I drove ~1100km to and from the mountains last weekend in the Scoob. Thanks to no gas stations along a big chunk, I drove 100km/k religiously while on the Icefields Parkway, and arrived in Rocky Mountain House on my way home to fill 'er up. Even through the mountain terrain, keeping my speed low paid off, with a stretched tank (only 6L left!) at 9L/100km. Sounds poor for highway, but with my roof box on and a full load of people/gear going over passes again and again, that's really good.
Wanting to get home fast, I turned up the wick, and spent the rest of the drive at 125-130km/h or so. In the previous tank, my fuel light came on at almost exactly 600km. The next tank? Only 410km until the light!! Ouch!!!
So, a 25-30% increase in speed meant a fuel consumption increase from 9L/100km to 13.2L/100km, or nearly 50%.
The relationship between power required and aerodynamic drag is exponential, meaning fuel consumption is also likely to be non-linear as well.
Before you guys drop your jaws at the 13L/100km highway number, that is with my ski box on the roof, which I find generally adds 1-2L/100km in consumption, BUT, at speeds over 115km/h or so, the penalty begins to grow significantly. The fact the roof box has such a pronounced effect on consumption also highlights how important aerodynamics are.
