It's a bunch of little things that all come together:

1) Proper air pressure in the tires.
2) Proper IP timing
3) Head reconditioned with new valves and seats less than 50K ago.
4) Maximum speed: 62 mph.
5) Minimum use of brakes
6) Acceleration at approx. 2100 rpm
7) no a/c
8) no winter fuel
9) 85% highway mileage (at 62 mph).
10) Use of 6 oz. Power Service per tank for the past three years


I do have a suspicion that the Power Service is working because there was a noticeable increase in mileage on the SD this year and no other variable has changed other than a slightly reduced speed (65 down to 62).