My buddies and I do LOTS of highway driving in the summer to go out to the mountain biking spots. Multiple 400km return trips a week are not abnormal for us. When we were in our twenties, we would go all out, driving 130km/h out there. Since we've all at least turned 40, we've slowed down a lot, taking 10 extra minutes, but driving the speed limit, or perhaps 5 over, all in the name of fuel economy. Every little bit helps. Savings go back into the bike budgets.