I just took my 2010 Express Van on a 4000 mile trip from Texas to Yellowstone and back. I did manual calculations on each tank of gas to compare to the Van's computer readings. In general the numbers were within 1%, the worst being 1.6%. The gallons used being slightly lower and the mpg being slightly higher. The worst being 1.6 gallons more being put into the tank than the computer showed as used. The worst MPG was 1.6 more MPG computed than my calculation. So I would say that using the computer to tell where you need to fill up next is reasonable if you allow 2 gallons for possible computer error. My worst mpg was 15 (first tank, 75 mph generally uphill) and my best was 19 (decided van was happier running 70) for runs in excess of 350 miles.

 

We did that same trip a couple of years ago... the kids still talk about it
Yeah I would agree that there are plenty of variables involved to consider the on board system 100% accurate, I think GM only intended it for information only...
I monitored and wrote mine down for several years after getting my 07 and it was with in the 2% margin for sure... I kinda got lax about it and stopped tracking it about a year and a half ago...

 

That is good mileage. My van is a 2002 with the vortec 350, and I get 13 city 15 hwy, and maybe 16 coming down from the mountains....
My wife has a 2007 Chev hhr, and the mileage info on the computer is almost bang on.

 

I have the 6 liter LS engine. These are considerable more efficient and powerfull than the older vortex engines. The 6 speed automatic transmission with the option of paddle shifting is great for cruising in the mountains!

 

Yup. I was hopeful that this newer van body style would be better on fuel, (my last one was a 95) but I was shocked to see it so crappy.
The only thing I have over the new engines is the repair and parts cost. I got u there...lol