Need help on understanding gas milage
I installed a GM 350 290 horse crate engine in a 1968 Firebird. It has headers , flowmaster exhaust , Holley 650 with vacume secondary , Eldelbrock RPM intake , turbo 350 and a 2/73 rear end . Driving at 75 I am tacking at 2600. I drove from Dayton Ohio to Wheeling WVA over the week end and was suprised to only get 10 miles a gallon. The cam is flat tappet with 450 intake and 460 exhaust with a 222 duration. Question where am I going wrong . The jetting on the Holley is 72 primary and a 74 secondary with a 6.5 power valve and secondary long yellow spring. I am lost.