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Gearing.
When on long trips, I switch my BMW's differential from a 3.46:1 ratio to a 2.93:1 ratio. I swap out the whole center section in an hour. Of course, this means that you need to have two on hand. This gains ~2 MPG. Another benefit is that the car is significantly quieter.
It's not an "eco mod," but I saw my '57 Chevy's around-town MPG go from ~13 to ~15 when I swapped from a TH350 (non-lockup 3-speed auto) to a TKO 600 (5-speed manual with 0.82 5th gear). Since I could get 18 MPG if I held it under 65 MPH on a long trip with the TH350, I'm sure it'd hit 20+ MPG on a long trip with the TKO 600.
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