A couple of rules of thumb, gathered from various sources that know a lot more about car engines than I ever will:
Aim for 40-45% of engine redline
Aim for RPM at (60% of RPM at which engine produces maximum horsepower)
In several cars I've done the math on, these two formulas produce results pretty close to each other. For my own car, it works out to (3200 to 3600RPM) or 3480RPM, depending on which formula I use. Over ten years of driving this car, I have noticed that maximum fuel economy is achieved at 3500 RPM. Not coincidentally, my engine is most efficient right where it should be, according to the experts. So use either of the above formulas, and you should be pretty close to where you want to be. If you can't find the horsepower-torque curves, you should at least be able to dig up the manufacturer's published horsepower specs. for the engine, showing (for always list this by maximum horsepower RPM, so just multiply that RPM by .6 to get a pretty good estimate of what to aim for.
Unfortunately, it might not be that easy if you own a slushbox. :( But I gathered from what you wrote that you have a real tranny, so you should be all set. -Dave
