If you are willing to make a few simple splices in your wiring harness, the ~$40 MPGuino that has been developed in the Instrumentation forum might be worth a look. If you are not comfortable, you can wait a few months for what will hopefully be the next iteration: an OBD-II model.
If you are doing mostly city, instrumentation will be your best investment. The next best thing would be losing all the mass you can out of the car. Remove the spare & jack, clean out the trunk, and rip out the seats if you want to go wild (do you really
need four seats
).
As for the highway, I'd look into rear wheel skirts, wheel boattails, wheel splitters ("spats"), and underpaneling. All of these mods can be done cheaply with coroplast.
Lastly, a mod may still help even if it doesn't register. A 0.5% gain may be imperceptible. If you add enough small gains together, you'll eventually see a noticeable change.
- LostCause