First truck ... the truck computer says I'm getting 8.4 mpg?

Just purchased a 2000 dodge 2500 with a  360 v8, 265x75x16 tires, quad
cab, 8' bed, I was thinking about the diesel but since it would be
used just one in a while, I went the gas engine route.

 I expected around 16mpg highway with the 4.10 rear doing about 55,
but the truck's computer is saying I'm getting 8.4 mpg with an
unloaded bed? I went out on the highway and cruised about 55 for 10
miles and still reading a tenth better at 8.5 mpg.  When I fill up I
will hand calculate... but how accurate is it to read the mileage from
the on board computer?

Is there any sensor or other issue that could justify such a bad gas
mileage if I find out that it really is that bad?

Thanks in advance.