I always felt like 5th was for the open road 70+. At 90, I'm wondering where 6th is. When I'm in stop and go traffic I catch myself reving the engine when I stop and go. I'm sure that sucks the gas more than if I was cruising.
The 1800C is a long-stroke engine that was made to be run at relatively low RPM. I can cruise at 35 in 5th gear and accelerate away smoothly without downshifting. You certainly don't want to lug it, but it's not a short-stroke engine that needs the RPM to be kept way up there. If the engine is running smoothly and will accelerate away smoothly it's not lugging.