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Old 02-12-2011, 04:20 PM
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1940 Chevy 4 dr sedan rearend ratio questions

Hi I need some help settting up my street rod. I have a 1940 chevy 4 door sedan with a 1964 327 and a th350 transmission. I have 15" tires and think I have a rear end gear ratio that is incorrect. When in third gear the Rpms are to high for freeway cruzing. It feels like it's still in second gear but it's not. I am not sure what rear end is in the car. It was my fathers car and I have been rebuilding it for 6 years. I want it to have a good cruising rpm. How do accomplish this.

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Old 02-12-2011, 04:44 PM
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Do you still have the old torque tube rear end in it? '40 Chevys had 4.11 gears in the rear end. You can swap the chunk from a '53 Chevy with the Powerglide transmission into the '40. These have a 3.55 gear in them.
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Old 02-13-2011, 01:40 AM
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If you needed a numerically lower Ratio, an O/D Trans could be used in place of the TH350.
TH2004R is the same length as the TH350, Trans mount would need to be moved back and TV Cable & Bracket + Lockup Kit or DIY wiring for the TCC/TorqueConverterClutch is needed.

Trans-------1st----2nd----3rd-----4th---------------------------------
TH350------2.52---1.52---1.00----------------------------------------
TH2004R----2.74---1.57---1.00----0.67-------------------------------
4.11 X .67 4th= 2.75
3500rpm, 4.11 Gear, 1.00 3rd, 26" Tire = 65.8mph
2300rpm, 4.11 Gear, .67 4th, 26" Tire = 64.7mph
3000rpm, 3.55 Gear, 1.00 3rd, 26" Tire = 65.3mph
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Old 02-13-2011, 02:01 AM
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1940 chevy 4 dr sedan rearend ratio question

Open the rearend, count the number of teeth on the ring and divide by the number of teeth of the pinion. The product is the ratio of your rear end. most likely you have a 4.56 rear end. If you want to keep it simple and have good wheel clearance , use a whole rear end of a 1967 to 69 Camaro. If hard to find use one of a 1965 to 69 Nova. If hard to find use one of a 1978 to 86 Monte Carlo, Olsds Cutlas or Buick Regal. That is your best choice if is just for crusing.
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Old 02-16-2011, 12:27 AM
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Thanks I will try to find a new rear end.
Thanks for the info. I thought it would come to this. Is it possible to just change the gears ?
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Old 02-16-2011, 09:08 AM
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If you have a 327 with TH-350 it is unlikely you still have the torque tube and stock rear end in the car. If you take a couple pictures and post them we can probably identify the rear and give you options on which way to proceed. I suspect its probably a GM 10 bolt, 12 bolt, or one of the smaller Ford rears but without pictures its a guessing game. If it is one of these you can pretty easily change gear ratios and you won't need to swap the rear out for something else.... but we need pictures to tell for sure.

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Old 02-16-2011, 11:08 AM
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Sorry i am new at this forum and can't figure out how to post a pic. But

I did pull some #s off of it. GM 3725889 B38

Iam pretty sure that it has 10 bolts but i will have to take a closer look to sure. I hope this info will help identify what it is
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Old 02-16-2011, 01:33 PM
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There is more than just one GM 10 bolt. They come in different versions based on the ring gear size. Also the smaller the ring gear size the more limited you are on gear ratios but no matter which one you may have they all have ratios available that will be compatible with good street driving. The numbers you gave mean nothing to me but here are pics of the three basic types. If the rear cover on your differential looks like one of these it probably is a 10 bolt and if so any competent driveline mechanic can replace or change the gearing for you.



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Old 02-16-2011, 01:49 PM
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55-64 Pumkin #
Lots of Info here>> http://www.google.com/search?hl=en&s...&aqi=&aql=&oq=
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Old 02-16-2011, 04:58 PM
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Quote:
Originally Posted by SSedan64
Based on this info chances are your rear won't have a removable rear cover and it will look pretty much like this stock '55 Chevy rear.



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Old 02-16-2011, 07:17 PM
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Yes that is correct. It does not have a rear cover and it is the same as the picture.
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