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I’ve been tracking my fuel consumption for months, and I can’t figure out why my highway mileage is consistently worse than what’s advertised. My car’s trip computer and my manual calculations at the pump both show I’m getting about 6 miles per gallon less on long trips, even while using cruise control.

Yeah I had something similar. On highway runs my car would report a bigger drop than the sticker promised, even with cruise on. It felt like wind and speed were the big culprits. A sustained crosswind or passing big trucks would push the mpg down more than I expected. If the tires were a little low or I had extra gear, the gap would grow. Drag and aero drag from things on the roof add up fast in real life.

Do you think the speed range is the real issue? The sticker assumes calm weather and a light load at moderate highway speeds. At 65 versus 75, air resistance climbs a lot, and that can eat into efficiency. Not sure if that explains your whole difference, but it’s worth considering.

I did a quick in house test last month: windows up, AC off, tires inflated to spec. On a 50 mile stretch at about the same time of day, I picked up about 2 mpg compared to when I left the windows cracked and the system running. It wasn’t dramatic, but it was measurable.

Another angle I chased briefly was load. I took off a roof rack and any useless junk from the cargo area for a week. The highway number ticked up a touch, but not by much. Still, it made me believe aero and weight matter more than the sticker suggests in the real world.

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