Tire Dimensions Made Simple - Discount Tire
Here is a link to a tire calculator that will you can try different combinations of tire sizes to get the desired specs.
Stock tire size the discoveries came with were 29" tires, which would put the disco at a height of 78". This gives you a difference between your disco's height and your garage of only 6" with your disco at stock!
A popular tire size is 275/65R18 which would give you 32" tires and would increase your height (tires alone) by 3", therefore that gives you a total of 3" left to the top of the garage door.
So the question is do you a really snug fit? Then get the 3" lift. If you want an 1" of clearance then get the OME 2" lift.
*Take into consideration this is with no roof rack. So if you have one this will affect your outcome as well.*
*That calculator will also tell you how much your speedo will be off if you upsize your tires*
I do not have any advice of different tires or mpg so I will let someone else chime in with that.