300D 2.5 gas milage

Hey guys, I got a question for you all, I just returned from Canada and my car is getting 31-32 mpg highway. I looked at the car specs for my car and its supposed to get 26mpg city and 31 mpg highway. This is good news, but consider the following:

1) my injection pump is leaking diesel, I had it rebuilt by the MB dealer a year ago and it started leaking again a few months ago. so basically I am loosing fuel physically

2) I am only running on 4 cylinder according to a compression test a couple months ago. usually when this is the case, i thought your gas milage is supposed to decrease not increase?

So I have two negative factors but still getting the gas milage I am supposed to, I have no modifications or additives in the fuel.

The question, 1) what is going on? 2) what kind of gas milage are other 300D 2.5 turbo owners getting? This was a 3,500 mile trip driving 80 mph, with zero oil consumption on a 295,000 mile engine.