Well, you do have to be careful due to the fact that as speed (and therefore RPM) goes higher, required power to maintain that speed increases as well.
So you might come into the situation where, when you're cruising in top gear, at whatever speed 2500rpm is, you require the 31.2 torque level, but the speed that is 3500rpm requires 46.8 torque to maintain. With that, even though the engine is most efficient at low throttle at 3500rpm, 2500rpm is better to cruise at, because it still uses less gas.
Since I do not know what speeds each RPM translates to in top gear, and I have no idea what torque output is required to maintain each speed, you really need to get some form of instant feedback MPG device. Either a scanguage (or equivalent), or an MPGuino.
In summation... There are far, far too many variables to be able to say what RPM, and therefore speed, your car is most fuel efficient at. If you have that saturn 1.9, I can easily tell you that your ENGINE is most efficient at 2500rpm and approx 3/4 throttle. But your car as a whole is a different matter.
The only way to know for sure is with an instant feedback MPG device.