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I agree that low rpm gives best FE for cruising. What I want to point out is, the manufacturer won't build the gear box optimized for cruising for another good reason.
In fact, the question of the topic is wrong. It should be "what is the ideal gear setting to cruise?" Different models cruise at different rpm with top gear and same speed. I want to cruise at 65mph with 2000rpm. However, my car doesn't allow me. @65mph, the top gear is 3250rpm. We want that but doesn't mean we can have it. What we can choose is what gear we are using at the given speed.
Cruising = constant speed =minimum power needed. And, highest gear = lowest rpm = minimum power. So, we should choose highest gear to cruise!!
Of course, we won't cruise at 10mph. Maybe 25mph or 30mph for minimum speed? If the top gear runs, that is ok. Just be careful when you break (easy to kill the engine) or go up hill. When power is needed, shift to lower gear and go, don't stay in top gear in low speed. That won't save any gas. (faster reaching the speed you want, faster starting cruising, faster start to save gas.)
The above case is for flat road. If cruising uphill, depends on the power of the engine, how steep of the hill, and the speed of cruising. Sometimes can't cruise on top gear, then, choose the next highest one.
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