Weight reduction - not always good?

[Bill in Houston]

Try this on for size. I have two Honda Elements. One weighs 5000 lbs with me and my stuff in it. The other weighs 200 lbs due to the extreme lightening measures I have taken.

In the heavy Element, I drive over to Austin at 90 mph. Because it is so heavy, I can shut off my engine, or just coast in neutral, on the relatively shallow downhills, and then apply power going up hills. My engine runs about 50% of the time at about 40% efficiency.

In my featherweight Element, I take the same trip, also at 90 mph. I don't really notice the lower rolling resistance because I am going at a speed where aero losses are a huge percentage of my overall power requirement. Since my car weighs 200 lbs, it has very little kinetic or potential energy, and I never ever get to coast. So, my engine runs at 25% efficiency 100% of the time.

In the contest above, my heavy E gets better mileage than my light E.

A few assumptions are required to make this work.
-I am able to operate my engine at 40% efficiency when I want to (i.e. I know where the best point on the BSFC chart is and I can always hit it.
-I am driving at a speed where rolling resistance is small compared to aero resistance.
-I pretty much never ever hit the brakes.

I think most of us could buy that if the assumptions are followed, I really could do better in the heavy Element.

If you buy all that, LCA, then my work is done.

BUT, the real world is different from my little situation I made up. We don't go 90 mph, and we do have to touch the brakes, and we can't operate right on the max efficiency point. So the question is, on the particular trip you are making today, what is your optimum weight? For stop and go, drive the light Element. For a nice long highway trip on rolling hills, maybe drive the heavy E.