MPG variation with Cruise Control at 72

My wife and I recently took a trip from Ark. to Indiana and back, mostly on Interstates with a speed limit of 70, during low traffic periods, so I tried this: I set the cruise to 72 (my best guess is 71 actual MPH), reset Trip A, and went for as long as I safely could without changing the speed.
Terrain was flat to Interstate-hilly. Very little net gain or loss of altitude. Nothing like the long ups and downs of the Rocky Mountain west, or even the Appalachins.
I only counted things where I could get 10+ miles without braking out of cruise. On a few longer segments, I had a couple of points where I braked slightly and quickly got back up to speed, without changing the trip consumption much.
My computer generally gives MPGs about 2.5 mpg higher than actual, so I use that number here.
Temperatures were 91-100F, so AC was always on. Winds were pretty calm, except for two of the runs, one of which was the worst in MPG terms.


Best segment: 53 mpg shown--estimated 50.5 actual, over about 40 miles. I'm guessing a bit of net altitude loss.

Worst segment: 47.5 mpg shown--estimated 45.0 actual. There was some cross- or headwind. Maybe a very slight net altitude gain.

So that's a fair amount of variation--do other folks see this much change on the basis of wind and hilliness?

An average here would not make much sense, because I don't meet Bob Wilson's criteria for scientific testing--didn't do both ways over the same terrain, same conditions. However, I would say that a typical segment would have been in the 49 mpg actual range +/- 1 mpg.

Any comments welcome.

 

Right. i screwed 53-2.5 up, didn't I?

I just went back and looked at Bob Wilson's famous chart:

. . .



He indicates 50 MPG at 70 MPH after discounting for the computer error in calculating MPGs. Bob--Was that on flat ground with little wind?
So it would seem my results are not all that different from his.

BTW on this whole trip of 1513 miles, my wife and I got 49.1 mpg. Given the 51 city/ 48 hwy estimates and the fact that we drove about 4/5 of the trip on the highway, I would say this is the first time we have beaten the EPA estimates over a significant distance.

My wife (who doesn't particularly like driving the Prius because the driver's seat doesn't come far enough forward for her and because she hates the split rear window) is naturally better at getting good MPGs than I am even after a lot of driving. She has a gentler touch on the gas pedal, probably speeds up and slows down a bit less than I do (even though I use cruise and she doesn't) and maybe drives on average a little slower than I do.
Ken