318 Gas mileage?

My 318 consists of near stock compression, XE268 cam (.477/.480 lift), 318 heads, Performer intake, 4160 600 cfm Holley with 70 front jets, headers, dual 2.5" exhaust, 904 with shift kit and 2500 stall, 3.21 gears.

Normally gets 12-14 mixed milage (50/50) and 18-20 on all hwy driving.

Its got to be tuned, jetted, and timed properly.