Lightweight wheels improve both traction and ride.

The fundamental of lightweight wheels is that they have less force when bouncing up from the road. Then the shock controls the wheel easier and faster and the tire stays in contact with the road for longer periods of time. Also the lightweight wheel puts less force into the chassis.

Aftermarket alloy wheels are usually aluminum, as are most current OEM wheels, but aftermarket wheels are often slimmer and trimmer than OEM wheels. Since OEM wheels are thicker and more broad-faced then they can use less expensive alloys and less expensive manufacturing processes.

The best alloy wheels are forged but this forging is often a high-pressure hot-forging. The second best alloy wheels are flow-formed cast wheels. There is a classic three-piece wheel that is made in a machine shop and not forged or cast either one.

Many premium aftermarket wheels are 6061 aluminum which machines very well and is okay for welding. There is one brand of forged aftermarket wheel which is suspected of being an aluminum-silicon alloy because they are very lightweight but thicker and more broad-faced than other forged wheels.

Now aluminum-silicon alloys are becoming competitive with carbon-fiber for reducing aircraft weight and so aluminum-silicon alloys are a rising fundamental for crafts and vehicles.

Wheel price is low

If you buy for fuel economy, then scuffed wheels from a scrapyard search service or classified ads are OK.

"$600 for a set of alloy wheels. Given the costs I saw at Tiretown ... prices were over that per wheel, let alone per set"

Ebay here in the UK has a good way of narrowing-down a quick search. I found one seller charging the same amount for old alloys as people charge for new plastic covers - fifteen UK pounds x 4 reduced from twenty-five x 4. I don't know how much I'd pay on a scrapyard search service or to someone with an add saying "breaking XXX vehicle close to you" but it would be the same kind of amount I suppose. Partshark, a scrapper search site, has one add for my type of car with a price of "please call" and courier fees to pay on top. Anyway it could be way-under the $600 per wheel break-even point quoted further up the thread.

If you enjoy economy or protecting the environment, you might not factor-in the time spent learning what wheels fit your car, buying wheels, and reselling if you get it wrong, but then there's the cost of changing the wheels for real with blown-up tyres on them. That's either an unfamiliar, time-consuming job with difficulties in wheel-balancing or a job you pay other people to do. There are web sites about cheap tyres that note the price of tyre fitting - often mobile tyre fitting - which is about the same per wheel as the wheels themselves. I guess that tyre fitters double the price if you want them to change a wheel as well.

Here's a suggestion. If anyone here runs a scrapyard parts ordering service, why not add a category for ugly scuffed alloy wheels for sale to misers?

Or if anyone runs a mobile tyre fittings service why not add an "scuffed old alloy wheels if available" option to the price list?

Theory

The calculation in the other answer is rather naive. I haven't done the calculation myself, but there are a few things to consider. First of all, lighter wheels mean more than just lower overall vehicle weight. Advantages are:

• lower overall vehicle weight: lower inertia of the car
• lower weight of the wheel (rotating part): lower rotational inertia
• (probably) different aerodynamic behavior
• lower unsprung weight

Weight distribution in the wheel also makes a huge difference. If the weight is more to the center, there's even less rotational inertia. Therefore, smaller diameter wheels are better than large diameter wheels! Clearly then, saving 10 kg in weight on wheels has a much higher impact on fuel economy than saving 10 kg by removing the backseat or something. A (itself quite naive) calculation suggests a factor of 1.5 difference, based purely on the rotational inertia.

You're especially going to notice the difference when accelerating and decelerating a lot (because that's when weight matters), so in city driving your savings will be higher than in constant high-speed highway driving (where drag matters). It's also worth to note that there will be less wear on break pads, discs and tires. Which is, again, most pronounced in city driving.

Practice

A test was conducted on a racetrack with 3 different wheel configurations, documented here. They also did a road test to measure fuel economy, but the main goal was better lap times. Obviously, the lighter wheels yielded better lap times. Unfortunately though, the road test actually registered worse fuel economy for the lightweight wheels. There's a few things that explain this result, having to do with the fact that this test aimed to optimize the car for lap times, not fuel economy. Those are the wheel size (the lightweight wheel had a larger diameter) and different tires (the stock tires were optimized for low rolling resistance). The stock wheels were already light as well, 20.5 pounds, while the lightweight wheels weighed 17 pounds, so the difference wasn't that major. There may also have been a difference in driving style. I wonder what the results would be with identical wheel size, the same tires and controlled driving conditions.

However, it's still interesting to note that the differences in fuel economy were quite measurable, about 2%. It's hard to determine what that would mean for real-life driving situations, but I could imagine a set of carefully chosen wheels saving a few tens of bucks on a yearly basis. That'd still mean you'd have to drive the car for at least a couple of years to pay for the wheels (assuming you receive the same amount when selling the car), but the lifespan of most cars probably makes the investment worthwhile.

NB: I'm not thinking about an expensive set of wheels here, I bought a set of less than €300 for my wife's compact car, with the same diameter as the original set. They obviously aren't magnesium wheels though, those are way more expensive.

A quick look doesn't show a clearcut weight difference. The claim is more for a 'rounder' wheel and a 'stiffer wheel'

First order approximation fuel economy depends on the weight of the vehicle, given the same engine. Suppose that your vehicle weighs 1 metric ton -- 1000 kg -- 2200 lbs.

Suppose that an alloy wheel was 5 kg lighter. That would be 20 kg (25 if you have a full size alloy spare) lighter.

20 kg is 1/50 of 1000, so 2% lighter vehicle.

So you get 2% better fuel economy.

So each $100 worth of wheel set cost requires the use of $5000 (2% of 5000 = $100) fuel to offset it's cost.

If a typical midsize car gets 30 mpg, and gas cost $3 per gallon, then your fuel cost is 10 cents per mile. $5000 worth of gas is 50,000 miles. If your car lasts 6 times as long -- 300,000 miles, then a 2% fuel savings would pay for an expenditure of $600 for a set of alloy wheels.

Given the costs I saw at Tiretown, this is a non starter. Prices were over that per wheel, let alone per set.

Note: A higher mileage car would take even longer to pay off. But even a 10 mpg muscle car would require 100,000 miles to get to this same point -- which still isn't enough.

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So financially it's not a win. How about environmentally.

Steel is common. It's cheap. The processes that make it turn fossil fuel coal to make coke (essentially pure carbon) which in turn removes the oxygen from the ore to leave iron. Mix stuff back in to get steel.

Aluminum is expensive. Huge amount of embodied energy, but most of that energy is hydro-electric. This in turn means that bauxite ore is shipped by ocean bulk cargo ships to where the electricity is cheap.

Most magnesium now is extracted from sea water, also a high energy cost.

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The big difference in price come from several factors:

• A steel wheel is made by stamping out two halfs of the wheel, then welding them together.

• An alloy wheel is cast as a single piece, I think, then in machined to spec. This makes for a rounder wheel. Complex support ribs can be moulded in that make it a stiffer wheel.

• Snob appeal. They're shiny, and bright, and pretty, and fashionable, and they are a way to say, "look at me, I'm cool" This always commands a premium price.