I was told by the guys at the local Goodyear store that these "low rolling resistance" tires designed to increase fuel economy are a hoax... in their words. They said that throughout the life of the tire you may save about ONE tank of gas, which isn't enough savings to justify the additional expense of the tires in the first place.
They said "Buy cheap tires and keep them properly inflated - don't buy into the hype".
Get the proper ratio tire... that's about it. Low rolling resistance tires are a scam and they hurt performance. They can also be damaged by high speed and there braking performance is disgusting. How they allow them on the road is puzzling.
You could install a lower gear... but 400 miles a tank isn't bad.