/6 Possibilities

Did you ever mention the year of your car. Being from the east and hearing all the restrictions on Caly cars I was wondering what your emissions requirement are. After that you can increase the power and keep your mileage--or improve it. Any mod that makes it more efficient will increase both. Free flowing exhaust, better carb(motorcraft 2100 is a good choice, plentiful,super simple, and lots of jetting options) mild cam, head mill and port cleanup work wonders. No one has mentioned the tranny yet. If you can locate a later one with the lockup and lower 1st gear ratio you can increase the stall speed without losing the mpg at cruise from converter slippage. If you know what tire diameter you will be using you can tailor your rear gear choice to get the cruise rpm's you want.

Good luck