The Stovebolt Site Forums The Shop Area Engine Shop 1957 6 cyl not getting proper gas mileage

Hello,

I have a 1957 Chevrolet passenger car with the original 235 6 cylinder. The engine has been rebuilt and I very recently put new bearings on the crank. I just installed a factory overdrive trans in place of the 3 speed manual trans.

Today, with the overdrive, I drove on a trip to Indiana and only got 15 mpg. This was between mostly 65-70 mph, with some city involved but not much.

I have heard various others say that with a 6 cylinder and overdrive they have been getting 20 or better mpg.

I'm looking for possible reasons my mileage may be off. I believe the the carb to be adjusted properly. The only other thing I can think of is that I still have original style points and haven't converted to the electronic style that replaces the points.

Any ideas?

Thanks,
brandon

15 mpg isn't too bad. What year truck and what size are we talking about?

**EDIT** I read your post wrong... thought you had a 57 passenger motor in a truck. I see you have a car. My bad.

I lean towards doubt that the reported 20 mpg you read about was acquired at 65 to 70 mpg. You don't mention your rear end ratio or tire size which would be major factors as well. One other point that's is a little unclear is your transmission. Is it an automatic or a manual with a overdrive? those all play a part in getting you to 20mpg...

I think anyone claiming 20 mpg or better with combined driving between city and highway driving in a Stovebolt with a 235, should redo their math !!!??

I get about 15 mpg with my 55.1 3100.
it's 235, 3psd OD running on E10 fuel.

regards,
Leon

Brandon - these guys have it about right, 20mpg would be really great with a 235 .... and I'll add this: if your 57 car originally had the 3 speed it would have 3.55:1 rear gears [with an OD it would have 4.11:1, we loved those back in the day for street racing] - if you're running an OD with the 3.55 gears you're likely bogging it down frequently on the highway, too low an RPM for good mileage, 15 would be about expected

Bill

I think you could get a little better mileage then that on a car. Check your timing & try advanceing it a little more then specs. Make sure the vacuum advance is working. Take the top off the carb & check the float level & make sure the power valve is working correctly. Remove the carb & make sure there is a psssage in the gssket for vacuum to reach the hole in the bottom of the carb. With the engine warm & running slowly turn the idle mixture in till the engine starts to slow down. Turn it back out just enough to speed it up (about 1/2 turn). With proper tune up you should increase the mileage.

You need to figure out the rpm at 65 to 70 mph, if it was pretty low, say 2000 or less, then 15 would be about right. Your stock engine makes very little torque at low rpm so you are required to step harder on the peddle to keep the speed up. I would bet the engine likes to run about 2300 rpm, and if the wind isn't to strong, 16-18 shouldn't be out of the question.

What is the rear gear ratio?
What about tire size?

Just don't expect better milage because the rpm is lower. New cars and trucks are designed for low rpm, yours wasn't.

Joe

One thing to think about...if the engine is newly rebuilt, it's probably still fairly tight, and may need a few thousand miles to loosen up just a bit. That extra drag will take a toll on your gas mileage as well.

Chris

The torque peak on my essentially stock 55.1 235 is 2000 rpm. I'd expect that's also about its most efficient operating speed.

regards,
Leon

Thanks for all the replies everyone.

I have the original 3.55 gears under the car. It came with a 3 speed manual and I replaced it with a factory 1957 3 speed overdrive.

At the moment I can't remember the size of the tires, but they're smaller than stock (they look a little small and I want to get some stock sized ones when these wear out)

On the way home from out trip that I just now got back from, we drove about 150 or so miles and kept it at 65 mph and still averaged around 15 mpg, but the roads were fairly hilly in Kentucky. Do you believe 55 mph would make a big difference.

I don't have a tach, but I could figure the rpm by getting all my numbers like tire size and the final drive ratio of the overdrive unit.

I'm going to install a pertronix unit to replace the points. Does anyone think this will make a difference?

I was thinking that it would get AT LEAST 18 mpg, but I guess not.

I did adjust the carb before we left on the way home and was hoping that would make a difference because it was running a bit rich, but it didn't make any difference. I suppose I need to really inspect the carb and make sure there's no seepage anywhere.

The engine itself was rebuilt a few years ago, but only the main bearings were replaced recently.

Thanks,
Brandon

Your gears are too high for the overdrive.

I think you should get 18, at least on a long drive. I would think you should get 14-15 on short trips. I doubt an electronic ignition would help you if the old system is working correctly. Like someone else said, make sure your vacuum advance is working and it wouldn't hurt to check out the carburetor. I will have to disagree about the rpm being too low. I think if you can turn it 16-1800 you will get the best mileage. Those old girls had a lot of torque at low rpm. Most of your newer cars run below 2000 rpm at 60 mph.

Make sure the jetting is correct on the carb.

How are you calculating mileage? Is your odometer accurate?

What was the fuel mileage before the overdrive?

Timing, vacuum advance, correct metering in carb, and fuel type all play a big part in fuel mileage. High ethanol content will kill mileage. Is the carb the correct one for this engine? or is it a general replacement?

Remember, you can not compare your car to a new car, rpm is not the only factor. Your '57 is a brick compared to the worn soap bar shape of new cars and trucks, that alone is most of the fuel mileage. 55 mph will increase your mileage greatly.

Joe

1,800-1,900 RPM at 65 MPH? 1,400-1,500 at 50MPH?

Thanks for the suggestions everyone, I'll start checking into them.

I'm not sure what the fuel mileage was before the overdrive. My speedometer doesn't work so I borrowed a GPS and checked the mileage that way. I presumed it would probably be more accurate than a speedometer anyway.

I going to start with checking the timing and advance and making sure the carb has no leaks at all.

Is there a more efficient carb than the original one that some people use?

Thanks,
Brandon

Also, I modified the air cleaner. I shortened it and rewelded it to accept a paper element.

[IMG]http://i1171.photobucket.com/albums/r548/AprilMay37/IMG_1892.jpg[/IMG]

Thanks,
Brandon

Top mileage will not be obtained driving over hills, that is just the way it is.

I generally get pretty good mileage in my bus with the 235, about 18 or so, but I drive slow, I find that peak economy for my set up, is about 30-35 mph. Anything over that, and the milage drops.

Now, I used to have a '49 Pontiac, with a flathead six, it got 19 mpg, at 50 mph. Any faster, than that, and the mileage dropped like Keanu Reeves from an airplane.

One more tip for increasing MPG :
Install a large face Vacuum guage where it can be in your vision while not taking your eyes from the road, and drive to hold maximum vacuum reading proportionate to your urge go.
Also will need loud radio or earplugs to cancel the horn noises as the rude motorists declare you a "barnyard driver" or worse because of your less than brisk speed and acceleration.
Actually driving with the maximum vacuum goal will make you a more efficient driver as you learn how to think of road conditions ahead to keep the vacuum high.
A pleasant study and practice on the daily drive but can drive your passengers nuts.

Just returned from a 900 mile round trip in my daily driver as listed below. Did a random mpg on a tank of gas and got 18.02 for full highway miles at 60-75 mph.

Not to be redundant but I can confirm those who said the rear ratio is too high for an OD with a stock 235-6 cylinder. It is straining the engine to pull the car at 65 and it would even be worse at 55MPH. Get some 4:11 gears in your booty. And get a tach and vacuum guage to prove the point.

Christopher is talking Canadian gallons.

I checked today and it appears that my vacuum advance is not working. The centrifugal advance seems to be worked, but no movement with the advance.

Where can I get a vacuum advance for a '57 235 6 cylinder? The local place I get all my '57 parts from doesn't have them.

Thanks,
Brandon

Try NAPA or CarQuest

Have you pulled it off and cleaned it out then checked for air flow or no go to be sure the diaphragm is whats wrong with it?

Wow, that's A LOT cheaper than what I've been finding. Thanks.

I haven't had a chance yet to take it off, but when I do I'm going t check and make sure the line itself has vacuum.

Brandon