Looking for help/advice on truck purchase

[RStewart]

I appreciate the breakdown of this. I never would've thought that a truck would get better fuel mileage running 70 vs 60 when heavy.
I guess this does make sense though. I used to drive a 1997 KW T600, 430/470 Detroit, 10 speed, & 3.90 rears. Set the cruise at 75 mph (1800 rpm) & the truck avg 6.5-6.75 mpg. My current DD15 won't do that good at that speed.

These particular trucks are geared all wrong. The 386s have 3.36s & the 579 has 3.25s. I guess I could run in 10th instead of using all the gears.

That's interesting that the sweet spot is so high. My dd15 sweet spot is around 1350 rpm. My truck has a straight 10 & 2.64 rears. At 60 & a light load with minimal wind & hills I can get high 9 mpgs to low 10s. With a heavy load, wind, & hills I've seen it drop into the high 5s but usually mid 6s.