Sabbathrocker

In simple terms, a larger crank (3.25"-3.40") will give you more cubic inches. More cubic inches requires more air to feed. More air to feed equals more fuel needed. All else being equal, gas mileage will decrease a bit.

I gained about 3 MPG after a H/C/I swap on a 302. The engine could breathe and run easier, which equated to less gas used. Now, if I then added a 3.25"-3.40" crank (requires new rods/pistons as well), then gas mileage would decrease, but total power output would increase.

It is a catch 22 situation. Good Luck!