No guarantees on this but check your air filter. It may be dirty and that can have a negative impact on you MPG. When I bought my current Z( originally an Arizona car) the filter was filthy. MPG improved by about 2. A taller 6th gear is good in theory but in practice on our car you'd probably just have to downshift more often to maintain speed. EPA is 38 highway, so at 55 you're already beating that.
Mine's supercharged, so I have a harder time getting 40mpg than you, but I did get over 50mpg when I drove 50-55 mph most of that tank of gas.... Gave up on MPG's in place of fun. Changed my outlook on life.
Tire type and (slightly more) air pressure help too, but more trips of shorter distance dropped the MPG's, as with any car, but the percentage of drop in my 1st Gen Insight and CR-Z was more noticeable. For instance, if I didn't drive the CR-Z to lunch (another short start/stop cycle, where the car didn't have a chance to fully warm up,) the MPG's wouldn't drop the 2-5 MPG's for that day of work of a 50 mile round trip, compared to NOT starting/stopping for that extra lunch trip.
I think if you just raise the tire pressures from 30 psi to 36 psi, you'll see a bump up in MPG's, but keeping it mostly in NORMAL mode and taking off a bit briskly to get up to cruising speed (and maintaining it) is better than taking all the time in the world to get up to speed, thinking you're being frugal. What I mean by that is, the IMA assist is more present and helpful, the more aggressive you use the throttle on take off, without being too aggressive, that is.