Not sure about the features that your trip computer can display, but a handy one to monitor is the 'instant mpg' function. This is a handy tool to adjust your driving habits.
If you don't have that function, here is a great tool that can help out...
http://www.scangauge.com/products/scangaugee/
Myself and a few others have these... they have a heap of other functions that monitor the cars general 'health' as well.
6 speed auto....
Many cars I drive are 5 & 6 speed autos. I found there is no need to have the car in 'D' when driving around town at speeds up to 50km/h, the cars run much better when selected in 3rd gear, which also eliminates the tranny changing up and down between 3rd and 4th constantly.
They still operate through the gears up to 3rd with no problem.
In your case the car may sit better in 4th, it all depends on the ratio set up, you might want to experiment a bit.
42 mpg.... I reckon you're doing well already.
Anyway....best of luck mate.