The thing about bigger tires = more gas milage isnt always true. I 30 inch tires on my truck from 27 inch, dropped my Engine revs to 1700 rpm at 70ish mph. Milage went from 18 to between 15 and 17. I never see over drive unless im flat and level or down hill lol.
Probably an easy explanation there. Engines have different powerbands-wherein an engine makes' it's best balance of HP and torque, between certain RPM's in the engine's operating limits. For example, My old '91 Formula 350 Firebird always seemed to make the best power between 2000 and 4500 RPM. Running lower than 2k seemed like the engine was a little dead, from the amount of throttle I was giving it. Over 4500, and it felt like it was running out of breath, even though it would run up to 5500. I got my best MPG when I ran with the Cruise set at around 2100-2300, in OD.
What I believe has happened to you Superpug, is that by going to a taller tire, you have in effect, changed the final drive ratio, and thus changing the operating band of your engine at the same speeds'. Before your tire change, your motor was probably running in it's sweet spot-making the easiest power for a given throttle setting. You made the same power to pull you down the road, with the least possible throttle. Whenever you put on taller tires, figure that for every additional inch taller, you go +-10% taller with your gears; by going from 27's to 30's, you in effect went to 30% taller gears. That has forced your motor to operate for a given speed, at an RPM that might be too low for it to make enough power to pull down the road, without excessive throttle/fuel usage, and lower Manifold vacuum as a result. Your best fix to run the 30's would be to either go with a deeper rear end gear, or run a different Tranny with a less-severe overdrive.
Almost forgot, running a vacuum guage in cab could help you on your mileage. Generally, running as much Intake manifold Vacuum as possible, for a given speed, is a good way to maximize your mileage.
