Tire selection will depend mostly on price, life, load range, and RR (if I can find info on it). Tire size is pretty flexible. I would like to take up more of the wheel well to improve aerodynamics, aesthetics, and gearing, but I'm not super picky.
Back to the original question: Load vs. RPM on diesel engine efficiency? Turbo diesels in particular....
There are a few threads on gassavers.org that are interesting, but I was looking for some more insight.
Rpm! - Gas Savers - Fuel Efficiency Forum
Diesel ecomony? - Gas Savers - Fuel Efficiency Forum