it is unlikely you will get any mpg increase from coasting down hills(sounds like a good one for mythbusters)
False. When your engine is idling, you're burning fuel at a pretty slow rate. For common passenger cars, I think it's around half a gallon an hour. Lets say a full gallon for the 4.0l Jeep engine.
Now, lets say your coasting in neutral down a moderate hill at 49 miles per hour. For the period of time that you are going that speed with the engine in neutral (burning fuel at the arbitrary rate of a gallon an hour), you will be getting 49 MPG. 49 miles per hour, one gallon per hour, 49 miles per gallon.
If the hill was steep enough, you could even gain some speed and increase the MPG even further.
(I don't know exactly what Jeeps use per hour in neutral, though Sweeney can probably answer that question for us with his scan gauge.)
Now, if you were coasting in gear, you would have to coast at the correct speed and gear to keep the engine RPM's from dropping below the fuel cut off point. I doubt, on Jeeps, that it is much lower then about 2000 RPMs. If you're coasting in gear higher then that, the engine is burning NO fuel and you are effectively getting infinite MPG. Free travel.
Now, the problem with engine braking is that it's engine BRAKING. As in, there's something (all that metal crap rubbing against each other in the engine) impeding you from moving forward as fast as you should be going under other circumstances. You are actively loosing kinetic energy (speed) to maintain the engine RPMs.
Thus, I only engine brake when the hill is steep enough that if I was in neutral, I'd be going too fast from a legal and/or safety prospective and would be stepping on the brake anyway.
'97 TJ, 2.5l 4 cylinder. 20-22 MPG city/highway combined.