I have a 1989 L98 Corvette (with 3.07's) My dad when he drove it used to get between 17-25mpg even when it had 3.07's (bought the car in 95 with them already in it) (got 17-25 mpg about 10 to 15 years ago before it sat). I recently got it from my dad. It has 92k miles. It sat for a like 2 or 3 years. The whole fuel system ended up getting replaced. Then came across an issue again. It sat but would be started once and a while to keep it going. But then I recently fixed it up. I installed a new o2 sensor by ACDELCO last year cause it literally blew apart. But about 2 months ago I replaced head gaskets which was causing my over heating issue. Got done with the job and Timing is set to 6 degrees BTDC replaced spark plugs with NGK 6341 Iridium lx and taylor cable plug wires. i cannot figure out this bad gas mileage issue. I have been getting between 11-13.5 mpg. The car runs smooth and doesn't smell like its running rich either. ( I replaced cap and rotor about a week ago and still no change) Injectors were replaced when the fuel system was done about 7 years ago. ( Has alot of sitting and idling after fuel system was re done) I have ran seafoam and such through the car (gas and intake treatment) and still no improvement. Below are pictures of the #1 Sparkplug. Notice that one side seems to be more burned on the white bit than the other. My guess it seems like the injectors are shooting a stream rather than spraying. But that is just a guess. Any help would be very much appreciated. Thank you