Kind of a loaded question.
Here's some (but not all) things to consider.
MPG will be affected by:
1. Transmission gearing -- the 200R4 I'm using has a .68 4th.
2. Rear end ratio
3. Diameter of rear tires
Figure out what you have versus what you want and gather up the pieces.
Once you know, trans gearing, rear end gearing, drive tire diameter and weight of vehicle call a cam munufacturer and get their recommendations.
