EDIT, updated graph, it isn't linear
Looking at the bsfc chart might help too:
Brake Specific Fuel Consumption (BSFC) Maps - EcoModder
Given that you still need about the same power to go 60mph, I made a guess at your starting point (2000 rpm at 4/9ths thottle?) , marked by the arrow.
I then multiplied 78Nm by 2000, and got 156000, then I plotted 156000/rpm with the other dots. All the dots represent the same power level, rpm*Nm = 156000.
So the ideal for cruise would be to be smack dab in the middle of that 250 island, say 2500RPM @124.8Nm for this saturn. But our engines are all generally too big for that, Some consolation is that you can (with a stick shift anyway or some ingenuity) centre your shifts and maintain your throttle so that you
accelerate in the bsfc peak area anyway (and stay off the brakes).
But for cruise considerations, and assuming no other mods than gearing, you want to pick the gearing that will get you the lowest bsfc number. It looks like you are about 1/2 way between 275 and 250 at 1500 RPM, so maybe a %5 improvement in MPG there (1-(275/262.5)). For the ratio above (1750 RPM) it looks like maybe 1/3 closer to 250 so maybe a %3 improvement. There is probably a more precise way to interpret the distance between rings, but you get the idea hopefully.
Of course getting your starting point right is the real trick
