[Patrick]

My thought is to get a BSFC chart for your engine, and try to get the rpm in the middle of the low-consumption "island" when you're at your cruising speed. This will give you a little leeway for accelerating and passing.

EDIT: See attachment. IMO, for the Saturn shown, you'd want to cruise at 2500 rpm (blue dot). If you speed up or slow down between 1500 and 3500 rpm (green line), you're still in the sweet spot. Likewise if you need more or less torque (between about 93 and 145 Nm) for climbing or descending hills or accelerating (red line), you're still in the sweet spot. You're still pretty good down to about 75 Nm torque because you're still in the 275gm zone. However, if you've modded your car so that you only need about 47 Nm of torque at cruise (I don't know how likely that is, but would guess it's probably rare), then you might want to gear it for 3500 rpm to keep you in the 300gm zone. Somebody please correct me if I'm wrong.