Since moving from Spokane to the West side of Washington State in 1992, I've done the 280 mile drive many, many times. I used to, uh, push the speed limit and could often do the drive, door-to-door, in about 3:45.
Over the last few years, a few things have happened. First, I've gotten a little older, and a little more risk-averse. Second, I've gotten back into bike racing. And third, I've gotten more concerned with my energy usage.
My old driving pattern was to stick it just under 80 and cruise across the state. In my Audi, this was almost exactly one tank of gas (~14 gals, ~20 mpg). And hardly any speeding tickets.
In 2006, I did a stage race in Spokane, and headed back home directly from the last stage of the race. As soon as I got on road, it was hard not to see the traffic like a stage race. My gut response was to find the biggest, fastest vehicle I could and cuddle up behind it at the minimum safe distance, and just hang out. Since then it's become a new distraction, and an effective method.
The trip over to Spokane on Wednesday took about 4:15 with one stop. I used somewhere between 2/3 and 3/4 of a full tank, which is 14.4 gallons. On the way back to Seattle this morning (the direction that's into the wind usually), I used 11 gallons of gas, which works out to almost 26mph - and almost 30% improvement!
The process is simple, and actually makes me appreciate those ridiculously large SUVs just a little. Basically, all you do is look for any vehicle - the bigger the better - going about the speed that you'd like to go. When they go past, get in behind them at a safe distance, and just stay there. If your car has a "live" MPG reading, you can really see the difference. Behind another car, just cruising along, I'll often see readings of 30 mpg or better. If it's slightly downhill, that number will go even higher. This offsets times when it's lower due to an uphill or when you're searching for a new pull.