Wheel and tyre confusion

[dmt257]

In a nutshell:

Honda Insight with 185/55/16 stated as 61.4 UK mpg.

Same car with 175/65/15 stated as 64.2 UK mpg.

I currently have the 16'' rims. Should I go for the 175/65/15 wheels or wider as I keep reading they will improve economy more?

Would the difference in rim and tyre size account for all of that FE difference? I assume there wouldn't need to be any changes to the CVT gearbox for example? The cars equipment spec is almost identical aside from a few items which wouldn't affect economy (wheel size excluded).

2.8 mpg sounds awesome

[redneck]

.

You may find this helpful.

http://ecomodder.com/forum/showthrea...ing-30307.html



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[Frank Lee]

I don't know, but now I can't get "Mother and Child Reunion" outta my head.

[mcrews]

tire diameter is?????

[Cobb]

I have an insight and went up a tire size.

175/65r15 to 185/65r15. 2% increase in size, 2% increase in mpg.

[dmt257]

Hmm. So where on earth is this extra 2.8 mpg coming from when the car moves from 185/55/16 to 175/65/15? The weight difference is surely minimal.

I'm considering a set of used lightweight racing wheels and 195 tyres too.

Why would Honda not put wider tyres on them from the factory?

[RedDevil]

As the tire is a bit narrower it would have slightly less aerodynamic resistance and less weight.

But most effect will come from the sidewall height.
Every tire flexes where it hist the road, and the higher the sidewall is, the easier it can accomodate the flexing.
Bigger rims mean you need a smaller sidewall on the tire to keep the same circumference. Big diameter rims mean small sidewalls, so high flexing and rolling resistance.

I run 40+ PSI in my tires; the sidewalls hardly bulge, much less than at 34 PSI or whatever Honda recommends. It is also less sensitive to sidewind and corners better.

Just changed my Turanza 185/55/16 summer tires on alloys for WinterContact 175/65/15 winter tires on steelies; run both at the same pressure, hardly any difference between them in economy and handling, except when it rains or snows

PS take a look at:
http://ecomodder.com/forum/showthrea...els-23509.html

[CapriRacer]

Originally Posted by dmt257

.......Honda Insight with 185/55/16 stated as 61.4 UK mpg.

Same car with 175/65/15 stated as 64.2 UK mpg.......

I'll bet there is more than just a tire and wheel change going on. Typically, these sorts of changes have other optional things that add weight.

Originally Posted by dmt257

.......I currently have the 16'' rims. Should I go for the 175/65/15 wheels or wider as I keep reading they will improve economy more?.....

There is more than just wider in this change. There is also larger wheel diameter (directionally better), and a lower aspect ratio (directionally worse), but the big difference is the tire itself. To my mind, THAT ought to be the deciding factor.

[cosmick]

Originally Posted by RedDevil

Bigger rims mean you need a smaller sidewall on the tire to keep the same circumference. Big diameter rims mean small sidewalls, so high flexing and rolling resistance.

False. Less sidewall to flex guarantees less flex.
Also, If you need say, 24 inches of total tire outside diamerer, it's much lighter to get it it from an 18" wheel and short tires than from a 13" wheel and tall tires. That's been proven enough times. Big wheels came for big brakes, and there's no such thing as too much brakes, but this is a nice side benefit. Go big or go home.

[RedDevil]

Originally Posted by cosmick

False. Less sidewall to flex guarantees less flex.
Also, If you need say, 24 inches of total tire outside diamerer, it's much lighter to get it it from an 18" wheel and short tires than from a 13" wheel and tall tires. That's been proven enough times. Big wheels came for big brakes, and there's no such thing as too much brakes, but this is a nice side benefit. Go big or go home.

False?
The sidewall does not hold the tire up, as you seem to suppose. Tire pressure does.
The tire will flex no matter whether the sidewall is high or low; just enough until the contact patch is large enough to compensate the extra force on the wheel.

In a high sidewall it has ample room to flex so it does not cause much friction; the bend radius is large.
In a low sidewall flexing the same distance means the sidewall has to bend sharply, which stresses the material highly and causes a lot of friction.

Originally Posted by cosmick

it's much lighter to get it it from an 18" wheel and short tires than from a 13" wheel and tall tires. That's been proven enough times. Big wheels came for big brakes, and there's no such thing as too much brakes, but this is a nice side benefit. Go big or go home.

Yeah, as if a 18" rim is just as light as a 13" rim. I challenge you to show your proof. What about the go home thing, is that aimed at me?

I have a strong example of the opposite position; check out the Prius link in my previous post.