Roof Racks and MPG: How Drag Impacts Gas Mileage
- DOE provides estimated fuel economy reductions from roof racks and cargo boxes.
- The impact can be larger at higher speeds (drag increases with speed).
- You can translate a percentage MPG drop into dollars with a simple fuel-cost formula.
What We Know (Sourced)
DOE’s fuel-saving guidance explains that roof racks and cargo boxes can reduce fuel economy and provides estimated reduction ranges for different types of driving. Source: DOE — Driving More Efficiently.
DOE also recommends removing roof racks and other external items when not needed as part of its broader Fuel Economy guidance.
Why Roof Racks Hurt MPG
The short version: adding equipment on the roof changes airflow and increases resistance. At steady highway speeds, that extra resistance can translate into more fuel burned per mile—which shows up as lower MPG.
Because the impact is often speed-sensitive, this topic pairs naturally with: Speed and fuel economy above 50 mph.
How to Estimate Your Cost
There are two easy ways to estimate the fuel cost penalty:
- Use DOE’s percentage range and apply it to your baseline MPG (rough estimate).
- Measure your own MPG over a few similar trips with and without the rack (more accurate).
Use our Fuel Cost Calculator to run both scenarios quickly.
What to Do Next
- Remove the rack/box when you don’t need it (especially for long highway stretches).
- Pack smarter: if you only need extra space occasionally, consider alternatives (we compare options here: roof box vs rear cargo carrier).
- Keep tires inflated and reduce other efficiency losses (see: tire pressure and fuel economy).
Why It Matters
If you do a lot of highway miles, roof drag can turn into real gallons over a year. A simple cost-per-mile approach helps you decide whether the convenience is worth the expense. For converting efficiency into dollars, see: Cost per mile (fuel only).
Estimate the Cost of Your Next Trip
Use distance, MPG, and gas price to estimate fuel cost with and without a rack.
Use the Fuel Cost CalculatorFrequently Asked Questions
How much can a roof rack reduce MPG?
DOE provides estimated fuel economy reduction ranges for roof racks and cargo boxes depending on driving conditions. See DOE — Driving More Efficiently.
Is the MPG penalty worse on highways?
It often can be, because aerodynamic drag becomes more important at higher speeds. DOE’s guidance includes highway and interstate estimates for cargo boxes.
Does removing a roof rack always save money?
It depends on how often you drive at speed and how big the MPG penalty is for your setup. If you rarely drive long highway miles, the savings may be small. Use a scenario calculation with your miles and gas price.
Is there an alternative to roof storage that’s more efficient?
Often, yes. DOE notes different impacts for roof-mounted vs rear-mounted cargo solutions. See our comparison: roof box vs rear cargo carrier.