this thread was started in 2008. Twelve years later it's still going.
I copied this from one of my other post but added a few comments.
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Ok I'm going to give you the short story
A few years back I installed a dyno-ed 530 hp 408 in a Cobra for Mike. He drove it 130 miles home and called me about the gas mileage. 6.7 MPG.
I told him to bring it back and I would let Coach tune the carb. (Coach is my carb guy. I like EFI.)
We called the shop (Keith Craft )that had build the motor and they told us they only tune for wide open throttle because they did not know what air filter, alternator, headers, exhausts system, etc that we would use.
Mike brought the car back and Coach tuned the carb. Trip back home he got about 13 mpg.
The next year I drove it to the London Cobra Show. Round trip 1150 miles and I got 16.7 or 17. something, don't remember for sure.
Moral of the story is check the power valve and get the carb. tuned. There is more to tuning than adjusting the idle screw. Jet size, air fuel ratio, cam plays a big part.
Air Filter is a big issue on fuel consumption.
I had a 351 W one time that would not rev over 4300 rpms in any gear. Stopped on the side of the road and removed the 8" air filter and the motor would rev to 6,000 in any gear.