My first fun data graph
 

https://i.imgur.com/hOLS6OU.png

It's a pretty wide picture, so I linked it instead of embedding.

I got an OBD-II bluetooth dongle and the Torque app. I logged a trip from downtown Wichita to El Dorado, Kansas, then imported the data to Excel and made a line graph depicting speed and instant MPG. For the first part of the highway drive (mostly flat, with some gentle slopes) I put the cruise control on at 60 MPH. Then I did some pulse and glide between 60 and 50 MPH without shifting into neutral. After a few miles of that, I did more P&G except that I put it in neutral for each deceleration.

My averages (which are totally off base, but give an interesting data spread):

Cruise at 60 = 42.7 mpg
P&G no neutral = 84.1 mpg
with neutral = 91.3 mpg

Data was collected twice per second for about thirty minutes.

So this is in a 2000 Pontiac Grand Prix 3.1L V6 Automatic. The only modification I've made is to the air filter, which is a low restriction cone filter. It's my wife's car and she doesn't want me making any changes to it that she can see or feel, so I just haven't done much to it at all.

The bizarre mpg readings might be proportional to real world usage, but they aren't even close to what I put in the tank compared to distance traveled. That cruise at 60 mph is probably closer to 30 mpg than 42. The best I ever got in the car was 33 mpg (calculated at the pump), which was with pretty constant P&G on a familiar stretch of highway. So I have two ratios now that aren't very close: 42/30 or 91/33.

Still, I think seeing the difference in fuel consumption depending on driving style is pretty striking in the graph.

Nice data collection...showing VERY interesting difference between "with" and "without" neutral during P&G.

Yeah, that surprised me quite a lot. I'm wondering if maybe when it has zero throttle that the fuel mixture goes super lean, and when it is in neutral, it runs a regular ratio for idle.

Or perhaps there is some resistance on a neutral gear in the transmission? My knowledge of automatic transmissions is pretty low, and this car has a somewhat faulty one anyway.