When I had my diesel pickup, I averaged about 27-28 vs. the 15-17mpg the gas version of the same vehicle got... had the gas truck been an automatic it would have been even worse. I had a good mixture of city and back road/highway driving.
Now, that can vary with what cars you are comparing and stuff... but for the most part a diesel is more fuel efficient than a gas model by a decent margin.
Except VW. Dieselgate killed their vibe.