You might like this program. It basically lets you play with the effects of different aero modifications. It's really designed for bicycling. But it does allow you to determine what sort of effect chopping off your side mirrors, etc.
1) Keep in mind that it doesn't directly indicate HP to MPG. This is something you need to experiment with yourself to get a chart.
2) It also can be tough to figure out the Cd effect of different changes. However, it puts a scope of effect on a change. It also indicates how drastically important aerodynamics are, and how big of a difference a few mph makes.
Note: you also need to estimate the effect of your tires. Hands down the Bridgestone RE92 is the BEST tire you can use and I researched to get the rolling resistence of it. You guys might consider running this tire if you don't already.