When I got my stock rig a month ago I was blown away to be getting over 20 mpg's going over the Cascades to Bend and tooling around at 60 mph...
I noticed my speedo read low though...like 4 mph slow at 60 mph. I threw on 6 ply AT3's at 265/75/16 and have brought the speedometer back to accurate, or a lot closer. I have been using a phone app to track my distance and the odometer is reading about 1 mile less every 30 miles traveled...so I clock 29 when I really went 30. Had the same thing happen in my 04 taco, but would clock 9 miles for every 10 driven.
End of the day...I am running just shy of 18 mpg with a 50/50 mix of highway and city.
I highly recommend checking your odometer against gps, seems to get off on these guys. I didn't do it with stock tires, should have...I bet it was clocking more miles traveled than actual.