I shift to 6th at 30MPH (1000RPM)
Any lower and I bog. I'm in 5th at 20. My goal is to stay at or below 1500RPM on the tach. Heck, I'd love it if I could idle slower than 800RPM while coasting. The way I figure it, if I use X amount of gas at idle, I use 20% more at 1000RPM and 100% more or double at 1600RPM.
Jay