Seems a little thirsty... how are you driving? Above 55 mph economy will drop. Aggressive driving will also kill economy.
9000 lbs doesn't seem that heavy for rigs like ours. Ambos are all 9k or more.
On the highway I'll get more than double that... it being diesel shouldn't make that big a difference. Last highway jaunt I calculated it at 21 mpg. Around town it'll easily get 12 mpg.
Yes, it is known that the Windsor 5.8s are thirsty under load and like to run at high RPM, out of overdrive. And it does rev really very well, sounds awesome, at the expense of mpg. For the 100-mile test drive, I drove it like that pushing it, flirting if it would even start to knock. It drives nice on overdrive, at the expense of driving in the slowest lane and everyone passing me.
So it seems there's a consensus that 8mpg is too low for other owners of a rig this kind around 9000#. My Cummins-powered 4x4 25ft Class A motorhome at 12,500# gets at least 12 mpg all day.
Nevertheless, for now, I accept the mpg is down at the bottom of the spectrum. I hope that with the timing bump and this next full tank will yield at least 10 mpg. But I won't be holding my breath. It is what it is
