My theory is that you see some improvement in slowing down, but honestly, it isn't the speed per se, but the time it takes to get there that is most damaging to mpg.
Say if you drive 75 mph, and you accelerate at 7.5 mph/sec, it will take you 10 seconds to get there. But if you accelerate at 7.5 mph/sec and only go to 65, you get there a little over a second shorter time, and that time is significant in the amount of fuel burned. Same with coasting. If you see a stop sign up ahead, and coast 1 mile to get to it, vs staying at 65 right up to 100 feet from it, you have 5180 feet of no fuel being burned vs nearly a mile of fuel being burned still, just to heat it up in the brakes.
I insist that there is little fuel economy savings at 65 vs 75, as a highway trip would show. If you get up on the highway for say 200 miles, nonstop pm cruise control, at 65 vs 75, I would you would not see much, if any, mpg difference. I speculate that you would see around .2 to .3 mpg difference if you are in hilly or windy terrain, not the 10% or more the politicians and so called self professed experts tell us. Now, if you are driving city/highway, and going from 0-75 often, yes, this will have more significant impact on fuel economy. But constant speed on the highway will not affect it as drastically as we are led to believe.
More depends on how much you are on the loud pedal than not. If you can be gentle (say no less than 9 inches vacuum under acceleration) under acceleration, vs heavy throttle starts to keep up with that Ferrari, you will do much more to improve your mileage. I also allow only the first 20 feet or so in first the plan for more acceleration in second and third. Traffic supporting me driving like this, I don't impede traffic flow, and will not, that ticks me off.
My experience with my 3.42's, mech fan and all, is 70 mph I get 23 mpg. Which is EPA rating for my car on the highway. Even going 75 I don't see much drop if any in my average mpg readings (and you know I track it carefully).
Look at fueleconomy.gov for the garage, you can enter in your own values there, even have it calc it for you and track it. There is only 2 Fleetwoods in there (as of last month), mine and another. I have over 50+ tanks of fuel in there with a solid 19 mpg AVERAGE for ALL driving since Feb 2005. Over 20K miles.
The resolution on fuel economy for less than 1 tank is poor, don't judge it by that. Take several full tank (try to fill to same point in tank) readings and average all the miles divided by the gallons. This is more accurate, vs a 100 mile trip. You will then see tanks like 12 mpg. But likely the next one might be 20. Average the 2, and it will be more representative oif reality.
Fuel economy is tough to characterize accurately. There are a ton of variables. Best we can do is shoot for the highest resolution that we can get, which is long term accurate tracking of conditions and type of fuel used, driving style (note it, if you were throttle happy that day beating that Hemi Ram again) or driving like a grandma, temp, weather, where you got fuel, is it ethanol or not (this DOES has a deterimental effect on mileage, between .4-.8 mpg less AVERAGE, I have a LOT of data to prove this, avoid if possible, and badger your politicians to stop advocating it, it is making a fuel economy loss for all Americans to the tune of millions more gallons of fuel needed a year, just for political gains!). So accurate tracking of these things will give you a better understanding of what affects mileage and what does not.
