Im switching from a 225 to a 318, is there a big difference in fuel cost?

SCREW THAT! I GAINED MILEAGE!

In my '74 Duster;

The /6 I had was getting about 15 MPG Hwy on a good day.

I swapped in a '79 318 and LU 904 tranny along with a swap from the small 7-1/4 rear to a 8-1/4 rear with 3.21's and it's a sure grip equiped rear to boot.

The following engine mods still allowed for 18 Hwy. MPG's on a stock tire size.

Unsilenced air cleaner up top a 600 Holley and later a 625 Carter with no mileage loose there ethier.
The first intake was a LD4B and now is a OE 360 intake. Equal at this point in time.
Headers (Hooker) into a Jegs 2-1/2 exhaust and a Crane Cam, Hyd @ 050 216-228/.454/.480 on a 112 c-line and a electric fan, orange box fired through a OE vacuum advance distibutor.

She turns around 3000 - 3300 on the Hwy.

IF I crank up the timing, it'll need better than 87 octane but, 92 does it, it adds not only performance but it also added another MPG and a half.

(Currently last run @ 15.14 @ 89 mph on 87 octane)

No head milling, no larger valves, idles and drives to Grand-ma's likeing. Quick enuff to get on the Hwy or pass on it. The gears hold it back, but also keep it nice enuff to frive anywhere at anytime. And I do so.

Dump that 6!