2-4 hp is correct. However, that does not equate to 3-5mpg. It would be less than .5 mpg. As mentioned, the easiest way to improve mpg is to slow down. The most painless way is to add pressure to your tires.

Other ideas would be to remove your side mirrors. (a Honda Insight has only one side mirror). Windshield wipers, roof rack.

Next step would be to fabricate some smooth covers for your headlights. Next could be a full belly pan underneath the truck.

Oh, I know, a torpedo shaped appendage to the back of the truck.

Do what Ford does. Use 5W20 oil. That alone would give you 0.25 to 0.5 mpg gain.

How about some tall thin tires.

Now we need some good pics. <img src="/forums/images/graemlins/kewl.gif" alt="" />