Last night after getting home, it was obvious how little wind there was outside. Now the question became... Shall we make a mileage run or not.
After supper, I jumped in the car and headed out to check a road that was exceptionally flat for at least one mile. After driving about 8 miles, I found there was quite a bit of traffic there. In fact too much to attempt any runs.
Next another longer road, but not a flat, was checked and again found way too much traffic at 7:00pm.
So I drove all the way back home and checked the congestion of another road, that while possessing a flat section of about two miles long, was slightly up-hill the entire way. Lo-and-behold, this road had little traffic, with about 1 or two cars every minute or so. This road looked doable, so I turned around at a good spot, got up speed and pressed the Insight "reset" button for the Fuel Consumption Display.
I repeated this process three times each direction, recording the fuel mileage after each run. At this point in time I had the tail installed and now it was time to drive back home to remove it, and place the stock Honda lights back on. I did not bother putting the rear bumper facia back on as this would take too much time, since the rear wheel covers need to come off as well just to mount it. This also meant that I had no rear license plate or illumination light.
I found that the two miles really only allowed me to track mileage for about 1.3 miles or so, and the Insight seems to only update the FCD display every 0.6 miles or so. I could not use the ScanGauge to get accurate readings since the lean-burn feature of the engine plays havoc with this reading.
It seems that the only way to really get solid readings would be to discover a flat, unpopulated road that is 5 miles long or so, and drive there for a testing period. That would also mean detaching the tail and leaving somewhere while the "stock" configuration is tested.
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Last night's conditions:
Speed: 56 mph
Temp: 52°F
Wind: Calm (maybe 3 mph wind at times)
Road: runs mostly East-West
- Freshly paved five years ago
- Reasonably flat with slight grade (10 ft over two miles)
- Low traffic in my direction with several cars traveling in opposite direction
- At no point was I closer than 1/4 mile to car in front of me in same direction
- I tried to hug the right side of lane to minimize wind disturbance
- Only had time for A-B testing, not A-B-A
- Test section flat only for two miles
- Marginally long enough for run of 1.5 miles
- Insight has limitation of display updating every 0.6 miles or so
- Finding longer road is highly desirable (Insight limitation)
The tail shows an increase in fuel economy of 10.27% at 56 mph over a short course of 1.3 miles.
This table shows the difference is fuel consumption with and without the tail. Note that underside of the car is 90% smooth, and the original design of the body shape is highly optimized by Honda, so the improvement due to the tail is smaller than MetroMPG's, as has been expected by several forum members.
But how is this mileage increase affected at other speeds?
The following table makes several assumptions about the relation between rolling drag (Crr) and aero drag. It assumes that the mix between the two are equal at 45 mph. With this assumption made, we can then establish how much fuel we will save at other speeds besides 56 mph.
Note that the table below works on the premise that Crr changes directly with road speed, and that air drag changes as the square of the speed.
And this is a graph of basically the same information in the table.
EDIT: parting note: some of the table and graph information still looks funny to me, and I will post another, more clear description later, but in the mean time I hope this helps.
Jim.