear and not dependent on how close to the rated load you are. You could be above it or below it and the RRC is still fairly linear - and for a given inflation pressure, the same value.

While as a tire engineer, I am happy that is so - keeping people safe should be my first priority - that doesn't change the laws of physics. It's just the way it works.

Capri
I always try to embrace all opinions.

However

Orig poster went from 15 to 17 to decrease mpg
Do you own and familiar with that particular vehicle
really not seeing the basis in experience that going
from 185/65R-15 to 175/65-15 would decrease RR,MPG, and performance.
What I think may happen is one of two which is changes nothing or possible improvement.........
I also have found a tire dealer who would mount the tire.......

[TexasCotton]

Originally Posted by RedDevil

Wyatt did mention it in his second post:

If the rims were an inch wider, so were the tires. That does have an impact of economy, but not enough to be solely responsible for the difference in FE. IMHO.

IMHO
I am going to try the 175/65R-15 and take off OEM 185/65R-15.
I value all opinions. However first hand knowledge is always a better indicator than others basis/speculation/ and opinion

[TexasCotton]

BTW
The change from 185mm to 175mm is less than 1/2 inch ...........not a dramatic change.....

[rmay635703]

Originally Posted by TexasCotton

IMHO
I am going to try the 175/65R-15 and take off OEM 185/65R-15.
I value all opinions. However first hand knowledge is always a better indicator than others basis/speculation/ and opinion

On my cobalt I went from 195/65's to 185/65s, my revs per mile are almost negligible (apparently the cheap 195's rolled almost as much a s the 185) (aka well under 5%) odometer is off under 2 miles per every hundred run

My fuel economy went up relatively dramatically but weather and heat have more to do with it.

Also the goodyear "energy saver" tires included with the car stock really weren't energy savers at all might also affect it.

I have a feeling if you could find narrow tires that were taller AND energy savers (good luck) you would probably gain FE.

AKA stock 195/60's to 185/65's for example, in my case the cheapness of the brand/type of tire made the difference end up being less than expected in terms of ODO error.

Now if one could find energy saver 165/80s then you gain aero while maintaining most of the load capacity and the same odo/gearing

But alas nobody makes 80 or 85 form factors anymore for decent new tires. just one off antiques.

Something I often wondered was if one where to downsize rims, but have taller tires what affect that might have since the load capacity would go up AKA 195/60r15s to 195/85r14 for example.

Ah well.

[Frank Lee]

Ahhh, there's a wider tire. Bet it rolls easier.


No? Too much load capacity?

[rmay635703]

Originally Posted by Frank Lee

Ahhh, there's a wider tire. Bet it rolls easier.

No? Too much load capacity?

http://forum.ecomodder.com/images/smilies/tongue.gif
yep the unwritten rule that capri and much of the wider folks gloss over is that "MASS" can have rather dramatic unwanted affects that would make any RR change small & meaningless.

You need the right tool for the job, if a narrow tire supports the weight correctly, its probably the right tire for the application.

[CapriRacer]

Originally Posted by TexasCotton

Capri
I always try to embrace all opinions.

However

Orig poster went from 15 to 17 to decrease mpg
Do you own and familiar with that particular vehicle
really not seeing the basis in experience that going
from 185/65R-15 to 175/65-15 would decrease RR,MPG, and performance.
What I think may happen is one of two which is changes nothing or possible improvement.........
I also have found a tire dealer who would mount the tire.......

First, my expertise is in tires - and of that I have a ton in that area.

I am basing the change in tire size on the information contained in the web page:

Barry's Tire Tech

Please note the 5th chart - the one marked "Rolling Resistance Coefficient". RRC is the rolling resistance of the tire divided by its test load. You multiply that by the actual load on the tire to get the tire's actual Rolling Resistance Force. It is RRC should be used for comparison of different tire sizes.

Why not the values from the 4th slide? - the one marked "Rolling Resistance Force"? Because the test loads are different - and not the actual load your vehicle puts on the tires. The load on the tires is the same for any particular vehicle regardless of what tire size is applied.

If you look at the 5th slide, you should notice the small tires (the combination of width, aspect ratio, and rim diameter) have the worst values, while the large tires have the best.

Yes, this hardly covers ALL the tire sizes available, but it does illustrate a point.

You should also notice that earlier slides on the page point to HUGE ranges of RR for a given size. This means that tire selection is more important than size.

It also means that folks who go from one tire size to another and report changes in fuel economy aren't showing the entire picture. It is important to remember that different tires can have HUGE differences in RR - enough to completely overwhelm any changes in tire size.

[rmay635703]

Originally Posted by CapriRacer

It also means that folks who go from one tire size to another and report changes in fuel economy aren't showing the entire picture. It is important to remember that different tires can have HUGE differences in RR - enough to completely overwhelm any changes in tire size.

A+

I went to the Bridgestone Ecopias from OEM Goodyear Integrities, rather big RR change.

In my case I saved $25 per tire going to 185 instead of 195, and my load rating was more than sufficient.

I think "small" changes in width & load cap coupled with a better RR tire are where the gains are to be had.

Increasing width significantly in my experience leads to more troubles unless you are overloading a pickup or suburban and need the weight rating as those tires don't seem to "roll" very well as they grow.

We really need the euro RR ratings to be posted on all our tires, would make it much easier to get both the weight rating and RR we want.