The EPA does not even produce fuel economy estimates for vehicles as large as the F-350, but data I can find gives about 13-14 mpg in real world use, perhaps about 20 mpg on the highway in ideal circumstances. Meanwhile the 2021 Outback (which is not particularly fuel efficient) is EPA rated at 26 city/33 highway, which is significantly better.
When you add in that diesel emits 13% more CO2 per gallon than gasoline, that's not even close, the f250/f350 is emitting at least 75% more CO2 per mile.
A WRX has a combined mpg score of ~23mpg. So even the worst case scenario (a tuner car) and the best average you provide (20mpg) we see that the WRX get takes 4.3 gallons to go 100 miles while the f350 takes 5. The ridiculous tuner car is 15% more efficient than the _unloaded_ f350 with the most efficient drive train.
I'm not sure why you're being downvoted. Diesel pickups tend to get high teens unloaded, depending how they're driven. Mine gets around 20mpg on the highway.
Diesel is more energy dense, so better to compare it to a VW TDI of 40+ MPG. You are still hauling around all that weight. I have always wanted a VW (Rabbit) pickup.
I don’t know why there are no diesel-electric trucks. If you need torque, do what trains do.
