I have had a 455 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 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

 

Mileage is probably more a function of vehicle weight and driving habits. The 403 is a short stroke, big bore motor. The 455 has a longer stroke, smaller bore, and more torque. I suspect you'll be happier with the BBO.

 

A 403 will probably get better mileage. I have never personally ran a BBO, love my SBO's too much. The 455's massive low end torque is legendary but the 403 is no slouch.

 

Less cubes should usually = less fuel used in the same vehicle, with the same driver.
Will you really notice the difference in city mpg though.....I dunno....

Going to see alot more people stepping down with $4.50 premium in the near future again.
Those that don't will be looking at it sit more because of the fuel prices.

 

Im by far not an expert. But it could go both ways 403=less power and more rpms witch means more gas being used. 455=more torqe and hp less rpms but it might be pretty close to even my opinion is stay with the 455 there stronger and better in my opinion

 

If you are worried about mileage, you need a fuel injected small block!. I don't know about the 403, but my 455 4spd makes very regular stops to the gas station. I couldn't tell you if it gets 6 or 8 mpg around town, but compared to the other cars I drive, including an 8 liter viper, its mileage is terrible. Only my boat gets worse.

 

As CWRacer mentioned the mileage with a SBO or a BBO will be lousy compared to a newer car. Just freshen up the 455 in your car and use another car as your daily driver.

 

I don't know what transmission you have but maybe a 4-speed automatic would be worthwhile for you. The overdrive should give a noticeable improvement in mileage.

 

If there's nothing wrong with the 455, instead of the expense of the 403 rebuild and swap out, etc etc etc why not get that EFI system from Holley.

It's expenisve, like 2 grand, but better MPGs and you don't need a a laptop to set it up

 

My 70 Toronado got 12mpg around town and 19mpg on the highway. That's with a slightly bigger cam than stock and ported heads. Nothing to rave about but decent for a car weighing 4600 pounds empty. As others have said here it's probably more about your driving style and how efficient the two engines are. I have no idea how bigger bores or longer strokes effect gas mileage but I would think a smaller engine should get better gas mileage with all else being equal.

 

What is this strange thing called "mileage" you speak of?

It's just like high dollar restos, if you have to ask how much it's going to be you can't afford it.
I just put gas in and whatever money is left I eat with.
The more the gas prices go up the more weight I lose, thus get better gas mileage because the car is lighter, thus save money and get more food and gain weight.
It's a vicious circle but all works out in the end.

 

More about rear end gearing IMO. The 455 has the punch to make a 2.50 rear livable, just don't expect more than a chirp out of it.

 

You, me, my alter ego and redgoat think alike