I live where the weather in the winter gets cold, and the summer gets hot.
I've had (3) 3.9 Dakotas.
One 1990 standard cab, long box, 2wd with a 5 speed. That Chassis is under my 48 Plymouth coupe and gets a consistent 17 mpg mixed city and highway, and a consistent 20-22 on the highway at 60-65 mph. It normally gets driven when the outside temps reach above 40 degrees.
The second Dakota was a 93 standard cab, long box, auto trans, 4x4. Most of its driving was in town, during the winter, with temps anywhere between -20 and +40 degrees. Because it was primarilly driven in the winter,and used around town, it spent much of its driving time before, or right up to the point, the motor reached operating temp before it was shut off only to sit until it was completely cold again. That one got a pretty consistent 10-13 mpg. The few times it was driven on the highway, it got 16-17 mpg at 65 mph. If you drove faster, the mpg dropped like a rock.
The 3rd Dakota is a 96 extended cab, short box, auto, 2wd. This one is mostly a winter ride as well, and spends most of its time in town. It also gets between 10 and 13 mpg. Mileage drops to that 10 when the outside temp drops.
Sometime this summer I intend to put a 49 Dodge truck sheet metal on a 93 Dakota chassis with the 96 318, OD auto, 4x4 drive train (I have 44K mile drive train donor with a rotted out frame, and 93 rolling frame that came with the sheet metal.) I expect I will probably get in the 13-17 mpg range. That truck will be a year round driver. I don't generally get trucks to use for their gas mileage, I tend to use them as trucks and the gas mileage is secondary. Gene