We could compare two rigs now that are similar, the Colorado 2.8L diesel and the Ranger 2.3L EB, modern diesel vs modern small displacement turbo. While there is no arguing the diesel gets better mileage, it's nowhere near double. We are talking 19-23 MPG vs 25-29 MPG, which works out to be 4-5 MPG difference. Remove all the emissions stuff from the diesel and you probably in the low 30's, but sadly, those times are long gone.
I've really come to appreciate the 600 mile range of my Discovery 5 TDV6 (1-3/4 tanks of fuel Atlanta to upstate New York) so I'm waiting for a diesel Defender. Having a boatload of torque at 1500 rpm is addicting...........
.....Both the P360 and P400 variants better the old supercharged V-6's EPA city, highway, and combined fuel economy ratings by 2 mpg, although our 5384-pound test car (201 pounds heavier than our last V-6 Sport) averaged just 16 mpg during its time with us. ........."
Case in point - 26mpg for Jack's travels, which with the lighter weight of the new Defender would probably be near 30mpg vs 16mpg for the P400 MHEV which would also benefit from a lighter vehicle and guessing 20mpg. That's + 50 to 60% for diesel, roughly apples to apples. Not insignificant. Now when loaded and/or towing I wouldn't be surprised to see that jump to near double.