The honest answer is that most bolt-on aerodynamic gadgets do very little, because they are added to a shape the manufacturer already optimized, and a few (wind deflectors) cost you a small amount instead. Smooth wheel covers have a real, testable mechanism. Bed covers and tailgate devices are genuinely studied but contested. The one aerodynamic change nearly every driver can make for free is removing a roof rack or box that isn't in use.
Reviewed: August 2026. Figures below are attributed by name to their source, with the year stated where the source gives one. Scheduled for recheck in six months.
Why a bolt-on part fights an already-optimized shape
Modern vehicles are aerodynamically shaped from the factory, not by accident. Manufacturers spend heavily in wind tunnels and computational modeling to smooth the specific airflow over their specific body, mirrors, underbody and wheels, because aerodynamic drag becomes the dominant force working against a car at highway speed. That matters directly for anything bolted on afterward: the vehicle is not a blank shape waiting to be improved, it is a system already tuned as a whole. A small add-on part interacts with airflow the factory shape was designed around, and because it was never part of that original design, it can just as easily disrupt smooth airflow as improve it. This is the honest starting point for judging any aftermarket aerodynamic accessory: it has very little room left to help, a real chance of doing nothing, and in several categories below, a documented chance of making fuel economy slightly worse rather than better.
Sorting the category at a glance
Not every product sold as an aerodynamic upgrade behaves the same way, and lumping them together into one verdict is where most coverage of this topic goes wrong.
| Product type | What it's actually for | Aerodynamic effect |
|---|---|---|
| Vortex generators | A genuine, engineered fix in specific applications such as tractor-trailer fleets | Real in that context; unverifiable as a stick-on retrofit on a passenger car |
| Wind and bug deflectors | Keeping insects and debris off the windshield | Usually a small drag penalty, not a saving; sold for a different job entirely |
| Smooth wheel covers | Reducing turbulence at the wheels | The one category with a demonstrated real-world mechanism |
| Bed covers and tailgate devices (pickups) | Cargo protection and weather sealing, sometimes marketed on fuel savings | Genuinely studied; results contested and vehicle-dependent |
| Anything claiming a large percentage | Marketing | Treat as a claim to verify, not a fact |
Vortex generators: real physics, unverifiable on a retrofit
Vortex generators are small fins that create controlled turbulence to keep airflow attached to a surface longer before it separates, and the physics behind them is real. The North American Council for Freight Efficiency, an industry research group that tests aerodynamic devices for commercial trucking fleets, has studied vortex generators and similar add-ons on tractor-trailers, where the flat, boxy trailer shape leaves genuine room for a well-placed device to help and where fleets can measure the result across many vehicles and routes. None of that transfers cleanly to a small vortex-generator strip stuck to a passenger car's roof or hatch. A production car's roofline was not left as an unoptimized cargo-box shape the way a trailer's is, so there is far less separated airflow for a small fin to fix, and no independent, named testing on ordinary passenger vehicles was found to confirm a measurable result. Treat passenger-car vortex-generator kits as unverified, not proven.
Wind and bug deflectors: solving a different problem, at a small cost
A hood-mounted wind or bug deflector is designed to divert insects and road debris up and over the windshield, not to improve fuel economy, and it typically comes at a small aerodynamic cost rather than a benefit. Wind-tunnel testing attributed to Canada's National Research Council, cited across multiple automotive outlets, found that bug deflectors reduce fuel economy by roughly 1.5 to 3 percent on average, a difference most drivers will not notice trip to trip but one that shows up over a full tank. The mechanism is straightforward: the deflector disrupts the smooth (laminar) airflow the factory hood and windshield angle were designed to maintain, pushing it into a more turbulent state before it reaches the windshield. The effect is largest at highway speed and negligible in city driving under roughly 40 mph. None of this makes a deflector a bad product; it is a windshield-protection accessory that happens to cost a little aerodynamically, not a fuel-saving device that happens to also protect the glass.
Smooth wheel covers: the one family with a real mechanism
Wheels create more aerodynamic drag than most drivers expect: an open, spoked wheel churns turbulent air as it spins, and a smooth cover over that wheel reduces that turbulence. This is the one product family here with a demonstrated, real-world mechanism, and it is why some efficiency-focused production vehicles ship with wheel covers from the factory rather than leaving the alloy exposed, Toyota's hybrid Prius and Tesla's Model 3 and Y among them. Car and Driver's own testing of Tesla's factory aero wheel covers on a Model 3 found a real, measurable gain, roughly 3.1 percent, about 10 additional miles of range on a car rated for around 312 miles. That figure describes a specific vehicle with covers engineered by the manufacturer for that exact wheel, not a universal result for any aftermarket cover on any wheel, but it is the clearest evidence in this whole category that the underlying mechanism is genuine rather than cosmetic.
Bed covers and tailgate devices on pickups: studied and contested
This is the most genuinely contested category here, and the honest answer is that results depend heavily on the specific truck, the specific cover, and the test conditions. Consumer Reports tested a 2013 Dodge Ram V8 at 65 mph and found 22.3 mpg with the tailgate up, 21.5 mpg with it down, and 21.4 mpg with a soft tonneau cover fitted, meaning it found no mileage gain from that cover on that truck, and cautioned the result was one truck and one cover, not a universal finding. Other testing has gone the other way: a comparison published by The Fast Lane Truck reported 21.14 mpg with a tonneau cover fitted against 19.53 mpg without, on a different Ram model. SEMA's 2007 fuel economy study, still cited as one of the more comprehensive tests run on this question, found effects that varied by vehicle rather than pointing to one consistent answer. Do not expect a bed cover to deliver a guaranteed percentage; expect a result specific to your truck that only you can measure by testing your own baseline before and after.
The biggest win is subtractive: take it off when you're not using it
Every add-on in this article works against an already-optimized shape at best, and none of it compares to the one change that is entirely free: taking off roof-mounted gear that is not currently being used. fueleconomy.gov's own testing found that a large, blunt roof-top cargo box can reduce fuel economy by 2 to 8 percent in city driving, 6 to 17 percent on the highway, and 10 to 25 percent at interstate speeds of 65 to 75 mph, and an empty roof rack left mounted between trips still costs several percent on its own. A roof box, a bike carrier, a roof-mounted flag or a kayak rack all do the same thing: they sit directly in the airflow the factory shape was built to manage cleanly. Removing them when they are not in use is the single most reliable aerodynamic change most drivers can make, and it applies to how to improve fuel economy honestly generally, not just to aerodynamics specifically.
This piece narrows the aerodynamics slice of our broader look at EV and efficiency gadgets worth it and its companion on fuel-saving gadgets that do nothing. If a seller is pitching a specific device with a specific percentage, run it through our framework for judging an eco car gadget's claim against its mechanism, and remember that a big headline number is exactly the pattern our look at peak versus working ratings warns about on gadgets generally.
FAQ
Do vortex generators work on a car?
The physics is real and used in engineered trucking applications, but no independent, named testing on ordinary passenger cars was found to confirm a measurable result from a stick-on retrofit kit. Treat passenger-car claims as unverified.
Do wind or bug deflectors hurt gas mileage?
Wind-tunnel testing attributed to Canada's National Research Council, cited widely, found a roughly 1.5 to 3 percent reduction in fuel economy at highway speed, a small effect most drivers will not notice trip to trip.
Do wheel covers actually improve fuel economy?
Yes, this is the category with the clearest evidence: Car and Driver documented a real gain from Tesla's factory aero wheel covers on a Model 3. That result describes a manufacturer-engineered cover on a specific wheel, not any generic aftermarket cover.
Do tonneau covers or tailgate up versus down really change gas mileage on a pickup?
Results are genuinely contested. Consumer Reports found no gain from a soft cover on one Ram; other testing found a meaningful gain on a different Ram. Expect a result specific to your truck, not a universal number.
Should I drive with my tailgate up or down?
Consumer Reports' testing found tailgate up returned better fuel economy than tailgate down on the truck it tested, which matches the general engineering explanation that a closed tailgate lets air pass over the bed more cleanly than a dropped one disrupting it. Results can vary by truck.
What is the single biggest aerodynamic change I can make?
Removing roof-mounted gear, a rack, box, carrier or flag, when it is not in use. It costs nothing, and fueleconomy.gov's own testing shows it is worth more than any bolt-on aerodynamic accessory covered here.
Is it worth buying any aerodynamic accessory just for fuel economy?
For most drivers, no. The one category with a demonstrated mechanism, smooth wheel covers, was measured on a manufacturer-engineered setup, and the rest of the category ranges from unverified to a small net cost. If fuel economy specifically is the goal, the free options in our guide to improving it honestly do more.