Something to think about this this. The ability of a car to maintain a constant speed is more related to drag and gearing than anything else (highway mileage). Weight sucks fuel big time for acceleration, but if gearing and drag are good, the maintenance of speed is pretty frugal. So if the vehicle that you are looking for the transplant from gets 60 mpg and your goal is 100, you need to determine how you will get there.
I do not have definitive proof, but the drag CD on an open topped spyder might actually be WORSE than the much larger fronted but pretty slippery shape of a Jetta. Gearing can play a factor, but I'm not sure how many options you will have for that if you utilize the Jettas gearbox. I suspect they are already pretty leggy in the gear department so I don't think you'll get huge gains.
Yes, the weight is less, so the maintenance of speed does require less energy (but less than most people think). But the question is, given the unknown drag CD, is the weight difference going to be able to get you to 100 mpg? I've seen diesel motorcycles that get pretty close to 100 mpg, but they are definately much smaller and much lighter than any spyder.
Just throwing out some thoughts. Not trying to talk you out of it, but if your goal is 100 mpg, I think that this is the wrong direction. A very small displacement diesel/turbo-diesel acting as a generator to charge a battery pack for a vehicle primarily motivated by electric force is a more likely answer. Think of something in the sub 1-liter range - maybe more like 500 or so CC. Go lithium ion in the battery packs to save weight. Look very hard at the side pods for battery pack placement - lots of dead space. Consider Thunder Sky for a battery pack supplier. Good system.
angela