Hills vs. gently rolling hills.
 

Flat ground doesn't really exist, so I won't comment on this. But I have observed that, it seems, significant hills are required to achieve long enough coasts to really bump your mpg up over what you could otherwise do while DWL on gently rolling hills. Does this make sence?
I've recorded 54 mpg in mountains of western PA, but only 51mpg in the rolling hills of the the Carolinas coasts. Are these two mpg numbers too close to declare a clear winner for better terrain? What have you observed?

I think the difference depends mainly on your aero and rolling resistance. The less resistance you have one the car, the more you can take advantage of hills with less inclination. I've noticed this especially with my recent wheel swap.

Part of the issue I think with slight downgrades that you can't EOC on is that even if it would only take 1hp to maintain speed the car needs to put out like 4 or 5 hp just to overcome all the losses in the engine and transmission, which lowers potential mileage around those sorts of hills.

I would think that slightly rolling would be better. Be able to keep it in high gear. Long downhills are nice, but I don't gain what I would lose climbing the steep hill.

Others may have different results.

No, I think you have it. My nightly climb is to 4,675 feet over 14.7 miles. I never get it all back on the downhill, as my best tank (climbing the mountain only once) in the Saturn was 1.29 mpg higher than my best effort tank with the nightly climb.

I drove 7238 miles last month on a cross-country trip in my Scion, and got my best mileage in the mountain west (going both directions)

My worst mileage was in the flat lowlands of the southeast, from Texas through Louisiana, Mississippi, Alabama, Georgia, and Florida. I attribute that to E10 fuel, having to run the AC, and using cruise control a couple of days.

All-in-all, I averaged 44.7 mpg over 35 days. My best day I got 59 mpg in the morning driving 165 miles from Gallup NM to Tuba City AZ, and 50+ mpg in the afternoon continuing on to St. George UT - a total of 54+ mpg avg over 370 miles. I averaged 51.5 mpg for the last 1213 miles from Gallup back to the Bay Area. The best roads were long downhills. Even if I couldn't keep my speed up coasting down them, I was able to gain mileage on every downhill pulse and glide cycle - more than I lost climbing the uphills in the first place. On one memorable downhill from Donner Summit to Reno, I averaged 75 mpg for 40 miles.

When I recorded the 54 mpg tank, it was without the SG. On this most recent venture I had the scan gauge and noted that I easily got up to 53.5 mpg in the hills of south east PA/DE/MD, however when I hit the gently rolling hills of Virginia and the Carolina's, I struggled to get 50 mpg. I'm driving an automatic, so no EOC, but I do coast in gear as much as possible and thats where this observation comes in. It was quite frustrating watching my mpg tick down from 53.5 to 51 as I would climb those gently rolling hills only to find I could not coast enough distance to make up the loss. The best technique I could come up with was to minimize the loss going up, going as slow as 45mpg so I wouldn't take a big hit, then gas it a bit to get up past 50mph then just feather the throttle for as long as possible, but it still took what seemed like 30 times the distance of the climb to gain back the loss and if there were several hills back to back, it was just a loss. I'm looking for advice on tackling this type of terian and to see if others have noticed this????

A guy in the hills with an Escort claimed over 100 mpg (or was it 400?). Must have been the hills.

Are you speaking of COZX2 from Gasavers?

Yup

Yea, I pretty much read everything he wrote on there. Being at 10,000 feet and driving in those hills along with his technical expertise and persistance to learn how to pulse and glide optimally got him the 100+ mpg numbers. I have copied many of his mods :)

SVOBoy, how much do those insight wheels wiegh?