The trick to getting 18+mpg is full synthetics everyewhere, a good tune up, good gas (Shell 93 octane), the windows rolled up and the cruise set at 65mph.
We went on a 650 mile trip one Sat last year with my '97 and we got 22mpg with the wife, me andthe two kids and the Coleman cooler packed with food and Pepsi.
Steady speed and a light foot is the key. 75mph and you cant hold that speed for longer than a couple of miles, you get higher RPM which means more gas, more wind resistance which means more gas, and having to speed up and slow down all the time waste's gas, slowing down is wasted energy.
The #1 thing you can do to increase MPG is to slow down, it is free to do and will guarantee you results.

 

I have a 99 Discovery I. We have had it about 18 months. So far, new water pump, plug wires, oil cooler lines, front crank seal, tires. I bought several extra remotes and several gallons of simple green to clean up the oil leaks (still need to fix transmission lines, oil pan sealand some sealon the transfer case......). We get 12-14 in town, hills and stop and go, and 18-20 on the highway at 75 in So Cal conditions. the truck looks like it came off of the showroom floor, is fun to drive, has airbags and abs, seats 5 and goes to the mountains or the beach with equal aplomb. Our 1999 Landcruiser has 165k miles on it also take premium gas and has taken numerous trips to Utah, Wyoming and Montana. Runs like a top, nothing done to it ever but oil changes at 3500 miles and gas, 14 around town and 16-17 on the highway, ususally at 80 with a family of 5 plus lots of gear and a Packasport 115 Carrier on top. If I needed to chose between the two, Landcruiser hands down. And it does not and never has leaked one drop of oil of any kind. It also holds its value like it holds its oil, wish I could say that about the Land Rover.......Phil