1. Yes, because on most cars it is not "adding" frontal area- it's simply pushing that same air to "cleaner" portions of the car (sides, not underneath).
2. Generally we say if you have an air dam you don't need to do belly pans. The former takes much less time and effort for nearly the same result. Then again, a belly pan will be easier and more effective on a body-on-frame vehicle than on a unibody.
Buy a roll of lawn/garden edging and attach that to the bottom of your bumper. If your Suburban has the stock air dam already, add the edging underneath it, as the stock one isn't deep enough (I've studied the parents' Yukon).
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biggest (changeable) factors in increasing economy would be weight, frontal area, drag, and rolling resistance (both mechanical and tires).
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Weight is hard to shave off in meaningful portions, and it can cost a lot. Get lighter wheels if possible, and remove the spare tire.
Frontal area is VERY hard to shave off. Deleting the mirrors and roof rack and installing lowering suspension are about all you can realistically do.
Aerodynamic drag is HUGE. My Ford Probe got 46% better highway MPG with JUST aero. 41mpg over 28 EPA. If you're going to put your time into something, put it into aero.
Rolling resistance is also important. Get thinner oil, different trans/diff fluid. There are threads on this. Tires can get expensive, though 10% improvement with LRR tires seems to be normal. And yes, whatever tires you have, pump them up to max sidewall!
I'm moving to Detroit in a couple weeks so if you need a hand don't be shy
Cheers!
PS- You'll be lucky to break 20mpg with a Suburban.