Tonneau
 

After hearing the MythBuster's episode referenced for the nth time, I decided it was time to test it out myself, on my own particular vehicle.

Results:
Inconclusive.

I ran several tests.
For the first, I held a constant speed (55mph) until one exact spot on the freeway, then held in the clutch and saw how many miles I got before slowing to 50mph.
After 2 runs each, the difference over about 2 miles was only .1 (cover on was better by .2 once, exactly the same once)

Then I tried accelerating to 75 on level ground, and then coasting back down to 50 and timing how long it took. Did this one in both direction so that any effect from prevailing winds would be neutral, and started at the same point on the highway in case of any slight road gradients. Cover on won again, but only by an average of 1 second (over 20 seconds - possibly significant as a %, but well under the margin of error introduced by the guy pushing the stop watch buttons).

So, I figured I had to bite the bullet and go ahead and waste a bunch of gas running a long test.

I went about 80 miles, out and back on the same roads, at a higher speed than usual (since aero differences should be more marked at higher speed), starting and ending at the same fueling station.

On the first run, between it being an extremely hot day, having to go through some hills, running faster than usual, and having just replaced the stock fan with an electric, it began to overheat so I had to stop and remove the grill blocking. I made sure to exit and stop the engine at the same point on run 2, but of course that leaves a lot of variation. Also, on run 2, judging by the angle of the trees, there was a substantial tail wind on the home stretch.

Between those two, the accuracy of the entire test is suspect, but I wasn't up for spending the 2nd half of my day repeating them.

In the end, cover off won, by a total of .1 gallons less fuel used (the minimum unit the pump measures, and most likely within the variability of when the pump clicks off).

Looks like its gonna take a much longer road trip to test it properly.
In the mean time I will assume that if it makes a difference, its a small one, and that if there is stuff in the back (which there usually is) it will smooth airflow by having it on.

One bright point of the failed tests:
my average mileage, even though I was running at a higher speed than usual) was my highest yet, 24.08mpg
(this is the first mpg check since adding belly pan, and removing wipers, stock fan, mechanical vacuum pump, and alternator belt)

I've said it before and I'll say it again, it is very difficult to get conclusive "test" results on the open road, especially when you are trying to validate such a small variable.

Shoulda used that gas on a weekend getaway or something.

True enough, but I don't have a wind tunnel handy.

Next test will be an excuse for a camping trip.
Hmm. Actually, it will have to be two, won't it? Even better!

It's the tough thing about MPG.

A quick test proves nothing. Drag racing, sled pulling, even Bonneville: A couple minutes at the most and you know who's who and what' what. Not true of MPG. You need AT VERY LEAST 750 miles to verify results. Long tests lets all the variables wash out.

Or .... long tests let changes in the weather skew the results even more.

You all know where I stand on on-road testing:

- vehicle warmed up (drivetrain stable, not just water temp)
- cruise control, set once, cancelled with the brake
- deserted road (runs thrown out where you are overtaken, or catch up to another vehicle - aero interference)
- bi-directional runs
- A-B-A immediately following one another
- multiple runs per set of A, B, and A
- readings taken with a fuel economy computer

And even this isn't always good enough.

On-road testing is really the only practical way to test if you aren't Bill Gates. Besides we all operate on-road. If a person finds something that works on-road, everybody benefits.

In a three-week test you can be sure you will see every weather and traffic condition. Winds from ever direction. Yes, you can go overboard letting comparative testing strech into different seasons. My test season is mid-May to early September.

Yes, it would be optimal to rent Talladega Speedway for tests, but until you hit the lottery, that is simply not practical.

There has been alot of debate on this subject on all the forums I belong to. The only conclusive test for me is that I like my tonneau simply for the fact that I can put items in my truck bed and keep them dry and "out of site out of mind". Does it save me $ in MPG? I don't really know. If it does it would take a very long time to save me enough money to pay for the cost of it. I think its personal preference as to whether you like them or not. The debate will continue on I'm sure.:D

At the very least now I know it doesn't make it substantially worse, so I can enjoy the other benifits

tonneau
 

My test minimum is a 170-mile round trip,back-to-back,mods-on,mods-off,as fast as I can do the change,so temps, wind,baro press,etc. can't vary much.And I mfully warm up the rig before I begin,with at least 22 miles of driving on the highway to reach equilibrium temps for all fluid and lubes.

I tilted the cover.

Not quite an Aerocap.
I wanted to maintain the ability to see out the back window. With the tonneau halfway up the back window, just below eye level, I don't lose any of the visibility of the area right behind the tailgate.
I also still have the ability to roll it up when necessary to accommodate big loads.
My "test" was only 120 miles, and I didn't bother with a-b-a at all (was also calibrating/testing GPS vs odometer) but I did set a new record by almost 2mpg.

http://ecomodder.com/forum/showthrea...tml#post144152

Even if it was a lucky coincidence, the new arrangement gives me more space under the cover, so I can keep it closed for bigger loads, and it gives a little bit of wall to keep things stable as they pile up. And of course I choose to believe that the extra 2mpg was because of the tilt.

http://www.instructables.com/files/d...VL6.MEDIUM.jpg

http://www.instructables.com/files/d...VL5.MEDIUM.jpg