Gentlemen:
Sorry to step in the middle of this dart throwing contest,
, but I think there's yet another variable that needs to be considered. That is the driver - IF the driver drives a numerically low differential ratio'ed car gently and not too fast, the engine won't be loaded to the point that it's being fed more fuel than it can use. (IE, the diesel car won't smoke, the gasoline car won't be running on full throttle enrichment). If the driver is mostly in a hurry and has a heavier footed style of driving, the numerically low differential ratio'ed car won't give nearly the mileage that it could be capable of, and the numerically high differential ratio will be better suited to this driving style, as the engine will be operating within it's design parameters.
A good example is a pickup truck with a small engine towing a heavy load, compared to the same truck with a larger displacement engine towing the same load. The larger engine will usually give better mileage because the engine can handle the load. I'm sure most of us have experienced that situation. My brother has a GMC pickup with the 4.3L V6, I have the same truck with the 2.8L V6. Same transmission & rear end ratio. He consistently gets better mileage. Why? Because my truck just doesn't have the poop to pull the truck in OD at faster freeway speeds, and his does.
So, there are many factors to consider... maybe Tom is a gentle footed driver.
Regards,