It is a good theory, engine turns less = better mileage, but physics works against you. What is one thing that many of the most fuel efficient vehicles have in common? Ex. trains and economy cars. They have small wheel sizes but make up for it through gearing. It is much easier to spin a gear than a tall tire.
For the ratio, that is also not an absolute rule. The peak gearing for mileage depends on the vehicle's weight. A 3.55 gear ratio may get great mileage on a ford ranger but it will get worse mileage on a F250 of the tire sizes are the same. Like you said, the lower ratio delivers less power to the ground. Take out too much power and you have to make the engine work harder to move the truck attached to it.
Also you might want to look at the pictures I have included in this thread
http://forum.gopowerhungry.com/conve...t-mileage.html