Let's clear up the concept of average speed first:
v<code>[SUB]avg[/SUB] = (s<code> - s<code>[SUB]0[/SUB]) / (t<code> - t<code>[SUB]0[/SUB]) = Δs/Δt</code></code></code></code></code>
In plain english, that means that average speed is total distance divided by total time taken. That means that what you are thinking is a 100 km/h average, is actually a 60 km/h average at best
Now that we have that out of the way, there is no way that a kei van like that can maintain 17-18 km/l going 100 km/h steady state. It's only possible if you are going downhill, have a tailwind, or tailgating a Daewoo at dangerous distances. Your best bet is to reduce your speed to the 75-85 range, or simply get something with a bigger engine and taller gear ratios.
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