Thread: Need help on understanding gas milage

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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.

too much carb and way big on the jets. plus at 2600 you wont get any real millage.

so about 200 miles and 20 gallons??????


Its hard to get ackward figures on a single segment/fillup unless the tank is full when started and then filled to same point when checking----------on trips I fill up before start, log milage and gallons at every fuel stop--figure by both segment and total miles/gallons at each and every stop-this way as distance and miles increase the total running numbers become more exact plus I can see what each segment does(mountains, curves, 2 lane , express/interstate)


But knowing the carb works pretty good around 68 jets primary, timing set to max without spark knock high vacume setting, no windows open, moderate acceleration and longer coasting to stops-----------

I can change out the carburator or change the jetting to a 68 primary and 70 secondary. As far as the RPMS goes all I can do is install a 700 r4 transmission. The rear end is a 2.73 now and I can not go any higher.

Your motor was produced as an 8.00:1 static compression ratio motor. The cam that GM used was meant for a 10.0:1 motor. Lack of cylinder pressure due to a late closing intake valve means you have to get down on the throttle more than you should have to if the components were matched better. More throttle angle equals less fuel mileage.

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