Something that might be useful when figuring out what weight you would be adding to the car, if using metal plate/sheeting:
Shapes Weight and Inertia Calculator
A suggestion for you too, if you're looking for durability, or build quality above coroplast and tape:
Consider go hybrid: use metal to build up mounting points, as a 'skeleton', then mount coroplast (or some other plastic sheeting) from there.
Myself, I've only worked on the front part of my car, and I've waiting for spring to arrive, to consider what to do for the rest.
So far, I've worked with:
- coroplast, which was rudely removed by a roadkill racoon I couldn't avoid on the highway.
- perf-steel, to cover over the same engine bay, and allow me to measure out, and eventually draw up CAD drawings so I can make a proper, metal fairing there.
Regarding that metal pan:
This past summer, I'd made a template for an lower engine cover for my Jetta, since the oil pan on the TDI models WAY TOO LOW, and is cast, and prone to causing catastrophic engine damage when you leak all 5L of oil on the road.
This pan was made out of perf-steel plate, 1/8" thick - not strong enough to prevent damage to the car, but it WAS easy to form. Using, say, 4" strips of this off of a sheet, you can wire tie, or drill to bolt this onto your car at the appropriate points, then mount something lighter (i.e. coroplast, old shower surround panels, etc.)
You could also mount other perf-steel panels securely in spots where heat and plastics don't mix, say, nearby your exhaust.
Using plate over the whole underside of the car WILL add a fair bit of weight, however. Myself, I'm only going to do this for the engine bay, since it -will- provide me protection for my engine, weight penalty be damned. A new engine is thousands of dollars, and the better streamlining will offset the added heft.
Aluminum plate (not accounting for the slight extra weight the diamond tread would add), at 1/8" thick is around 1.76 lbs per square foot, according to that calculator link.
Steel plate the same thickness: 5.1lbs/sq-foot
Perf-steel is around half holes, often more, so that's adding around 2.6 lbs/sq-foot (or less)
Your Echo, if that's the car that's going to get 'plated', is roughly 65" x 163". For a ballpark figure, we'll not count out the areas you'll not plate, like the wheel wells, or that the underside isn't a true box shape. This might come in a bit heavier, however, you'll have overlaps, and mounting hardware, brackets, etc., which probably balance it pretty close.
65" x 163" = 10595 square inches
10595/144 = 73.58 square feet
That means, given my 3 example metals:
Full Aluminum 1/8" plate pan: 129.5 lbs.
Steel 1/8" plate pan: 375.25 lbs.
Perf-steel 1/8" plate pan: 191.3 lbs.
Doing a hybrid, like I mentioned, a "skeleton" to mount coroplast or some other plastic, would come in at a much, much lower weight, for any of these choices.
I won't speculate too much on weight, but it would stand to reason that the full aluminum pan's weight wouldn't impact fuel economy as much as the aerodynamic gains would help.
I'd be more apt to say, a full steel plate pan might be closer to a wash, since it'd be like adding 2 more people in your car (or 3 very skinny ones, or 4-5 supermodels
).
Oh, and veering back on your original question about the diamond plating. If you're concerned about the checker pattern causing airflow disruption: face it smooth side down to the ground, patterned side up to the car, and any surface drag quibbles would be muted.