I was talking with a friend today about doing some FE testing and I told him that my plan was to run no faster then 45mph for the whole test so I could get killer results. He thought that was not a fair test, because most people would not run that slow.
He said it should be at 65mph because that's what people judge for the most part MPG. Plus he said it shows what the whole car is capable doing including the aero of the vehicle.
I'd say the biggest thing that people who aren't into hypermiling or modding for FE don't understand (or tolerate...) when speaking about gains is any time the car is either out of gear or moving with the engine off.
Leave it in gear, leave the engine running, prove mileage gains based on physical changes to the car alone (aero or mechanical)... then you'll have some believers.
I told a few friends about the HAI when I first did it and they were like, "Wait, you drove it normally. You didn't ever coast in neutral or cut the engine off and you got 43mpg... just from that HAI thing? DAMN! What's that car rated to get?" I told them 31 highway and one said, "can you help me build one for my car?" lol
When I do testing, I do it at the speed at which I'll be driving: 55mph (plus or minus a bit.)
In order to extrapolate the data to different speeds, you really need multiple data points. Some engine mods or general driving habits may scale linearly with speed... aero mods would probably not be so linear. For each change, a minimum of 3 data points (3 different speeds) would at least point to the type of scale to expect. Actual results will vary significantly from driver to driver and car to car (and with a whole slew of environmental variables.)
I guess it comes down to the fact that none of us are doing this in a controlled environment... there are so many variables involved that strictly controlling vehicle speed won't be enough to get repeatable/reliable results; we're ballparking it to the best of our ability. But that's NOT to say it isn't an important factor... doubling your mileage isn't actually that great if it only happens at 5 mph. =)
That is correct. I get readings from my scangauge that average about 35-40 MPG in my truck, but it only happens on flat roads when cruising at about 42-44 MPH. I can't do that on the highway. The best I can do at highway speeds is about 18-19 MPG. For this reason I actually get better mileage on 2 lane country roads than I do on the highway. I can putt down a country road at 2 or 3 MPH under the limit and not cause a safety hazard.
If you're trying to convince average drivers of a modification's efficacy, you should control your variables to match theirs as much as possible. That means driving like they do, the same speeds as they drive, same habits, etc.