A Toyota Corolla with a 1.8L engine requires 6.2 liters of gas to drive for 100 kilometers on a highway. But on city roads, it would require 7.9 liters of gas. A 2022 Ford F-150 requires 9.4 and 12.1 liters per 100 km for highway and city driving, respectively.
Something about intermittent power draw vs. constant draw.
I have the same effect on my electric dirtbike. I get a lot more mileage off-road up and down hills at 15-40km/h than I do on flat road at 70 km/h.