The engine will have to work harder to move the car at a given speed with larger diameter tires/wheels. You can do the same thing by changing the gear ratio in the rear end. There is a point where that effort will exceed the possible savings from lower RPMs. Note also that a lot of our Diesels don't see a significant change fuel economy with different RPM ranges - I don't understand why - chalk it up to MB engineers.
-- OK Don, KD5NRO Norman, OK "The Americans will always do the right thing... after they've exhausted all the alternatives." Sir Winston Churchill '90 300D, '87 300SDL, '81 240D, '78 450SLC, '97 Ply Grand Voyager
