GAS MILEAGE ....

Id like to know something that may sound dumb but I just am looking to get an idea that's all...as one can not really put a number on this question but a round about maybe..

What time of gas consumption do you think a decent 340 (approx. 400 hp) is getting on a full tank of gas.

Like I said, one can not say exactly as someone might be beating it daily while the other guy strolls easily with it and maybe opening it up once in a while..
This is a 340 six pack liker I said and at the time the rear gears were 2.76 (ouch lol) a 4 speed car.

Thanks for any ideas any help will be appreciated with all respects from me..

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