Why doesn't my highway fuel economy line up with the official figures?

[Evelyn.L]

I’ve been tracking my fuel consumption for the past six months, and I can’t seem to get my highway mileage to match what the official figures claim. My last long trip averaged about 6.5L/100km, but the advertised number is a fair bit lower. I’m wondering if it’s my driving style or maybe something with the car itself.

[AlexanderA]

Yeah I track this kind of thing too. On open highway I usually land around 5.5 to 6.0 L/100km when I cruise at 90–100 km/h. If I creep up to 110–120, it climbs fast into the 7s. Official figures assume calm winds, flat road, and no extra load, so real trips aren’t apples to apples with the sticker.

[Luke_M]

Mine got noticeably worse with a roof box and full luggage; highway numbers jumped to the low sevens. After taking off the roof box and lightening the load, I was often in the 5.7–6.0 range. I also found that keeping tires at the recommended pressure helped a bit—underinflated tires eat more fuel.

[Kevin_S]

I wonder if the measurement method is the culprit. My car’s trip computer sometimes shows 5.2 L/100km while a fill-to-fill check at the pump comes in closer to 6.0. The six-month average mixes city and highway, so it isn’t a clean apples-to-apples highway figure.

[AbigailS]

I tried Eco mode and a very light right foot on a highway run, and the dash numbers looked good, but the actual fill numbers didn’t line up. Then a stretch with a strong headwind reminded me wind and grade can swing the numbers a lot, even if you’re cruising steady.

[Adam.T]

Are you sure you’re comparing apples to apples? The official figure is from a test under ideal conditions, not six months of mixed driving.

[ScottJ]

Quick tip from my own detours: check tire pressure monthly, keep luggage light, and remove roof racks when not in use. Small changes add up over long trips.