403 Olds vs 455 Olds, mileage?

I have had a 455 Olds in the 56 for 12 years, now I need to build a new engine.
I do have a bunch of big block stuff.
However, a 403 Olds would probably be plenty of power, for what I need it to do.
Would a 403 get any better mileage (in spunky street trim) than a 455
(in similar trim)?
Years ago an adventure into dodge pickups left me disappointed with 360s,
a 383 magnum ran harder, and got better mileage.
So... what had anybody actually seen , in the real world?
Darrin

 

I'm thinking the 455 is gonna do better or at least the same. It won't have to work as hard as the 403.

What transmission are you running? There may be some room for improvement there. Maybe a 200r4 if you don't have overdrive already.

 

Have a 403 in my shop truck and had many 455's and can't say that the 403 would be any better. Run a 650 cfm holley on the 403 but it may get better mileage with a q-jet. My truck gets between 12/14 mpg

 

page 6 already, anyone else?

 

Maximizing MPG is a product of matching the peak torque RPM to your cruising speed.
Back in my muscle car days, I was a fan of 71-73 mustangs. I replaced the stock 351W and 3.08 gears with a 70 460 I had installed an Isky mile-a-more cam in and a set of 2.56 gears. My mileage went from 15 up to 23 on the highway, along with a good boost in street power despite the higher gears.

Same theory holds true for your oldsmobile. Be it a 350, 403, or 455. Figure out your cruising RPM, and then select a cam that offers peak torque at that RPM, and build the engine to perform it's best at that particular RPM.

The distributor also plays a big part of optimizing power at a particular RPM and allowing the engine to perform at it's most efficent level. GMC Bubba is the man to talk to about that.

Headers help, a good carb is a must. Dual plane manifold, etc, etc.

Over on the Cadillac forums, one guy followed that exact plan of attack, and he's getting 21 mpg out of a mid 70's 3 1/2 ton barge with a 500 cid engine.