Hammie, my '83 240D used to average about 28mpg on my 90 mile a day highway commute. They definately get better mpg on the highway.
Always keeping the engine low in the rpm range will result in it carboning up which is bad for it. Theres a balance to be attained here. Probably if you do some good highway driving and get the engine good and hot you won't have anything to worry about.

Weepy fuel lines are fuel getting lost as is not getting all the way to operating temp...