Estimate the extra gasoline and fuel cost of driving one trip or a year with a roof rack and cargo box.
Table of contents
How to use our Roof Rack and Cargo Box Fuel Cost Calculator
- Choose One trip or One year of cargo-box driving.
- Enter your usual Normal highway MPG before the cargo box (mpg) and your local Gas price ($ per gallon).
- For a trip, enter One-way trip distance (miles) and choose Trip direction; for a year, enter only Miles per year with cargo box installed (miles).
- Adjust Estimated highway MPG loss with cargo box (percent) if you have before-and-after driving data, then select Calculate.
- Sanity-check Miles included in this estimate before using the cost: a round trip should be twice the one-way distance, and annual miles should include only driving with the box installed.
Definitions
Normal highway MPG: Your vehicle's usual miles per gallon on highway driving before the cargo box is installed.
MPG loss: The percent decrease in fuel economy caused by the cargo box. A 10% loss changes 30 mpg to 27 mpg.
Cargo-box-installed miles: Miles driven while the cargo box is mounted, not all miles driven by the vehicle during a year.
Extra gasoline: Gallons used with the cargo box minus gallons used for the same distance without it.
6% to 17% cargo-box loss: A planning range for highway fuel-economy loss from a large blunt roof cargo box, used to show uncertainty around the editable estimate. [1]
Common mistakes and quick fixes
Mistake: Entering combined or city mileage in Normal highway MPG before the cargo box (mpg).
Fix: Use your usual highway MPG before installing the cargo box, from a highway trip screen or fill-up record.
Mistake: Typing the full out-and-back distance into One-way trip distance (miles) and also choosing Round trip.
Fix: Enter one leg only, then choose Round trip; check Miles included in this estimate.
Mistake: Using all vehicle miles for Miles per year with cargo box installed (miles).
Fix: Count only miles driven while the cargo box is actually on the vehicle.
Mistake: Leaving an old price in Gas price ($ per gallon).
Fix: Replace the example with the price you expect to pay for the fuel grade you use.
Mistake: Treating Estimated highway MPG loss with cargo box (percent) as a measured fact.
Fix: Use the 6% to 17% range shown in Extra fuel cost using 6% to 17% cargo-box loss to plan for uncertainty.
Limitations & Key Assumptions / Boundary Conditions
- This estimates gasoline use and gasoline cost only. It excludes the cargo box purchase, rental, installation, storage, maintenance, tolls, parking, insurance, and depreciation.
- The calculation assumes the same route, driving conditions, vehicle load, and fuel price with and without the cargo box.
- Actual MPG loss can differ because of vehicle shape, box size and shape, rack position, speed, wind, terrain, traffic, tire pressure, and driving style.
- The 6% to 17% range is a planning range, not a guarantee for a particular vehicle or trip.
- A 0-mile annual entry correctly produces $0 extra fuel cost because no cargo-box driving occurs.
Methodology
Calculation method
The calculator first chooses the miles for the selected period. A round trip doubles the one-way distance once; annual mode uses only miles driven with the cargo box installed.
selected miles = one-way miles x trip multiplier
estimated MPG with box = normal highway MPG x (1 - MPG loss / 100)
It compares gallons needed with the box against gallons needed without it over the same miles.
extra gallons = selected miles / estimated MPG with box - selected miles / normal highway MPG
extra fuel cost = extra gallons x gas price per gallon
The low-to-high cost range repeats the fuel-use calculation using 6% and 17% MPG loss. Those endpoints are a highway planning range for a large blunt roof cargo box. [1]
Worked example
For a 100-mile one-way route chosen as Round trip, the selected distance is 200 miles. With 30 mpg, a 11.5% loss, and gas at $4.00 per gallon, estimated MPG with the box is 26.55 mpg. The added use is about 0.87 gallons, and the estimated extra fuel cost is about $3.47.