Every bolt-on accessory changes how your truck moves through the world — and most of them cost you fuel. Roof racks catch the wind, lift kits and bigger tires add weight and rolling resistance, and steel bumpers push a wall of air down the highway. Individually the losses seem small. Stacked together, they can quietly erase 5 to 8 MPG.
Here is why that matters in real dollars: at $3.50–$4.00 per gallon, a 3 MPG loss on a truck that started at 18 MPG costs roughly $500–$700 per year at 15,000 miles. Before you drop money on the next upgrade, it helps to know what it will actually cost you at the pump. Here are the real numbers on the most popular truck and SUV modifications.
Why Fuel Economy Suffers When You Modify a Truck
Three forces decide how much fuel your truck burns: aerodynamic drag, rolling resistance, and weight. Nearly every popular modification pushes on at least one of them.
- Aerodynamic drag grows with the square of speed, so anything that disrupts airflow — racks, cargo boxes, bumpers — hits hardest at highway speeds.
- Rolling resistance rises with heavier, taller, more aggressive tires that flex more and grip more.
- Weight from steel armor, racks, and larger wheels forces the engine to work harder every time you accelerate.
Understanding which lever each mod pulls helps you predict the cost before you buy.
Roof Racks and Cargo Boxes: The Aerodynamic Tax
Roof-mounted gear is the single biggest aerodynamic offender most owners bolt on. An empty crossbar rack alone typically costs 1–3 MPG at highway speeds. Add a large cargo box — a Thule or Yakima-style enclosure — and you are looking at 3–8 MPG depending on the box profile and how fast you drive.
Not all roof gear is equal. Streamlined cargo boxes are dramatically better than flat cargo baskets or platform racks, which act like parachutes. Consumer Reports and countless owner highway tests confirm the same pattern: the taller and blunter the load, the worse the penalty.
The easy win here is simple — take the rack off when you are not using it. A rack that lives on the roof 51 weeks a year for two weeks of ski trips is burning fuel every single commute.
Lift Kits: It's the Tires, Not the Height
Lift kits get blamed for fuel loss, but the lift itself is rarely the main culprit — the tires you fit under it are. Here is the breakdown:
- 2-inch leveling kit: minimal to no measurable effect on its own.
- 3.5-inch suspension lift with 33-inch tires: roughly 1–3 MPG loss, mostly from the tires and slightly raised frontal area.
- 4–6 inch lift with 35-inch tires: 2–5 MPG loss.
The pattern is clear: tire size drives the number far more than lift height. A leveling kit on stock tires barely registers. The same kit under 35s tells a very different story at the pump.
Tire Size and Rolling Resistance: The Slow Bleed
Bigger, heavier, grippier tires flex more and demand more energy to keep rolling. Approximate highway penalties compared to stock:
- Stock 31-inch to 33-inch AT tires: about 1.5–2.5 MPG loss.
- 33-inch to 35-inch: another 1–2 MPG.
- 37-inch and larger: 3–5+ MPG loss.
- Mud-terrain vs. all-terrain (same size): MTs are typically 0.5–1.5 MPG worse thanks to those aggressive, chunky lugs.
There is a maintenance angle here that too many owners ignore. Larger tires are far more sensitive to under-inflation, and a soft tire adds rolling resistance on top of everything else. Keeping big AT or MT tires at their correct pressure is one of the only free ways to claw back a little of the fuel you lost. That starts with a gauge you can actually trust.
Steel Bumpers, Brush Guards, and Armor
Heavy-duty steel bumpers and brush guards deliver real protection off-road, but they carry a double penalty: added weight (50–200 lbs) plus aerodynamic disruption from that big flat bumper face pushing air. Expect roughly a 0.5–1.5 MPG loss depending on design and how much of the grille it blocks.
If overlanding and trail protection are your priority, this is often a trade worth making — just go in knowing armor isn't free at the pump.
The Accessories That Barely Cost You Anything
Good news: plenty of popular add-ons have almost no measurable fuel impact.
- Bed liner (spray-in or drop-in): negligible.
- Running boards / side steps: less than 0.5 MPG.
- Bedside steps and tie-down anchors: negligible.
If you love the look and function of these, add them with a clear conscience — your gas station habits won't change.
How to Modify Smart and Recover Lost MPG
You don't have to choose between a capable rig and reasonable fuel bills. A few strategies recover much of the loss:
- Run an aerodynamic tonneau cover. A hard or soft tonneau can improve MPG by roughly 2–4% versus an open bed by cleaning up the air behind the cab.
- Keep tire pressure dialed in. Proper inflation on larger tires directly cuts rolling resistance — the cheapest MPG you'll ever recover. Air up before every highway trip.
- Choose streamlined roof gear. A low-profile box beats a flat basket, and removing the rack entirely when it's empty beats both.
- Right-size your tires. If you don't truly need 37s, 33s or 35s deliver most of the capability for a fraction of the fuel penalty.
Airing back up after a trail run — or topping off before a road trip — is where a fast, portable compressor earns its keep, especially on big-diameter tires that take forever with a cheap pump.
The Bottom Line: Know the Cost Before You Bolt It On
None of this means you shouldn't build the truck you want. It means you should build it with your eyes open. A loaded roof box for two ski trips a year, 37s you rarely flex, and a steel bumper on a mall-crawler can stack into a very real 5–8 MPG hit — hundreds of dollars a year in fuel.
Match your modifications to how you actually use your rig, remove aerodynamic gear when it's idle, right-size your tires, and keep them properly inflated. Browse JACO's precision tire pressure gauges and portable air compressors to keep rolling resistance in check — the one MPG-saving habit that costs you nothing but a minute at the trailhead.
