Depends on how heavy the shoe I am wearing that particular day. Driving briskly This is what I've averaged:
with 4.10's on a 26 inch tire naturally aspirated I was lucky to get 15 on the highway, got like 12-13 in town.
Still on the 4.10's, but now on a 28 inch tire with a blower, I'm getting 14 in town, 19 on the highway.
Switching to 3.55's, and back to 26 inch tires (Less rotating mass/lighter), Expecting 15-16 in town, 21-23 highway.
If I'm wailing on it, I don't even TRY to keep track of it... I burned a quarter of a tank at the drag strip last time out (10-12 passes)