My '13 F250 6.7L with 2wd, 3.31 rear and Supercab got 11 mpg towing two Polaris RZR's on my 24' bumper pull tandem axle equipment trailer. My '13 Ecoboost gets 8 mpg for the same load.
The F250 was configured for the maximum fuel economy while my ecoboost is a King Ranch with the 6.5' bed, 4x4 and loaded with every option available.
I was not impressed with the towing fuel economy on my Powerstroke. I expected 14-15 mpg not 11. The Powerstroke got better unloaded fuel economy by about 3-4 mpg. In the city they are about the same. I run premium in the ecoboost so that makes diesel and gas about the same but with the diesel option costing $9000 I will not buy another one unless the things I tow get so heavy that I have too.