A friend of mine is freakishly tall (6’6" or maybe taller) and only fits inside big, gas-guzzling vehicles. He doesn’t have the scratch to upgrade to the hybrid SUV just yet, but he wants to maximize the fuel efficiency in his van. He is already looking at removing the unused seats, maintaining tire pressure/balance/alignment, and so forth. What I’d like to know about is making a custom belly-pan to cover all the nasty bumpy bits on the underside of his van.
(1) Will a large sheet of fiberglass, cut to size and bolted into place, serve well enough?
(1b) Will that actually reduce the drag at all?
(2) Should he drill drain holes anywhere?
(3) Is taping over the door seams going to have any appreciable benefit?
I have read on one article a few years ago that most cars running gear ends up running way hotter than normal if a bellypan is fitted (accellerating wear) and that’s pretty much the reason that very-few production cars have come with them, and why no companies will make them as accessories for regular cars. They do cut drag significantly, but they cause acccellerated wear on a lot of different parts underneath due to the higher temperatures those parts reached. Regular ground effects (an air dam and rocker panels) would probably be about as effective, and there are aftermarket companies that make those parts (for that particular vehicle I don’t know but anyway, I’d be surprised if nobody made a ground-effects kit for it).
Taping over the door seams is so minor a modification that I’d bet it wouldn’t amount any useful difference.
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There’s so much pressure and turbulence under a car that you have to count the area underneath as part of the “faceprint,” the hole you punch in the air. However, an air dam under the front bodywork forces some of the air around the sides, which reduces drag a bit.
Minor changes can mean a lot in fuel mileage. Remember the drip rail above the doors on cars in the 60’s? Now, they are internal, and that gives about 2 tenths of a MPG. Hood ornaments died for MPG. On cars that have them, you can gain about a tenth by turning the emblem sideways, like a fin.
Tape the door seams? Some cars use the door seams as an exhaust vent for the heater and A/C. Taping it could screw that up.
Really, though, it comes down to a cost/benefit question. If spending a thousand dollars on body mods can give you 3 more MPG, how many miles will it take to pay for that? If you trade up to a less thirsty ride, how many years of more efficient driving will it take to pay for the upgrade?