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Well that's the rub with fuel economy. Add some nice aero like your shell, but it adds weight. Aero is king at speed, and the faster you go the bigger the difference it makes. Weight is king for stop and go. The more you accelerate, the bigger the hit because of the weight. Of course you know that.
At Ford several years ago, weight was valued at (I don't remember the exact numbers) 1 pound weight savings in the car was worth $100,000 for their high volume family cars like the Taurus. Part of that equation included the tax and fleet ramifications of corporate fuel economy numbers. The $100K had more to do with facilities than car cost, of course, and the number was different for different vehicle models based on volume and fleet positioning. Anyway, I mention that because I'm guessing your cap probably weighs 80ish lbs. That doesn't seem like much, but if you think about the energy it takes to start and stop that mass over and over, then it adds up.
The interesting part, and one of the reasons I'm attracted to Ecomodder, is you can control what you do on the acceleration. I have to admit I love a WOT accel, and I have a car with more engine than sense, but I understand the consequences, so mostly I'm in the granny category. Size is most important (though that's often dictated by function), then technique, but after that, aero is the big fish -- which you've attacked with style.
Your shell should serve you very well as time goes on. The payback won't be fast in terms of savings, but you probably already have full payback in pride of workmanship and accomplishment! Congrats again on the great work! It's absolutely delicious.
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