89 F350 Reg Cab 4x4 460/zf5/4.10 Gets maybe 11 or so in urban traffic, maybe 12 cruising empty at 60-65mph. Mine likes premium. Same per gallon price as diesel, around here. Have not towed in years with this truck; seem to remember getting 7 to 8 towing an enclosed two-axle 6'x12' trailer weighing around 6000 lbs. But that was in the rockies.
02 F350 Reg Cab 4x4 V10/zf6/4.30 Gets 12.5 or so in urban freeway traffic, nearly 14 highway cruising empty. Best tank was a little over 15 running empty from Tehachapi to San Bernadino.
07 GMC K2500hd classic Duramax/allison/3.73 17 to 18 urban freeway traffic. 14 running empty at mostly 80-85 from Dallas to El Peso. El Peso to Dallas, back up to 17, running 80-85. Nearly 20 cruising empty at 60-65. You would need some serious tonnage or sail area on a trailer to even notice it, with this beast.