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what rear end ratio comes on a 1958 plymouth ?
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jerrybev
Posted 2023-05-03 6:11 PM (#629304)
Subject: what rear end ratio comes on a 1958 plymouth ?


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Location: Winston Salem, N.C.
what would be the rear end ration on the rear end of 1958 Plymouth 4 door with 318A engine?

reason for asking is:
(car gets 11.1 MPG and we are looking for ways to improve gas mileage on this car, the engine has had a complete rebuild 4000 miles ago.
I am going to post in the engine section of this site request for what are the timing specs. Working With the carb we got the vacuum up from 14 to 17. I think vacuum is supposed to go to 22 ?? Any suggestions appreciated.
thank you
Jerry Whitfield
winston salem North Carolina
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Shep
Posted 2023-05-03 8:40 PM (#629310 - in reply to #629304)
Subject: Re: what rear end ratio comes on a 1958 plymouth ?



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Location: Chestertown, NY ( near Lake George)
Make sure the vacuum advance is working, my friend with the same basic car gets 15mpg. 8 degrees initial and a working vacuum advance, 2.94 rear.
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Powerflite
Posted 2023-05-03 9:07 PM (#629311 - in reply to #629304)
Subject: Re: what rear end ratio comes on a 1958 plymouth ?



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Your statement is flawed. Saying that it gets 11.1 MPG is almost meaningless because you don't say at what speed, stop & go around town, flying on the freeway at 95mph, idling in a traffic jam, or what. 17" Hg vacuum isn't a bad number. Most old engines will get only up to around 18".

The gear ratio used is heavily dependent on the type of transmission used, which you don't tell us. But regardless, if your intention is to improve freeway mileage, then I suggest installing a 2.76:1 gear ratio that was never used in these old cars. That will require you to either shorten the axles by 1/8", or swap the center thrust button in the differentials. This is assuming that your rear doesn't have a sure grip. If you don't want to have to shorten the axles or swap the thrust button, then an old set of 2.93 gears is the highest gear set you can easily swap in. Keep in mind that a gear change will only modestly help fuel mileage around town, in stop & go. Assuming you have an automatic trans, most likely your current gear ratio is either a 3.15 or a 3.31. You can always measure it by counting rotations to find out for sure.
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