These Car Accessories Could Be Killing Your Gas Mileage

Since the 1970s, I have only owned station wagons or SUVs, plus I do not mess with the looks of my cars. However, I am guilty of adding factory accessories that negatively affect fuel mileage, such as roof rack cross rails, mud flaps, running boards, wind deflectors, and towing packages.

All these accessories alter aerodynamics, weight, rolling resistance and load-lessening MPG, but adding a little pizzazz to my cars, sort of like jewelry, giving them a polished look. And I’m not the only one.

According to Specialty the Equipment Market Association (SEMA), “U.S. consumers spent over $52 billion dollars accessorizing and modifying their vehicles” in 2024. Additionally, Research and Markets forecasts the Car Modification Market will grow to over $80 billion dollars by 2034. Custom car enthusiasts want to build something that reflects their style and taste. People customize their cars to express personality, increase performance, driving pleasure, or sometimes even to solve problems.

Unfortunately, adding accessories, even original equipment manufacturers (OEM) approved, and dealer-installed add-ons destroy gas mileage. Here, experts explain how common accessories quietly diminish MPG.

How Car Accessories Affect Fuel Economy

Manufacturers design and build cars to squeeze every mile out of a drop of gasoline to achieve maximum fuel economy. Everything from the headlights, hood, windshield angle, door handles to the antenna is mounted in such a way to increase cars’ aerodynamics.

Anything that adds drag or forces the engine to work harder eats away at MPG. For example, “JustAnswer” Auto Expert Chris Pyle, says, “Body kits and rear wings that state your car will become more aerodynamic, so you get better MPG. Actually, they grab more wind because anything that sticks out into the airflow increases drag, forcing you to apply more throttle, and making the engine work harder just to maintain the same speed as without these add-ons.”

Even glue on hood scopes causes turbulence that affects MPG.

“Mounted equipment can hinder vehicles from having a streamlined airflow over the vehicle cabin,” said Eliot Vancil at Fuel Logic LLC. Even small items such as window deflectors to deter wind buffeting or light bars affect fuel economy.

“While the effect of most add-ons is usually small, adding multiple accessories can quickly lower MPG,” said Vancil.

The Impact of Roof Racks

Because it sits in the cleanest, highest‑pressure airflow [area] on the vehicle, “even when not carrying cargo, car roof racks cause aerodynamic drag and reduce your gas mileage,” said Vancil. Even though I am a fan of roof racks, the increased head-on mass creates turbulence over the roof, forcing the engine to work harder to maintain speed, especially above 55 MPH.

To enhance my 1978 Pontiac LeMans station wagon, I added a factory rear wind deflector, even though I knew it would cut the fuel economy by one MPG. Added Vancil, “Roof racks can reduce fuel economy up to five MPG.” He suggests removing the crossbars when you are not using them. Always observe the roof rack’s weight limits and properly tie down the load.

Lift Kits and Vehicle Modifications

By raising the suspension, lift kits expose the body’s undercarriage, which creates more underbody turbulence. “If the vehicle frame geometry is changed due to a modification, the altered suspension [angles] increases rolling resistance and rotational mass,” said Vancil. Why do you think racing cars are so low to the ground? Being lower reduces air flow under the car, lowering turbulence and drag while increasing stability at high speed.

Even a small two-inch lift can result in one to three MPG loss. An aggressive off‑road modification [that includes heavier tires and added components] can result in a 20 percent MPG loss. Driving with the tailgate down doesn’t help improve fuel mileage either.

Heavy Aftermarket Accessories

Although they add visual appeal, as well as functionality, a custom truck bed storage system can weigh between 200 and 250 pounds, empty. Other weighty items, such as steel (chrome) bumpers, winches, skid plates, rock sliders, rollbars, cab covers, bull bars or cargo organizers all increase a vehicle’s weight.

Adding significant weight hurts fuel economy, particularly during city stop‑and‑go driving. “Every extra 100 pounds will lower MPG by one to two percent, or more,” said Pyle. He added, a full coverage package that protects the engine, transmission, transfer case and fuel tank of “off‑road armor easily can add 200 to 400 pounds [additional weight] before you even load gear.”

The added weight forces your engine to work harder, which means you’ll need to push the throttle harder [using more gas] to reach and hold the same speed you’d get without the added accessories. Like racing cars, less weight equals more speed with better fuel economy.

FAQ

Are larger tires always worse for fuel economy?

It depends. Yes, heavier wheels coupled with larger tires are almost always worse for fuel economy. Larger tires increase rotational mass, rolling resistance, and often have more aggressive tread, which reduces MPG. The biggest MPG losses come from wider, heavier, or off-road pattern tires. “A wider tread requires a greater amount of power to keep it moving at highway speed, potentially increasing fuel usage 10 percent,” said Vanci.

Although some factory wheels may lack visual appeal, OEM wheel designs are made to help achieve greater efficiency and safety. On the other hand, if tires with an overall larger diameter made from low rolling resistance compounds and are fitted correctly (called tire fitment, a tricky set of calculations that do not affect the delicate balance between tire performance and safety), this can increase MPG.

Is a hitch-mounted cargo carrier more efficient than a roof box?

Although a hitch-mounted cargo carrier may lower MPG up to two percent, a hitch-mounted cargo carrier is practically always more efficient than a roof box. Depending on the speed, size and shape of the roof box, expect to see a 10 to 25 percent MPG drop. Even modern aerodynamic rooftop cargo shells can cause up to a five MPG loss. A hitch carrier sits in the vehicle’s aerodynamic slipstream, causing minimal drag.

“A roof box is a large, blunt obstruction that just grabs more wind, causing resistance,” said Pyle. “Mounted equipment hampers vehicles from maintaining a smooth, streamlined airflow over the vehicle.”

About the Experts

Chris “Moose” Pyle is a master-certified technician with 20-plus years of automotive experience. He has also worked as an expert for JustAnswer since 2006, where he answered hundreds of car maintenance and repair questions a day. (Electronic communications June 16, 2026)

Eliot Vancil is CEO of Fuel Logic LLC, a Dallas, Texas-based company specializing in comprehensive fuel management solutions, including delivery of various diesel types and gasoline nationwide. (Electronic communications June 16, 2026)

Resources

Steve Haney, Corporate Technical Trainer at Bosch Mobility Aftermarket (June 16, 2026)

Jim DeLeo, Northeast Division Manager at Hunter Engineering Company (June 17, 2026)

Bodyshop Business: “SEMA Releases New Market Report on Consumer Trends in Aftermarket” (Published July 1, 2025)

Car and Driver: “Decked Drawers Pickup Truck Bed Storage System” (Aug 2024)

Research and Markets: “Car Modification Market Outlook 2025-2034: Market Share, and Growth Analysis By Type (Overall Modification, Local Modification), By Category (Performance, Cosmetic), By Vehicle, By Sales Channel Type” (Published Aug 2025)

U.S. Department of Energy: “Find and Compare Cars Fuel Information” (May 13, 2025)

U.S. Department of Energy: “Many Factors Affect MPG” (June 17, 2026)

The post These Car Accessories Could Be Killing Your Gas Mileage appeared first on Family Handyman.