Cruising RPM. can it be to low ?

I had a 2000 WS6 Trans Am LS1 with the 6-speed T56. At 65 mph it got BETTER mileage in 5th gear than 6th! At 70mph I could almost use 6th gear. Yes, you can lug the engine too much and your rod bearings will hate you for it. We are not talking diesels here.