C10, not C5, right? Also, do you know the gear ratio?
I'm not sure any of the regulars on here have a 292. You might try posting this over at the 67-72chevytrucks.com site also to see if anyone has that combo.
My only experience is with 4wd's. My 70 K5 with a 400 sbc and 4 speed gets probably about 11-12 mpg and my 69 K20 with the 307 sbc and 4 speed got about 10 and with the 454 and 4 speed it is still getting right about the same mpg.