Quote:
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.......
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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.