Inliners International Forums General Discussion MOPAR POOR GAS MILEAGE

I have a '38 ply. p6 coupe with a '46-'48 Dodge 230
The problem is the gas mileage is never more than 14 mpg.
It has new B'B Carter with correct jet, it has plenty of power, runs smoothly and burns very little oil. It's been tuned with new plugs and wires. I've tried diffrent carbs
but it doesn't change. Any suggestions?
P6'38

What's your gearing?
What speeds are you driving?
Tire pressure correct?
Little things can add up to poor mileage.

Gearing is stock '38 Plymouth. 410?I drive around town and out in the country at 40-50 mph? The tires are the correct diameter [28-29"]as the original stock tires were. The odometer is right on as I tested against other cars.
If I drive at 40-50 mph constantly, say 100 miles, just highway driving to test gas mileage, It's 13.5-14. Tire pressure is correct.
I've been dealing with this for awhile, so I think everything is in order. Except shouldn't I be getting 18-19 mph?

In 1938 they would get that easy.

However; you've added a larger engine, which probably has a little compression loss. What your getting is about right.

Are you ready for the AG salute??

Last summer, I rode with a buddy in his '54 Plymouth 2 door, 230 with 3 on the tree and OD. On a 560 mile trip, on fairly level ground, at 60-65 mph, he averaged 21.6 mpg. When I read your post I called him in Kentucky and asked if he remembered what we'd figured on the trip. (He keeps a log) Needed an excuse to talk to him, anyway. He said he can't remember the rear gear ratio, but he's turning about 1825 RPM at 62 mph.

It's your gear ratio that's killing your mileage.
4:10 is mighty steep unless your towing a trailer or pulling stumps.

If you can change to a 3:50 ratio or something close, I'll bet your mileage will be in the 17-20 range @ 55 mph!

I have a modified 292 bored .40 with 3-2's, cam, headers and an .075 OD with 3:55 gears and I get 18-20mpg @ 65mph at 1950 rpm's.

RapRap
1940 ChoppedChevy Coupe

I consistently get 19 on the highway with my '50 Plymouth with 230 and overdrive. And it's a hopped up flathead six to boot, I've done just about everthing to make the mileage worse .

An overdrive tranny would be the ticket, but Plymouth didn't come out with one until '42. A knowledgeable mechanic friend who also has a '38 coupe says an O.D. trans won't fit through the tunnel on the x-frame without mods. Changing rear end ratios would be better for me if I could find a 3:50 . A T-5 conversion would be great, but beyond my talent to make the mods.
So, it looks as if the rear end gear ratio is the culprit.
Thanks to you guys for all the input and if there are any other ideas, I'd like to see them.
P6'38

Does anyone Know the ratios that are availiable for '38 Plys. I think all mopar cars from '36 on up to the '40's used the same rear end set up.
If I could locate a 3:50 or something around that ratio, I could just change out the third member. that would make a big difference.
any help on this would be appreciated.
Thanks, P6'38

Contact Floyd @ 'French Lake' (320.274.8497) he has one, but can't tell exact ratio till It's removed etc..

Good luck.

Thanks John,
What is AG salute?
I figured my axle ratio is 3.90 still too high.
Jim P.

It's the annual (American Graffiti) 'salute' held in Pentaluma, Ca. A celebration held in honor of the film's history in the town. This year it wil begin on May 17 and go through the 20th.

Also; a "rear end" ratio of 3:90 to 1 is a low ratio and the 3:50 +/- will be 'higher' etc. A 'lower' one causes the engine to revolve more to go the same distance/more fuel to be consumed.

If you go the Salute get me an XL T-shirt & a Large one. Save the receipt & I'll pay ya back.

Happy trails.

Without to much trouble you should be able to switch third members but I think there wasn`t to many options for ratios but 3:90 to 4:11. They did have 3:54 in the 46-48 Chryslers but I don`t know if they inter-change. My 52 Windsor had 3:90 so I just switched to a Ford rear set up out of a Bronco then I could go with almost any combination that was avaliable for 9" Ford. The Bronco was out of early 70`s and was the 3300 lbs axel unit and it has the 3:25. I use it with the big six ( 265 ) and fluid drive and it pulls well with the higher gears. Lot less revs on the motor--roughly 15-18 percent less. Check this option if you like--I think it should be narrow enough to fit under your car. Any good speed shop should be able to do this without to much problem and shouldn`t cost an arm or leg. Good Luck...52er

Here's the latest info on the AG salute;

www.american-graffiti-salute.com/

Happy trails.

John, I am aware of AG salute.I did mean "too low at 3:90". I'll see what i can do about the t-shirts.

52er,
Thanks for the information, I'll check around. I'll have to find a site that list the ratios for the early Mopars.
P6'38

I did find on Allpar.com that plymouth made a 3.54 ratio for some '38 models. So there are probably others [ Dodge, DeSoto, Chrysler that had them too, and for a span of years]I.Just have to look around.
Thanks for all the help.
Jim

I'm sure that some pre-war Mopars had a taller ratio... I know someone here used that rear axle specifically to get a tall ratio when he built a Special back in '48...

Used a Wolseley 25hp engine with twin SUs, Vauxhall 30/98 gearbox, upside down Armstrong Siddeley chassis... but for some reason used a Chev front axle(?).

Point was, with that mechanical spec, he really needed a tall rear end, so he got a Dodge one.

Someone recently told me that lowering the float level a bit and advancing the dist. 2-3 degrees would help some. I'll check it out.
Jim

Dear Jim;

The 'float level' has nothing to do with fuel consumption, in general.

A little more 'advance' may help.

What do your plugs look like?? If "all is well" they will be 'tan' in color.

Good luck.

John,
The plugs are a nice tan. So everything seems to lead to the gear ratio. I will advance it a little.
Thanks, Jim

My '49 Dodge D30 (3.91 rear end) was getting about the same mileage right around 14. I added a second outlet to the exhaust and went to a holley 94 carb. My mileage is now right at 19 mpg and the ol' girl has really picked up some power. I too am looking at going to a 3.55 to 3.21 rear end to make her a little more comfortable as a daily driver. (Well I'm also looking for a new rear end that is a little narrower. I've got a set of 8" wheels I want to put under her. Any body got an old A body rearend?)

Why not slip an overdrive gearbox into it?

Then you can have the grunt when you need it and the lazy running when it's appropriate...

As for the wheels, you can get them changed to a greater inset can't you?

I'd really be interested to know what 8" wheels with a lot of offset do to the front end.

I'm a bit of a contrarian. I hear a lot about people putting in the overdrive unit and I'm sure they're happy with it. It's like the carb thing. Everybody was doing the 2 single barrel B&B's, I thought the Holley 94 would be much simpler with equal results. I believe I have gotten that. Positive offset wheels are an option but like you suggest it may make the handling a little whacky not to mention the added load to the bearings. The change to the '73-'76 A body rearend sounds like the simplest solution to me. The advantage gained will be modern brakes and E brake. I don't really feel I will lose much grunt since I have the fluid drive and there is no mechanical connection between the engine and the drivetrain now. I don't do interstate with this car and I am just looking to keep the engine a little less busy on secondary roads.

That's what I do. Keep the engine a little less busy. The tires on my P6 are on 15x7 rims [on the back]and are the same diameter as the originals. the rear axel ratio is 3:90.
The speed limit in 1938 was 45 mph. If you are going 45 mph on the freeway now, you're road kill.
An overdrive won't fit in the tunnel of the x-frame on a '38 without mods.
So for now, it's country driving on secondary roads. Still a lot of fun!


'

Hi,This is an old thread but if I could give my two cents.I have a '38 dodge w/the original eng.that has been upgraded to a 234 c.i. Have gotten as much as 32 m.p.g. while on a trip to Detroit to the Inliners conv. Av.25 on the entire trip. It has a 273 Camaro rear & a T-5 O/D trans. Turning around 2000 rpm @ 70 mph. It does 18 mpg in town & has a single b&b carb.You have to change something if you want less rpm & more mpg.Just finished a '51 ford coupe & av. 30.6 mpg on it on the trip to Tulsa...Good luck

I found out that'36, '37,'38 all used the same third member.All interchageable, probably goes for Dodge,Desoto and maybe Chrysler too.
The common ratios were 3:90, 4:10, but there were some 3:54's in there too.
The coupes used the lower ratios.

Sounds to me like a 3.23:1 A-body rear end is the go...

Sorry Ray, but what is an A-body?
Jim

An "A" body is a Dart,Barracuda,ect,, early stuff 64-71 or 72? IIRC??

To get the 5 on 4 1/2" bolt pattern you will need to go with the 73 to 76 Dodge Dart or Plymouth Duster axle.

In other words, late A-body...

Or, if you like, you could get one of the Australian axles. There's the Chrysler Centura, but it needs a lot of brackets cut off it (coil spring rear), or the 1970 and up Australian Valiant range. They have a host of ratios, they're made by Borg-Warner... 3.23, 2.92, 3.89. And if you swap in gears out of a Commodore you can have a 3.45, or out of a Nissan Skyline there's the 3.7 or another 3.89.

We have a wealth of that stuff here.

You could put on bigger rear tires to change your gear ratio easily and cheaply.

Or just forget about it. How much difference will it make to your family's budget if you get 18MPH instead of 14? And how much will it cost to get it?

Rusty,
I have big tires on the back with deep rims. However, the tire size is the same diameter as the oringinal 16" 650. They are 15's now. I like the way the low end torque of the dodge 230 pulls at low speeds around town without shfting down a lot.
I have decided to just enjoy the car and not hassel with the gas miliage. The difference in the cost of gas is not a big deal, I drive it all the time.
i'll try to get some pics on here to show it.
Thanks, Jim