Car geek question

[vettecat]

When I got my summer tires put on, the mechanic told me that winter tires cause higher gas consumption. But he couldn't tell me why, aside from thinking it was somehow related to the wider treads. Now I'm curious. Anyone know?

Comments

[michelel72.livejournal.com]

Grip/rolling resistance is probably one factor. I think "winter tires" are designed to be "stickier" -- to "grip the road better" -- which requires the car to expend more energy to push the vehicle forward. My Prius has specially designed Low Rolling Resistance all-weather tires as one of several factors meant to decrease fuel consumption (which is the only reason I know anything about the subject) ....

[lokiz-mom.livejournal.com]

there's more than one type of tire? what's winter? we rarely have that out here.

[vettecat.livejournal.com]

That makes perfect sense. Low-resistance tires probably mean it doesn't corner as well, but most Prius buyers probably don't care about that.

[vettecat.livejournal.com]

:-P
Yeah, winter tires have softer rubber and wider treads, and are designed for better grip when it's slippery (a.k.a. snowing). Some people use all-weather tires instead, but since my car is rear-wheel drive I find that changing the tires really does make a difference.

[c1.livejournal.com]

Several factors:

Number one, as has been touched on is the softer rubber. The aim here is to cause the tyre to deform as much as is possible-- create the largest possible contact patch and in so doing, you're providing the greatest possible amount of friction. Friction more or less equates with traction, and in a slide, wouldn't you like more friction? ;-)

Number two. Most winter tyres have what's called siping-- in the factory, thin slices are made in the treads, crosswise to the direction of travel: typically about half a dozen per nub. As the tyre comes in contact with the ground, these slices spread apart gecko-style, making the apparent size of the tread nub larger without adding any more rubber to the tyre. The associated benefit here is that the siped tread now has "fingers" and so that rubber will better conform to any irregularities in the road surface: again, more gription.

Number three. The treads themselves often feature gaps everywhere. Most summer tyres have enough gaps that water can escape during rainy weather, etc. but you'd rather have a smooth, hard tyre, because that will feature less rolling resistance. Having more gaps increases friction, but while that friction scores points in keeping you from sliding on snow and ice, that same friction docks you points in mileage.

My old Tercel had these wheels that were super narrow (I expect Priuses have something similar), bordering on the automotive equivalent of razor blades. I regularly got mileage in the high 40's in mixed driving. The Matrix has wider wheels, and along with being a heavier car, I'm in the doldrums of low 30's.

[totient]

All of these things are true. But the biggest reason you get worse mileage in the winter is that the air is denser then.

[deguspice.livejournal.com]

I thought winter tires were usually narrower than summer tires (especially if you also change rims).

[dda.livejournal.com]

They usually are because that puts more pressure on the actual tire surface (contact patch).

[(Anonymous)]

Besides that, the ethanol formulations of gasoline that are now the rage are providing less power in general. I've noticed that my car's mileage has been dinged by about 2 MPG since I bought it in '06 (independent of having the kayak on the roof.)

[vettecat.livejournal.com]

See, I knew someone would have the answer. :-) Thanks for the detailed explanation!

[vettecat.livejournal.com]

Really? Why so?

[vettecat.livejournal.com]

Kayak on the roof... OK, this has to be C....

[vettecat.livejournal.com]

I meant I think the treads are wider, not the tires.

[totient]

PV=nRT. When air is colder the molecules move more slowly and so more of them settle to the bottom of the atmosphere instead of being bounced back up to the top. Every degree F it gets colder is 1/500 or so more air your car has to move out of the way; the difference between a 30 degree day and an 80 degree day is a 10% difference in air resistance, and since that's the lion's share of the work your car is doing, a similar change in its mileage.

This effect totally drowns out the ethanol/no ethanol effect, as well as the difference between sticky winter tires and the kind of also sticky tires I rather think you're running in the summer.

[vettecat.livejournal.com]

Fascinating. Thanks for the explanation!