Do you mean new as in less than 100 miles on the clock?
Wait until you have 1000+ miles, or better yet 3000-8000+ miles on the
clock and then start measuring. A new engine, more so a diesel, needs
to "run in" before it starts to perform properly.
Also if you do mostly town driving you will not see much improvement
over a petrol engine. Diesels forte is motorway cruising.