Around here, diesel is usually 30-40% more expensive than 87 octane gas, so bear that in mind also. If buying a 3.5 EcoBoost 4x4 and leaving the stock tires on it, you can expect about 19-20 average if you run highway about 60-70% of the time. At least that's what mine averaged with the stock Goodyears for the first 40k miles. Some tanks were 20.5-21mpg. I lost about 2mpg going to 34" E-rated all terrains. If going diesel in a new half ton pickup, it's going to have to average around 24-25mpg to break even on the fuel costs versus regular unleaded gasoline.
