Danp76

I know these cars are terrible on gas, but if I decided to take the 350 out of my 1971 cutlass, and install a 455, how much worse would the gasmileage be? Obviously assuming everything is factory stock.

oldcutlass

Does it really matter? It would depend on how far you have the fun pedal pushed down, but figure somewhere between 7-8 around town and maybe 15-16 on highway.

Danp76

Is that with a 350 or with a 455?...is there a big difference in fuel economy between the two engines..if everything is stock? This is on a 71 olds cutlass convertible. I want to make it a 442 clone, and figured I would be way off if I keep the 350 engine, but if the gas mileage is better, maybe I should keep it? If both engines are about the same then I'm not sure what I'll do...

tru-blue 442

A lot would depend on the
rearend gears too.
Do you know what they are?

Danp76

I'm not sure what the gears are, I just know it's a cutlass convertible with a 350 4 barrel. I was going to clone it as a 442, but I don't want a tremendous gas hog if I'm going to enjoy it during the summer months.

MDchanic

So I guess you haven't used the search function and found the threads here, here, and here.

- Eric

oldcutlass

The mpg i quoted is with the 455, the 350 will get a bit better. Again, as mentioed previously gear ratio's, power adders, and driving habits will directly impact fuel economy.

These cars are not considered fuel efficient as I'm sure the links Eric provided above will dictate.

DeltaPace77

As mentioned, rear end gearing has a lot to do with it. My car with a far from stock 455, 3.73 axle and 4L60 4 speed od tranny, fuel milage isn't bad. Like cruising around on the highway in top gear with a 2.56 rear axle, but thing really moves. Think this is more what you should be concerned about.

hookem horns

Think about your usage to help guide the decision.
On a weekender racking up 2-3k miles/year, 3-4mpg might change your fuel cost by $2-300.
On a DD or if you do long cruises, rear end and an OD transmission are likely more relevant than cubic inches
If all miles are put on a 1/4 mile at a time, well....

HonestDave

So many variables, it bogles the mind. Gears, tranny, what year 455? High compression, low compression? Burns premium?
I've never thought of a 350 Olds as a gas hog. I tend to not change engines unless I absolutely have to. It leads to exhaust mods and motor mount mods sometimes. I try to make do with what I have and enjoy. I wouldn't change to a 455 unless you have really deep pockets, or don't plan to do much driving.
If you can afford to have the fun, then what the heck !

455man

I lost about 5 mpg going from a 350 to a 455. That was the only change. Where you start depends on a bunch of factors.

jfb

I have had a 68 with a 350 and it had about 11 to 13 to the gallon city with easy driving 10.5 to one compression and a 308 gear. my 71 is a 40 over mondello engine 342 gear and it did have a jm 18 20 cam about 10 to 12 city and around 10.75 to one compression and easy driving both were not to bad on gas i have had worse. how about a sport fury and 456 to 488 gears plus a six pack for around 300 well beaten but fun car 440 engine also about 7 to the gallon. by the way for anybodys opinion i have had 0 problems with my mondello products plus 0 durability issues if you follow there guidelines and recomendations and no problems performance or otherwise like shipping with them they are a little more to spend with but somewhere you have to pay for experience, you can pay out your nose maybe, or a little more and let them due to leg work for you.

tru-blue 442

Dan, if you will get under the car
and look at the axle tube on the
passenger side, there will be a code
stamped there. I maybe upside down.
It could be on the front or back side
of the tube, between the pumpkin and
shock bracket. post this code, and you
can get the help you need.