The stock tire on your 2002 TS is 195/60/15. Check the driver's side door sill for the silver sticker. This sticker includes the OEM tire size. The 195/60/15 has the familiar Impreza tire diameter of 25.0".
The 225/70/15 is just wrong for the 2002 TS. The diameter is 2.4" too large. This kills acceleration by making the wheel/tire gearing too tall and making the speedometer read almost 10% too slow. Suspension geometery is compromised as well.
I realize you don't want performance (you drive like a granny). Then why have all of that extra 225 width? You are not going to use it. The extra width and weight of the tires is costing you fuel.
If you want the fuel savings with lower RPMs at highway speed you should have bought a car with overdrive (5th gear) instead of trying to lower RPM with tires that are too large. And what about all that useless weight you are hauling around with 4 oversized 225/70/15 tires?
Just don't hit the accellerator so hard and if you have cruise control for the highway stints, use it. This will save fuel. I'm sure you have heard it before, but you can save fuel by keeping tires properly inflated.
Given the kind of driving you do I'd highly recommend the 195/60/15 Kumho Solus KH16. Look at the test results at Tire Rack. Some of the Kuhmo tires can be an excellent value. I just finished putting a new set of Solus KH21s on the minivan today and so far they are just great.