I always try and read these gas mileage threads since ill be getting a 3rd gen this late spring/early summer. Do higher speeds absolutely kill these vehicles mileage? My states interstate limit is 75. I always peg my cruise control at 80. Our excursion(bone stock v10) gets 11/12mpg on pure interstate driving @ 80 to 85mph.
Yes, higher speeds absolutely destroy your gas mileage in pretty much any vehicle. I used to follow hypermiling websites and I've tried a LOT of the tricks. Out of every trick and part I tried, the biggest difference in fuel mileage was achieved by changing my driving style, and lowering my highway speed. This is echoed by most hypermilers.
The drag coefficient of the gen 1 and 2 Montero's is horrendous...something like 0.50 or above. Once you pass 60mph or so, the decline in fuel economy for every 5mph grows with each increment...it's not linear.
The difference between a tank at 60mph and a tank at 80mph is probably around 50 miles cruising distance, maybe more.
I still think the OP is pretty much in line with what he should be getting...he's basically getting the same number as the Mitsubishi and EPA estimates for his vehicle (15 combined). Seeing as how he notes a cruising speed of 75mph (or, about 15mph/20 percent higher than optimal for economy), I think that's fairly normal. His old Tacoma was also in range of the EPA numbers.
http://www.fueleconomy.gov/feg/Powe...997&make=Mitsubishi&model=Montero&srchtyp=ymm
http://www.fueleconomy.gov/feg/bymodel/1999_Toyota_Tacoma.shtml
Larger tires on flat roads while cruising at a steady speed might improve the mileage - but 31's were still a factory OEM size. That size is listed in my owner's manual for the optional tires as new.
You might be able to get a *little* better, but not much. Though, from having been through the "max mileage possible" thing on a few cars, I'm pretty confident that driving habits and speed will produce better results than cleaning your injectors or things of that nature.