Re: 6.5 TD gas mileage
Well the first thing you need to do is quite putting gas in your diesel engine. Oil burners do not like gas.
With 4.10's expect about 14-16 in town, add +1-2 mpg ea for 3.73's and 3.42's. I get a low of 10-13 pulling my travel trailer, city 15 +/-1, a best on freeway of 18.2 from Seattle to Portland and back going 55 mph with cruise on. Otherwise I'm topping out in the 17.x range.
Your timing could be off, -1.94 is the best, injectors could be bad or dirty, injection pump not working correctly. Dirty/clogged fuel or air filter. Bad fuel, I have noticed differences between brands, and get better mpg's when purchased at one particular station here in town by over 1 mpg. Where do you live. You may have winter blend in the stations near you. Winter blends give you less BTU's per gal, hence less fuel mileage by about 1-2 mpg's, depending upon how much #1 is in the mixture.