I am not sure where some of you guys are getting where the the worse fuel economy of the gas HD's will be off set by the taxes. According to my math with an average of 15k miles a year, gasbuddy prices in western Washington, and the average fuel economy of each truck per fuelly.com; his best choice in regards to fuel savings would be the 1500 3.7L followed by the 1500 3.0L Ecodiesel and then the 2500 6.7L Cummins in third.
Ram 2500 6.7L CTD - 15.5 mpg at 967.74 gallons used with diesel at $3.55 = $3,435.48
Ram 2500 6.4L - 12.0 mpg at 1,250 gallons used with 89 octane at $3.35 = $4,187.50
Ram 2500 5.7L - 12.0 mpg at 1,250 gallons used with 89 octane at $3.35 = $4,187.50
Ram 1500 5.7L - 15.5 mpg at 967.74 gallons used with 89 octane at $3.35 = $3,241.93 + $440 taxes on $40k truck = $3,681.93
Ram 1500 3.7L - 18.5 mpg at 810.81 gallons used with 87 octane at $3.20 = $2,594.59 + $440 taxes on $40k truck = $3,034.59
Ram 1500 3.0L Ecodiesel - 22.0 mpg at 810.81 gallons used with diesel at $3.55 = $2,878.38 + $440 taxes on $40k truck = $3,318.38