Your 98 SC2 has only one coolant temp sensor. Pre-96 models had two. One for the gauge, one for the computer (a better system, IMO, but oh well)
28-30 MPG doesn't sound to bad for a SC2, but the coolant temp sensor could still be off. Unfortunately, the are prone to failure as the car gets older.
Changing the sensor is an easy job. It's located on the right side of the cylinder head, below and just behind the top radiator hose.
A few tips for changing it: 1) Loosen the coolant reservoir cap to let the pressure out of cooling system. Then re-tighten the cap. This will minimize coolant loss. Have the new sensor in hand ready to install when you remove the old one.
2) Change the sensor when the engine is cold (no burnt hands)
3) Inspect the electrical connector for corrosion on the terminals. If they are green and corroded, you'll need to splice in a new connector as well.