I don't post here much, I just like to read what all of you guys say but here are my 2 cents.
Make a car with less horsepower, this is one of the reasons why the Metro gets such good mileage.
Make a light car, once again the Metro is a perfect example of this.
Make a car with a Camless engine. I don't know how many of you guys have heard of this, I would imagine most of you had.
Here is a good camless engine link:
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Transmissions: I have little experience with the new CVT (Constantly Variable Transmission) I got to drive a Dodge product with this transmission in it but the car seemed fat and heavy, typical of most American Cars.
We got in a discussion at work about the mileage of new trucks, the guys at work were saying how these new trucks get the same mileage as trucks from the past. I pointed out that these new trucks are weighing a lot more and have at least 2 times the horse power. Imagine a new truck with only 150 horse power and a reg cab long box, I wonder what kind of mileage this combo would get. This would mean though that the truck wouldn't be able to pull 10,000lbs without slowing down going up a hill.