All of my older carbed trucks got between 12-15 mpg in mixed city/highway driving....didn't matter if they had a straight six,305,350,400 SB or a 454,and the mileage was about the same empty,or loaded to the hilt....my van and a '79 Bonanza with a 250 six got 18 mpg on some longer trips,but they are no better than a V8 in city driving for gas mileage..A 6.2 diesel will get maybe 5-10 mpg more,in my opinion its not worth the extra expense of having to buy more costy diesel fuel and put up with cold start issues and the higher cost of parts, that are getting scarce for them now though..I would prefer a diesel off road ,no carb to flood when climbing or desending steep grades and great low end torque...