On 7/19/2017 2:07 PM, Mad Roger wrote:
On Wed, 19 Jul 2017 11:22:48 -0700 (PDT),
trader_4 wrote:

That's crazy! Where on earth did that come from. 1" diameter increase
would only cause that change if you started with 4" O.D. tires.

Yeah, that number seems suspect to me too.

I made a mistake in quoting the loss of torque due to a one inch increase
in diameter. The 25% loss in torque was due to four inches in diameter.

That means the loss in torque at the contact patch is less than that for
one inch increase in diameter.

It's still a loss in torque though, which itself isn't doing the fuel
mileage any favors. The reliable reports (Cooper Tire for example) said
that the torque loss is the biggest factor where they said "most cars"
would see a negative impact from tires (unless they had torque to spare).

Here's a summary of the Car and Driver article by someone not me.
http://www.dunntire.com/blog/Does-ti...ct-gas-mileage
"What¢s immediately apparent from the results is that as the wheel-and-tire
packages get larger and heavier, acceleration and fuel economy suffer.
Specifically, [Car and Driver] cited a '10-percent drop in fuel economy'
from the 15s to the 19s."

And I don't believe less torque translates into lower fuel economy either.
Yes, it will accelerate slower with a larger diameter wheel, but that
just means you have less torque applied over a longer period. It's
not torque, it's energy that you need to look at.

Nobody can find a reliable reference which says anything better for larger
diameters other than saying that "most cars" will not find any increase in
gas mileage but that the torque curve is everything so "some cars" may get
better gas mileage if they have torque to spare.

Almost all reliable sources said there are many factors but the one factor
of torque loss is in all the reliable sources.


The torque loss is defined by the physics of that change. The mileage
resulting is determined by the characteristics of the vehicle, engine
and gearing.