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I'm not totally with it right now, but...
A BSFC map would be more useful to look at than a torque curve. You can see what kind of fuel consumption you'd have based on the engine load and rpm.
Peak torque would generally be where an engine is most efficient...but only if you need all the power it makes at that RPM. And only if you keep you engine out of "enrichment" mode...usually starting somewhere around ~80% load.
So to answer your original question...you'll get the best fuel economy when you're in as high of gear as you can get without lugging the engine. Why? because otherwise you'll be running at less than optimal(close to maximum) engine load. (Efficiency takes a nose dive the lower the engine load, unless you're running a diesel engine. EGR mitigates this somewhat, but far from completely).
This is where having a smaller engine comes in. If you want maximum efficiency, you want to run it at high load as much as possible; a small engine will have to work harder, more often, to keep the car moving. Whereas if you're cruising in your v8 with the throttle barely open, you'll almost always be in an inefficient area on the BSFC map.
Engine design does matter...but if you had two engines with the exact same design, other than displacement, the smaller one would come out on top every time...unless you're constantly overloading it (too much weight, too many hills, maintaining too high of a speed), which, again, would push it out of it's efficient zone and the larger engine would get pushing in to it.
Uhm, yeah. something like that.
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