Well, I've tried downsizing tires on the Blue Beast, from 265-75R16 to 235-85R16. The new tires are a hairsbreadth taller than stock but narrower. I haven't seen much improvement in mileage.
I have been thinking of going in the opposite direction. BFG makes huge all-terrain 305-75R16 tires. Now I know that the tread on these tires is not aerodynamic and they are heavier. But it will effectively gear the truck about 8% lower for a given speed. The effective axle ratio will go from 3.73 to 3.47.
What's the group's opinion? Will big mud tires make my mileage go up or down?
Capitalism: The cream rises. Socialism: The scum rises.
Did you adjust your odometer readings for the new taller tire? If you went 100 miles on the old tires...the new ones might be traveling 104 miles but the odometer will still read 100. Taller-skinnier tires at higher pressures would be you best bet.
2006 Jeep Liberty CRD...Founder of L.O.S.T.
OME 2.25" Lift w/ Toyo Open Country HTs 235/75/16s
ASFIR Alum Eng/Tranny/Transfercase/Fuel Skids
2002 Air Box Mod...Air Tabs (5) on Roof...(3)each behind rear windows
Partial Grill Block with Custom Air Scoop and 3" Open Catback Exhaust
Lambretta UNO150cc 4 Stroke Scooter
If they're rated at 80 PSI then you've already got E rated tires. If you have E rated Michelins aired up to 90 PSI then I think that's all you can do as far as tires goes... Look elsewhere for mods. You've already hit the point of diminishing returns on your tires. I don't see any signifigant FE gains from different tires.
I always figured diesels would be good candidate for larger wheels. They make insane amounts of torque. If you just going down the highway unloaded, or even a light load you may only be using 300 ft-lbs. of that 480 lb-fts. of torque your producing. That leaves you 180 ft-lbs. to play with, without increasing the amount of fuel its burning. I'd look into American Force Wheels. They make some mighty big wheels with basically street tires, instead of those insanely wide mudding tires.