gas savings vs. wheel weight
February 3, 2005 1:32 PM   Subscribe

From cycling I'm aware that rotational weight is the best to cut for efficiency, and when I think about my new car and ways to coax more mileage out of it, I wonder if getting a lighter set of wheels would improve that measurably? I can't tell if anyone has ever looked at gas savings vs. wheel weight before in my google searching. [+]

I believe my current stock wheels run about 22lbs each and I've seen some high performance wheels that go down to about 15lbs that fit on the car (as opposed to bling bling wheels that can approach 30lbs each), but they cost hundreds more, per wheel. What I'm wondering is -- can the weight savings offset the cost and can I save more than I spend in say a year of driving 12k miles on 5lb lighter wheels?

Or would the gas savings be much more slight than that?
posted by mathowie to Travel & Transportation (29 answers total)
I'm not a mechanic, but in a year, probably not. Don't know your mpg but 12k miles @ 25 mpg = 480 gallons = ~$1000 on gas (for an overestimated but round figure). So if you spent $500 on four new wheels, you'd have to save $500 on gas, which would mean doubling your mpg. It would ammortize out over years, but I'd doubt you'd get your money back at 12k miles/year, if you spent hundreds per wheel.

These people say that saving 1kg in wheel weight is the same as saving 8kg in body weight. But then, they're trying to sell you wheels.
posted by carter at 1:49 PM on February 3, 2005

Of course, I should have added:

posted by carter at 1:49 PM on February 3, 2005 [1 favorite]

It really makes a difference with bicycles since we're talking .25 horsepower. I'm no great cyclist but I can tell the difference between my Mavic Cosmos wheelset and my 36 spoke bulletproof touring wheelset with my eyes closed. With both bikes and cars, the major use of energy is overcoming wind resistance. My SWAG is that it would take years to break even with a lighter set of wheels. I just think that the potential fuel savings would be really tiny, but I've no data to back it up.
posted by fixedgear at 1:52 PM on February 3, 2005

I guess at the core, if I have a 25mpg car and lose ~2kg per wheel and drive it, will I see gas mileage shoot up to 26, 27, 28mpg or is the difference so slight that it will be unnoticeable.

I often hear people load up their car with heavy 20" chrome wheels and say "I lost a couple mpg, but they look great" so I figured the flipside might be true, that taking away 5lbs per wheel instead of adding it would result in a couple mpg change.
posted by mathowie at 1:55 PM on February 3, 2005

I know neither jack not squat about physics, so I could be completely wrong here, but I've heard that while an object is in motion, it weighs less than it does when it's not. If true, it sounds like this is a more complicated question than initially proposed.

Be gentle with me if I'm wrong :-)
posted by glyphlet at 2:01 PM on February 3, 2005

Here's one comparative test on BMW series 3. Both lighter and heavier wheels slightly reduced mpg (scroll down to table at the bottom). I guess it's a lot to do with profiles and tyres, and maybe the BMW was pretty well set up when it came out of the factory.
posted by carter at 2:06 PM on February 3, 2005

As far as how much you'd save in gas by reducing the wheel weight, this is the only comparable info I could find: "An extra 100 pounds in the trunk reduces a typical car's fuel economy by one to two percent." So it doesn't sound like you'd be saving too much. But there are some good tips on that site for improving fuel economy.
posted by sarahmelah at 2:07 PM on February 3, 2005

Don't other characteristics of a tire--like tread, stiffness, and even inflation pressure--have a pretty dramatic impact on MPG? I would think that however you changed your tires, weight is probably fourth or fifth most important of the things you can change to affect mileage, so it's probably not worrying about. You could easily pick a lighter tire that either had the wrong tread or was too stiff/too soft, and drown out the impact of the weight. And vice versa, pick a tire that's actually heavier, but still give better mileage.

(On preview, what carter basically said, I guess.)
posted by LairBob at 2:22 PM on February 3, 2005

["...not worth worrying about..."]
posted by LairBob at 2:23 PM on February 3, 2005

an easier way to squeeze a few more mpg from your car is to add a few more pounds of air to each tire than the recommended amount.
Be careful tho as the ride will suffer, handling will change and at high speeds, the tires might overheat if they are not speed rated.
posted by Fupped Duck at 2:30 PM on February 3, 2005

If you can get some nice aluminum or magnesium racing wheels for free, then sure, take em. Otherwise, don't bother. They're not going to make a significant difference, and lightweight aluminum and magnesium wheels are easy to damage anyway.

If you want to increase your mileage, concentrate on driving smoothly, and watch further ahead on the road to plan your actions further in advance. Don't drive too quickly (air resistence is a velocity squared function), and use cruise control if you don't have a steady foot. Don't tailgate. If you do these things, you should be able to get an extra 2 or 3 mpg, with a total cost of $0.

Make sure your car is tuned up properly, and has a clean air filter.

I'd recommend against increasing the tire pressure, as meddling with this will adversely affect tire performance and wear. Tire pressure is important though, so buy a tire pressure gauge and check the pressure in the morning, before you drive it. Manufacturer tire pressures are for "cold" tires, so if you drive even a few miles, you'll be a couple PSI off.
posted by mosch at 2:50 PM on February 3, 2005

The big advantage to lighter wheels on a car is that you reduce the unsprung weight, which can make a big difference in how well the car handles. Your wheels need to track the road -- ideally, they'd have no inertia, so the springs could hold them against the road. They do have inertia, so we have shocks, which keep the wheels from just bouncing against the springs. (A better name that "shock absorber" is "damper".)

When the shocks are worn out, you can have a wheel hit a bump, and then bounce several times. When a tire isn't touching the road, you have zero traction. This is why shocks are so important.

Cars with less unsprung weight are less affected by weaker shocks, since there's less mass (thus, less inertia) put into motion by the bump. Racing cars, esp ones that do more than turn left, put a premium on reducing unsprung weight, since losing contact with the road -- even with one wheel -- at speed will almost certainly result in the car leaving the track.

The other fact is that on most car wheels, the rim weight doesn't change much -- most of the savings is on the center of the wheel. On a bike, almost all of the wheel weight is in the tire and rim, which is why doing something as simple as changing to kevlar bead slicks can make such a huge difference in your acceleration. Note that your top speed won't increase as much on a bike, since wind resistance is such a huge factor in how fast you can go. (It's why almost anyone can ride at 15mph, but damn few can sustain 25mph for any length of time -- no matter how light the bike is.)
posted by eriko at 2:57 PM on February 3, 2005

In addition to higher pressure, you might want to look into low rolling-resistance tires. They're standard equipment on hybrids and other vehicles that use fuel mileage as a selling point (my Civic HX, for example).
posted by letitrain at 3:00 PM on February 3, 2005

Lightweight wheels should noticeably improve acceleration, because there is less mass to spin up. A lot of race cars take advantage of this idea by lightening everything in the drivetrain, like the flywheel, clutch, and rotating assembly in the motor. So with light wheels, the car should feel a little peppier when you accelerate.

However, once you're rolling on the highway, the difference is probably negligible. You are mainly fighting wind resistance at speeds over 60mph, so the concept of rotational inertia is not really the issue. If you managed to remove a few hundred pounds from the car, I bet you'd get better gas mileage. Or if you could magically make your car more aerodynamic. But I don't think losing 30lb of weight from the wheels will make a noticeable improvement.
posted by knave at 3:04 PM on February 3, 2005

If you're looking at this from a cost point of view, I think you'd be unlikely to outweigh the quite high cost of lightweight wheels with a few % savings in MPG. The lightest wheels are generally geared for performance, and as such they tend to be expensive and of exotic construction. Depending on the diameter of the wheel, you'll start spending $300+ per wheel if you want to be more than a five or so pounds lighter. Plus mounting, balancing, etc.

If you're really serious about this, and you could fit them over your brakes, you might look into smaller diameter wheels. I'm pretty (but not entirely) sure that the further you move the weight from the axis of rotation, the bigger the force required to move it. So larger wheels, in addition to requiring more material in general, have their bulk that much further out from the hub and take more effort to move. If your car has 17" wheels, for instance, and other versions of the same model came with 16" wheels, it may be possible to buy a light set of 16"ers and see some more benefit at lower cost. Smaller wheels are much cheaper as well.

Another thing most people don't think about is that tires can be really heavy too. They also sit that weight furthest out from the hub. They're also (generally) less expensive than wheels, and you have to change them periodically anyway. The downside is that it's pretty tough to get reliable information on how much different kinds of tires weigh. It's not uncommon for people to buy lighter wheels only to find that the new tires they bought for the wheels are heavier enough to more than make up the difference.

The biggest practical difference you can make in everyday MPG is driving conservatively and making sure your tires are properly inflated. Underinflated tires, which is what the majority of people drive around on, will significantly hurt MPG, as well as performance. Weight in the car makes less of a difference, but it can't hurt to clean out any crap in your backseat/trunk either.
posted by tirade at 3:15 PM on February 3, 2005

I've also sometimes wondered how much of a MPG difference could be made by increasing the overall diameter of the tires (not just the wheels) beyond the stock size. Changing the diameter of the tires directly affects the gearing of the car, so that with larger tires you'd weaken acceleration, but also lower highway cruising RPMs in top gear. Of course, you'd also throw off your spedometer/odometer a bit.

Anyone think that might work?
posted by tirade at 3:22 PM on February 3, 2005

I'm pretty sure I once heard Click and Clack say that using larger tires (i.e., bigger radius) would provide a small savings in fuel at the cost of a small loss in performance. I have no idea how you put a tire of larger radius on your car, but they seemed to be saying that it's possible. Of course, they could have been joking and I wouldn't know any better, but they sounded serious.
posted by anapestic at 3:39 PM on February 3, 2005

Yes, I think that would work. Back when I was in college, I blew out a tire and could only afford to buy two used tires that happened to be a little larger than standard. The mechanic told me that putting the larger tires on the front would improve my gas mileage slightly, but also throw off my odometer by a few mph.

Another upside was that it improved my clearance. My current vehicle rides a little low to the ground. Now that I'm grown up, I can afford four new tires at a time, so once my current treads wear down I plan to replace all four with larger versions.
posted by Jonasio at 3:40 PM on February 3, 2005

I third the whole larger radius tires having a benefit, something to do with the RPMs. I had a friend who swore by it. I don't know how much he actually saved in mileage though.

A word of caution, make sure the tires aren't too big for the wheel well. It seems obvious enough, but when the wheel is turned all the way to the left or right even an inch too much can cause it to scrape against the well.
posted by sarahmelah at 3:50 PM on February 3, 2005

Um, Tirade - smaller wheels generally take smaller diameter tires. If smaller wheels gave better mileage (which they wouldn't), then larger wheels would do the opposite.

The mechanic was right in his comment to Jonasio. Smaller tires (it's the tire OD that determines the final effective gear ratio) give better acceleration for a given power input, but reduce overall mileage, because the engine has to spin faster to produce a given mph, and has to turn more revolutions per mile traveled.

You could get smaller wheels and put on tires that have a higher aspect ratio (say, R75s instead of R60s), but the taller tires would be heavier, so I'd guess the net weight savings would be near zero. And there's no telling what adverse affect the change in tires or wheels would have on steering geometry. In short, I wouldn't do it.

You'd have to significantly improve your gas mileage over a long period to even approach paying for $1000 worth of wheels. Tirades suggestions about smooth driving, etc. really are the best way to save gas.
posted by Kirth Gerson at 3:59 PM on February 3, 2005

Um, Tirade - smaller wheels generally take smaller diameter tires. If smaller wheels gave better mileage (which they wouldn't), then larger wheels would do the opposite.

Not necessarily, since we're talking about different effects. First of all, smaller wheels do not necessarily mean smaller overall tire diameter. Most people, when they change the size of their wheels, get a tire with a different sidewall height. So if you were going from 16" to 17" wheels, you'd get a tire with a shorter sidewall so the overall diameter of the wheel+tire would stay the same.

Second, I only suggested smaller wheels would increase MPG because of less weight/resistance and equal diamater. Smaller diameter would shorten the gear ratios and thus increase acceleration, increase cruising RPMs, and thus decrease cruising MPG.

The reason large tires would work, as I suggested before, is because they effect the gearing of the car - the number of times the engine must turn compared to the number of times the transmission, the driveshaft, and ultimately the tires turn. Increase the diameter and you increase (or is it decrease?) the gear ratio. Bigger tire = fewer tire revolutions per engine revolutions.

The reason cars designed for high MPG don't come with comparitively larger tires, among other practical reasons, is that the manufacturer can set the gear ratios to whever they want when they're designing the transmission in the first place. But a car owner having custom gears designed would obviously be cost-prohibitive for something like this.
posted by tirade at 4:32 PM on February 3, 2005

To accelerate the car, you would have to start the wheels rolling, or produce a change in the state of rotation of the wheels.

Resistance of a solid body to a change in rotation is the moment of inertia. For a solid disk (such as a tire/wheel), this moment is given as I = mr**2, where m is the mass and r is the radius and I'm taking a huge liberty with the distribution of mass. Angular momentum is the product of the moment of inertia and the angular velocity (L = Iw). I'm assuming that angular momentum is what you mean by rotational weight.

A change in mass produces a change in I that is directly proportional. A change in the radius, however, produces a change proportional to the square of the radius. Smaller tires with the same mass would reduce I much more than lighter tires with the same radius. At first glance, it looks like reducing the radius of the tire/wheel is the best bet.

However, a smaller tire means it has to rotate faster to produce the same translational velocity as a larger tire (circumference being a function of radius and all that). The faster rotation means that the angular momentum increases. And not just the tire either -- there are axles and crankshafts and other things connected to that wheel that are all rotating with their own moments of inertia and angular momentums. Reducing the radius of the wheel produces an decrease in its rotational weight, but it will increase the rotational weight of other components in the car.

With a bicycle's lighter weight and simpler mechanisms (and different distribution of tire weight as eriko noted), changing the tires would have a much more noticeable effect than doing it on a car.
posted by joaquim at 4:37 PM on February 3, 2005

How about this:

It's not worth it, but do it anyway.

It's obvious that light wheels, particularly those that also have low rolling resistance, do boost milage. (I think letitrain had the best link.) It's unlikely that they will pay for themselves at the petrol pump, but they will reduce the amount of petrol you consume and surely that's a good thing.
posted by krisjohn at 4:39 PM on February 3, 2005

I've heard that while an object is in motion, it weighs less than it does when it's not.

Actually, it weighs more. But not by enough to affect your gas mileage unless you're driving close to the speed of light, which I assume is not the case for most people here.

As has been said here, and as I've heard elsewhere, lighter wheels can make a noticable difference in acceleration. Takes less work to get them rolling. So the difference in efficiency should be measurable if you're speeding up a lot. Driving at a constant speed, I guess it wouldn't make much difference. Maybe a little, since there would be less inertia for the suspension to absorb when you hit a pothole.

Anyway, there must be some reason why all those cars that aim for fuel efficiency have those tiny little 14" wheels.
posted by sfenders at 7:03 PM on February 3, 2005

It's not worth it, and don't bother. Changing your driving style to be more efficient instead will save more and cost you nothing.
posted by pmurray63 at 7:04 PM on February 3, 2005

Changing your wheel diameter has a serious effect on the computer which programs transmission shifting and throttle opening/fuel injector settings. The auto manufacturers set these up with the OEM wheels to deliver the best possible mileage, since in the US there is a significant penalty paid for high corporate "fleet" mileage.

The algorithms used to govern shifting speeds, loads, injector settings, timing, etc. are tweaked to fractions of a percent on a dynamometer and in wind tunnels by modern manufacturers with a view to maximizing fuel economy while delivering acceptable "performance" (acceleration and cornering).

It used to be true that putting larger wheels lowered the rpm and thereby increased gas mileage. That was in the days of carburetors. The reason was that the wider opening of the throttle plates gave rise to better mixing of fuel and air. With modern fuel injection, this is no longer a factor.

As eriko and others have pointed out, lower unsprung weight keeps the tires from bouncing needlessly. Quite apart from the traction/steering effects, a bouncing wheel consumes energy by converting the up and down motion into heat. That energy ultimately must come from the fuel. However, a lighter rim with its lower unsprung weight will transfer less bouncing energy to the rest of the car, so that over a rough road, the ride will be smoother.

Finally, a word about tire pressure. The recommended pressure is a compromise which has also been worked out by much testing and tweaking. OEM supplied tires also take into account vehicle spring stiffness, camber change upon deflection and other effects. If you overinflate the tires to reduce the rolling resistance, you will narrow the contact patch of the tire with the ground, thereby causing only the center portion of the tire to wear. In addition, the effectively "narrower" tire provides less steering and traction forces. Finally, the spring rate of the tire/shock/spring train will now be different than what was originally designed and your vehicle will ride and handle poorly.

Should you put too little air in the tires, the center or crown of the tread will bow inward and no longer contact the ground properly. This again will reduce traction and steering, but it will cause the outer edges of the tire to wear rapidly. It will also cause the sidewalls to flex more than they were designed to, resulting in overheating and premature failure.
posted by RMALCOLM at 8:43 PM on February 3, 2005

Wheel dimension is a part of an overall design of the car. And nowdays people strive to design cars to be as fuel efficient as possible, so second-guessing the engineers is probably not going to get you very far. Sometimes people go for things like looks or higher ground clearance (like me, in case of the latter), but almost always it results in compromising other performance aspects. So if it is fuel efficency that you are after, its probably best to keep the factory wheels. Also, what pmurray63 said.

On preview: RMALCOLM said it much better.
posted by c13 at 8:51 PM on February 3, 2005

As has been said here, and as I've heard elsewhere, lighter wheels can make a noticable difference in acceleration. Takes less work to get them rolling.

It does take less work: the kinetic energy in a circle rolling about its circumference is mv2. This is exactly twice the energy of a non-rotating mass moving at the same velocity, ½mv2.

So removing 5kg from your tires (if you could somehow remove the mass entirely from the part of the tire which is actually in contact in the road; removing it from the interior of the tire has less effect), would be the same as removing 10kg from the non-rotation portion of your car.

Important: this does not take into account the friction between the tires and the road. The "noticeable difference" that people observe in real-life is almost certainly due to the lower rolling resistance of the high-performance tires, not due to the lower weight alone (which, even when you take into account that the reduction in mass is effectively doubled, when compared to the same reduction in a non-rotating part of the car, is negligible compared to the total mass of the car). For the "low resistance, low weight" tires, it's the low resistance that's important, not the low weight.
posted by DevilsAdvocate at 9:38 AM on February 4, 2005

Okay, so depending on what you're starting with, you could save up to around 5kg per wheel. So that would be equivalent to 40kg of weight reduction: enough to be easily measurable. But 2kg per wheel, that'd be equivalent to 16kg of net weight, maybe 1% of the mass of a light-weight car.

As well as straight-line acceleration, it would help whenever the car is turning. Add in whatever gain there is from the suspension bouncing around a bit less, and maybe you could save something like 2% of the energy used to do things other than overcome the various kinds of resistance. All that remains is to figure out how much of the total that amounts to. It would vary widely with the type of driving, but I would guess it's less than 30% even for city driving.

If all that guessing is anywhere close, your 25mpg car might improve by something like 0.1mpg per kg of weight lost from the wheels.
posted by sfenders at 10:20 AM on February 4, 2005

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