Snow tires and gas mileage
December 23, 2018 1:45 PM Subscribe
Wondering why snow tires reduce gas mileage if driven on the same surface as summer tires. I assumed this had to do with friction but was told that I "don't know how tires work." Any help? Thanks.
Another thing related to winter mileage, though not tires---do you run the car to defrost the windows while you scrape, or to let it get warm inside? That can really add up too.
posted by kate4914 at 2:24 PM on December 23, 2018
posted by kate4914 at 2:24 PM on December 23, 2018
Just wanted to add: I know this isn't directly related to your question, but did the person who told you that you "don't know how tires work" bother to share their wisdom with you? Because I now have to wonder if they know how tires work.
Seriously, though- if they did give you an explanation, what was it? Are they claiming that there is some kind of real debate over the relationship of winter tire usage and gas mileage? Or that is has nothing to do with the tread and perhaps more to do with the tire weight? Might help us to better understand how much information you're looking for / how detailed you need the responses to be.
posted by nightrecordings at 2:33 PM on December 23, 2018 [2 favorites]
Seriously, though- if they did give you an explanation, what was it? Are they claiming that there is some kind of real debate over the relationship of winter tire usage and gas mileage? Or that is has nothing to do with the tread and perhaps more to do with the tire weight? Might help us to better understand how much information you're looking for / how detailed you need the responses to be.
posted by nightrecordings at 2:33 PM on December 23, 2018 [2 favorites]
Response by poster: Thanks for the responses! The explanation the other person offered is the following:
Snow tires have increased friction on snowy surfaces, which increases mpg because you have less slippage. They have lower mpg overall because most have high 'rolling resistance' which is the energy needed to keep the tire spinning. This loses energy from the deformation of the tire as it spins. Not all the energy used to deform the tire from the weight of the car is recovered when it reforms after spinning.
posted by essenescence at 2:58 PM on December 23, 2018
Snow tires have increased friction on snowy surfaces, which increases mpg because you have less slippage. They have lower mpg overall because most have high 'rolling resistance' which is the energy needed to keep the tire spinning. This loses energy from the deformation of the tire as it spins. Not all the energy used to deform the tire from the weight of the car is recovered when it reforms after spinning.
posted by essenescence at 2:58 PM on December 23, 2018
That's a fair explanation. Rolling resistance is not technically a friction loss, because your tires aren't sliding along the road.
posted by clawsoon at 3:02 PM on December 23, 2018
posted by clawsoon at 3:02 PM on December 23, 2018
Yes, you are right. It is friction, but with a qualifier. There is some friction between your tire and the road. You can hear this friction sometimes in a parking garage if you turn your wheel sharply and you hear a screech from the tires. That is external friction.
But most of the friction in your tires is internal. You can see when parked that the tires bulge out on the sides at the bottom. At the tire rotates the sidewalls have to bulge and deform at very high speed. This requires energy to deform the rubber. Some of this energy you get back when the tire un-deforms as it turns, just like a rubber band snapping back returns the energy you used to stretch it. But you don't get back all of the energy. Some is converted to heat in the rubber. That is why tires get hotter after driving for a while. It is the wiggling back and forth of the rubber, mostly the sidewalls, that causes most of the friction losses in your tires.
Since winter tires tend to be more flexible and have bigger rubber lugs for traction, then have more wiggling friction. They also make so-called Eco-tires, that are specially designed to reduce friction. This is done by changing the rubber formula, the tread design and the sidewalls. But it all comes down to friction, most of which is internal bending.
posted by JackFlash at 3:04 PM on December 23, 2018 [4 favorites]
But most of the friction in your tires is internal. You can see when parked that the tires bulge out on the sides at the bottom. At the tire rotates the sidewalls have to bulge and deform at very high speed. This requires energy to deform the rubber. Some of this energy you get back when the tire un-deforms as it turns, just like a rubber band snapping back returns the energy you used to stretch it. But you don't get back all of the energy. Some is converted to heat in the rubber. That is why tires get hotter after driving for a while. It is the wiggling back and forth of the rubber, mostly the sidewalls, that causes most of the friction losses in your tires.
Since winter tires tend to be more flexible and have bigger rubber lugs for traction, then have more wiggling friction. They also make so-called Eco-tires, that are specially designed to reduce friction. This is done by changing the rubber formula, the tread design and the sidewalls. But it all comes down to friction, most of which is internal bending.
posted by JackFlash at 3:04 PM on December 23, 2018 [4 favorites]
I used to think that my snow tires were reducing my mpg but then it turned out it was the fact that winter gas where I am has ethanol in it and THAT was reducing my mpg, it just showed up at almost the same time as I got my snows on. Don't know if this affects your discussion with the other person, but it was news to me.
posted by jessamyn at 8:19 PM on December 23, 2018 [3 favorites]
posted by jessamyn at 8:19 PM on December 23, 2018 [3 favorites]
The comments above by JackFlash and others are spot on. I'd just add that one component in the rolling resistance is the depth of tread. Deeper tread takes more energy to deform than does shallow tread.
Tires with worn down tread have the least rolling resistance and thus are most "fuel efficient". Some trailer truck tires are now being designed with purposefully thinner treads to increase the MPG. Yes, that does make them more vulnerable to injury or puncture, but it does add up to lots of fuel savings over the long haul (pun intended).
posted by tronec at 3:29 AM on December 24, 2018
Tires with worn down tread have the least rolling resistance and thus are most "fuel efficient". Some trailer truck tires are now being designed with purposefully thinner treads to increase the MPG. Yes, that does make them more vulnerable to injury or puncture, but it does add up to lots of fuel savings over the long haul (pun intended).
posted by tronec at 3:29 AM on December 24, 2018
This thread is closed to new comments.
Likewise, tires with low air create this same issue.
posted by nightrecordings at 2:18 PM on December 23, 2018 [4 favorites]