What wastes the most gas in a car?

A large, blunt roof-top cargo box, for example, can reduce fuel economy by around 2% to 8% in city driving, 6% to 17% on the highway, and 10% to 25% at Interstate speeds (65 mph to 75 mph).5

Rear-mount cargo boxes or trays reduce fuel economy by much less—only 1% or 2% in city driving and 1% to 5% on the highway.

If you need to use an external cargo container, removing it when it's not in use will save fuel and money.

Falling gasoline prices have, at least temporarily, slowed the improvement in the average fuel efficiency of cars Americans choose to buy. Since reaching a peak of 25.8 miles per gallon (mpg) in August of 2014, the average fuel economy of new cars sold in the U.S. market dropped to 25 mpg this past November, according to researchers at the University of Michigan's Transportation Research Institute.

Methodology: To identify the cars with the worst gas mileage, 24/7 Wall St. used data on estimated combined city and highway miles per gallon from the EPA. We only reviewed 2015 and 2016 models and used data on the 2016 model when available. We further screened by eliminating any duplicate models, opting for the model and trim with the worst mpg. If multiple trim levels of the same model have the same fuel efficiency, we only looked at one model.

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Manufacturer's suggested retail price (MSRP) data came from third-party sources, when not available from the manufacturer. The vehicle's specifications came from the EPA's reports. Gas-guzzler data came from the EPA's most recent (2013) report on which vehicles are exempt from the tax and the EPA's overview of the gas-guzzler tax.

These are the cars with the worst fuel efficiency.

10. Mercedes-Benz, G550

> Miles per gallon: 13.4
> MSRP: $120,825

Mercedes-Benz has been building the G550 for 36 years, and chances are the vehicle will celebrate its 40th year anniversary before the automaker decides to move on. For 2016, the SUV got a peppier and more fuel efficient engine. Despite the improvement, it is among the least fuel efficient vehicles on the market, getting an average 13.4 mpg. The G550 weighs 5,724 pounds, and the 4-liter, 416-horsepower engine can push that weight from 0 to 62 mph in 5.9 seconds.

9. Pagani, Huayra Coupe

> Miles per gallon: 13.2
> MSRP: $1.07 million

A boutique carmaker, Pagani has been selling hand-made cars for 14 years at a rate of less than 10 cars per year. The Italian automaker built the first of 100 Huayras in 2013, and even at around $1 million per vehicle, it sold all the cars. The Huayra, which can zip from 0 to 60 mph in just 3.0 seconds, has a top speed is 224 mph. The Coupe version sacrifices efficiency for speed as it gets an average 13.2 mpg.

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8. Ferrari, F12

> Miles per gallon: 13.1
> MSRP: $490,000

Ferrari plans to build only 799 units of the F12tdf. Named to honor the Tour de France auto race of the 1950s and 60s, the special edition was announced in October and sold out in a month. One of the fastest cars ever sold, the F12 accelerates from 0 to 60 mph in a mere 2.7 seconds and sports a 211 mph top speed. Though it gets an average of only 13.1 mpg, it still has better fuel economy than its slightly less expensive cousin, the Ferrari FF.

7. Lamborghini, Veneno Roadster

> Miles per gallon: 12.8
> MSRP: $4.5 million

Those who spend $4.5 million on a single automobile are paying for exclusivity as much as anything else. Lamborghini hand-crafted just nine Veneno roadsters — and all nine are already sold. The company claims a top speed of 221 mph for the Veneno and it accelerates from 0 to 62 mph in just 2.9 seconds.

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6. Ferrari, FF

> Miles per gallon: 12.8
> MSRP: $302,450

The EPA rates the FF as a midsize car — in the same class as the Honda Accord and Toyota Camry — because Ferrari offers it in a 4-seater version. Introduced in 2011, the FF of course is a high-end vehicle and one of the most expensive on the U.S. market, with an MSRP of around $300,000. The 12-cylinder engine generates 651 horsepower, and the car goes from 0 to 60 mph in 3.5 seconds and has a top speed of 208 mph.

5. Lamborghini, Aventador Roadster

> Miles per gallon: 12.2
> MSRP: $550,000 (estimated)

Lamborghini's supercar Aventador has been in production since the 2012 model year. The 750-4 SV roadster uses a similar V-12 engine to the Veneno, and like the Veneno, the car goes from 0 to 62 mph in just 2.9 seconds with a top speed of 217 mph. The car is slightly less fuel efficient, however, gets an EPA rating of 12.2 mpg combined city and highway.

4. GMC, Savana 2500 2WD Conversion Cargo

> Miles per gallon: 11.9
> MSRP: $32,410

Some of the vehicles on this list are luxury vehicles, with automakers often sacrificing fuel efficiency for performance and luxury. The GMC Savana 2500 2WD is not such a vehicle. The Savana is a work vehicle, offered as a passenger van and as a utility vehicle for hauling cargo. Although a version of the van has been sold for roughly two decades, the years of production have not yielded a more fuel-efficient cargo van. The Savana 2500 gets just 15 mpg on the highway and 10 mpg in the city.

3. Chevrolet, Express 2500 2WD Conversion Cargo

> Miles per gallon: 11.9
> MSRP: $30,550

The Chevrolet Express is mechanically identical to the GMC Savana — GM sells the same car under different names. Like the Savana, the Express focuses on effectively hauling cargo — at the apparent expense of fuel efficiency. The Express 2500 has 270 cubic feet of cargo space and is rated to haul 8,600 pounds of cargo.

2. Mercedes-Benz, AMG G65

> Miles per gallon: 11.6
> MSRP: $218,825

The Mercedes-Benz G65 traces its roots back to 1979, when it was designed as an off-road vehicle for military use. Eventually, the car gained tremendous appeal in the civilian consumer market. Since the original car was released, the German automaker has added some creature comforts and tailored the vehicle to the high-end market. Perhaps its roots as primarily a utility vehicle explain its incredibly low fuel efficiency of 11 mpg in the city, and 13 mpg on the highway.

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1. Bugatti, Veyron

> Miles per gallon: 10.4
> MSRP: $2.25 million

Built by luxury automaker Bugatti, the Veyron is one of the fastest cars in the world, capable of going from 0 to 60 mph in just 2.4 seconds and has a top speed of 254.04 mph. That is what a 16-cylinder, quad-turbocharged engine that generates 1,200-horsepower can achieve. Speed, however, does not typically translate into efficiency. The car gets just 10.4 mpg combined combined fuel economy. After marking its 10-year anniversary this year, production of the Veyron ceased, with a final production run of 450.

See the rest of the top 20 cars with the worst mileage:

Cars With the Worst Gas Mileage

More on the cars with the worst gas mileage

The recent Volkswagen TDI scandal — in which it came to light that the German manufacturer intentionally misrepresented the fuel efficiency of its diesel vehicles — highlights the importance of fuel efficiency to Americans, even in times like now, as gas prices are at a multi-year low. Much of the decrease in overall fuel economy is attributable to higher sales of pickup trucks and SUVs, cars that burn more fuel than smaller, lighter vehicles. Other extremely low efficiency luxury models continue to reach high-end buyers as well. The Bugatti Veyron gets roughly 15 miles per gallon on the highway and 8 miles per gallon in the city. Based on Environmental Protection Agency (EPA) estimates, 24/7 Wall St. reviewed the 20 cars with the worst gas mileage sold in the U.S.

The manufacturers or importers of most vehicles with a fuel economy below 22.5 mpg are assessed a gas-guzzler tax. Pickup trucks and SUVs are exempt from the tax because of a concern on the impact such a tax would have on farmers and construction companies.

If a car is considered a gas-guzzler when getting less than 22.5 mpg, a car getting less than 15 mpg certainly has to offer buyers extra special features to justify the lost fuel efficiency. In the majority of cases, what is being offered is extreme luxury or performance or both.

Of the 20 cars with the worst fuel efficiency, 14 cost at least $100,000, and four cost at least $500,000. Included on this list of low-efficiency vehicles are so-called hypercars such as the Lamborghini Aventador and the Ferrari FF. These vehicles are built to achieve peak speed and acceleration. Fuel efficiency is an afterthought.

The remainder of the cars with the worst fuel efficiency are built on light truck frames. Two are SUVs, two are large vans, and two are pickups. In all six cases, potential owners must choose to sacrifice fuel efficiency in order to gain utility.

24/7 Wall St. is a USA TODAY content partner offering financial news and commentary. Its content is produced independently of USA TODAY.

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