Speed vs. Fuel Consumption

When I was driving at 80 mph (~130kph), my Honda CR-V consumed half a tank of gas in about 2 hours. My gas tank capacity is about 15 gal (~57 l), that meant my fuel consumption is about 21 miles per gallon (mpg ~ 112 ml/km). On a long quiet stretch, I went up to 90 mph (~145kph). I went through the same 1/2 tank in about 1 hour 15 min. That's 15 mpg! A full 6 mpg drop! Sure, I'm going faster, but I'm also wasting more time and money topping up my gas tank (10-15 min) more often, every 2 1/2 hours instead of every 4 hours. Not to mention tempting fate and highway patrol. Sigh.