Know the cost before the first mile.

Fill the tank, reset the trip odometer, drive normally until you refuel, then divide the miles on the trip odometer by the gallons it took to fill up again. Do it over two or three tanks and average them. That number is usually 10–20% below the EPA combined figure for real mixed driving, and it is the one worth planning around.

Most manufacturers trigger it with roughly 1.5 to 2.5 gallons left, which at 28 mpg is 40 to 70 miles. It is a warning, not a plan: the exact reserve varies by model and the gauge gets less accurate at the bottom. Treat a quarter tank as your refuel point on unfamiliar roads.

The US Department of Energy puts the A/C penalty between 5% and 25% depending on outside temperature, humidity, traffic and how healthy the system is. The 12% used here is the median for mixed driving. On a hot day in stop-and-go traffic you will be at the top of that range; cruising on a mild highway you will be near the bottom.

Only at low speed. Below roughly 40 mph the drag penalty of open windows is small and the compressor is the bigger cost. Above that, open windows cost more in aerodynamic drag than the A/C costs in compressor load, so the windows-up, A/C-on option wins.

A gasoline gallon equivalent is the amount of compressed natural gas holding the same energy as one gallon of gasoline — 5.66 pounds under the DOE definition. Stations post CNG in GGE precisely so you can compare it against gasoline dollar for dollar. If a price is quoted per kilogram or per therm, convert it to GGE before comparing or the saving will be badly wrong.

Divide the installed cost by your annual fuel saving. At 15,000 miles a year, 28 mpg, $3.20 gasoline and $2.40 per GGE at 25 miles/GGE, the saving is around $275 a year — a $4,500 conversion takes well over a decade, which is longer than most tanks are certified for. High-mileage drivers are the only ones for whom the maths works.

The common guidance is a break every two hours or every 100 miles, and no more than about eight hours of driving in a day. The AAA Foundation research is what sits behind the 10-hour total-trip flag in this calculator: past that point, crash risk from fatigue climbs steeply regardless of how alert you feel.

Because average speed is not cruising speed. Every stoplight, toll booth, construction zone, on-ramp and fuel stop pulls the door-to-door average down. On a mixed interstate route a 70 mph limit typically produces a 58–63 mph door-to-door average once you include everything.

Divide the trip distance by your full-tank range, then round up — and plan on refuelling at about 80–90% of range rather than 100%, so you are choosing the station rather than taking whatever appears. Rural interstate stretches in the western US can run over 100 miles between stations.

Yes, more than most people expect. A roof box can cost 10–25% of your fuel economy at highway speed, and every 100 pounds of extra load costs roughly 1%. If you are packing heavy for the trip, drop your mpg input by 10% before planning your fuel stops.

Tire pressure does: under-inflation of 8 psi can cost around 3% of fuel economy and wears the shoulders out early. Tire size matters for a different reason — a larger overall diameter than stock makes the odometer and speedometer read low, which quietly inflates the mpg you calculate from them.