I was wanting to know what kind of gas mileage I can expect from a 350/400hp with a Muncie M-21/22 with 3.73 gears.

geez with all the H.P. you would think you'de be lucky if you got your MPG in the double digits!

my Nova routinely got 17 mpg with the Vortec/HOT cam/T10/3.70 combo.
of course, i had a qjet and a performer intake.

Both my orginal 302 and mule 355 always got about 15-16 . Since going to TK0, it jumped to 20-22 cruising 70.

Here is the buidup of my 350.

Holley 750, vacuum secondarys
Holley Street Dominator intake
Hooker Competition ceramic headers
AFR 195cc Eliminator heads
Comp cams Magnum 280/.480
Mallory Dual point with Accel supercoil
cast crank and pistons around 9:5:1 to 10:1:1 compression

probably low double digits with normal driving habits, which of course we all know will never happen. I have never even attempted to gauge my 302, but would guess 8 mpg at most, 3:73 and m21; anyone really care about gas mileage anyway?

Im more concerned about how much horsepower it makes.

I thought my H2 was bad at 10mpg...Than I put my 69 back on the road...I'm lucky to get 6. (3.73 at 2600rpm) If I put my foot in it...The gas gauge and the Tach go in sync. The tach goes up..and the gas gauge goes down. My dad, who built my motor for me just laughs. He tells me that I told him I wanted a bad boy..and he gave it to me.

high performance does not have to mean low mpg.
and high mpg does not have to mean low performance.