I like tractors
Tractors are great. They drive at a steady 25–30MPH, and (on account of their less–than–areodynamic design) they pull along a massive slipstream behind them. If one overtakes you whilst you're on a bike, you can usually tuck in behind it without much effort, after which you've basically got a free ride for as long as you and the tractor are going the same way.
The only slight downer is that you've got to stick about 5–10 feet away from the back of the tractor, which means either staring the 3–pronged spiky metal linkage thing in the face (knowing full well that if the tractor decides to stop suddenly, you'll be eating it), or sitting behind the nearside wheel and getting covered in crap every time it goes through a puddle. Small price to pay for the chance to sit back and relax, whilst travelling at speeds that would normally be pretty much unsustainable on the flat, though :–)