Restoring 1968 400 M20 4-Speed. Should I go with 3:42 or 3:90 gears?

Old October 24th, 2018, 12:01 PM
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Restoring 1968 400 M20 4-Speed. Should I go with 3:42 or 3:90 gears?

Getting ready to upgrade me rear end and have a hard time deciding which gears to go with 3:42 or 3:90. I want the car to be fun to drive but donít want to be to disappointed in the highway cruise ability. Appreciate any opinions on the matter.

Thanks Tim
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Old October 24th, 2018, 12:28 PM
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Originally Posted by Tjohn8573 View Post
Getting ready to upgrade me rear end and have a hard time deciding which gears to go with 3:42 or 3:90. I want the car to be fun to drive but donít want to be to disappointed in the highway cruise ability. Appreciate any opinions on the matter.

Thanks Tim
With 3:90 gears, you would be turning too high RPM to do any highway cruising at highway speeds. Around town they are great, ask me how I know about both.
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Old October 24th, 2018, 12:29 PM
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Ill give my opinion even though I haven't owned either: 3:91 would not be desirable for highway use, Im not sure 3:42 gears would be much better but they would be better. If its a choice between the two I would go 3:42. You could go with 3:91 to experience the high rpm at moderate highway speed and see how you feel about it then fork over the money for a 5 spd trans conversion. JMO
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Old October 24th, 2018, 12:40 PM
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3:42 would be the outside comfort limit on the highway for most. 3:23 quite tolerable. Go with OD trans & 3.90, 4.10 become quite tolerable. FWIW my car tachs 2300 rpm @ 70 mph, 2600 @ 80 mph with 3.90 gear & appx 27" tire (can't remember the size or actual diameter) in OD, converter locked up.

With M20 you will have a rather deep first gear. I had a 65 442 w/ M20 & 4.11, man was that a deep first gear.

Last edited by bccan; October 25th, 2018 at 09:58 AM.
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Old October 24th, 2018, 01:06 PM
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I have never driven a car with a 400 in it, so not sure how it would compare to a W31, but the 4 speed W31 I had with the 3.91 gear was bad ***!!!
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Old October 24th, 2018, 01:33 PM
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Its not that 391's would be too high rpms on the hiway because the stock engine would do it all day long. Its just to the point where most get uncomfortable or scared. If your uncomfortable with rpms well over 3k then go with the 342's.
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Old October 24th, 2018, 02:04 PM
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M20

Originally Posted by Tjohn8573 View Post
Getting ready to upgrade me rear end and have a hard time deciding which gears to go with 3:42 or 3:90. I want the car to be fun to drive but donít want to be to disappointed in the highway cruise ability. Appreciate any opinions on the matter.

Thanks Tim
HI TIM

With wide ratio M 20 transmission the 3.42 would be better. If you run taller than factory size tires it will be tolerable RPM on the highway.

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Old October 24th, 2018, 05:23 PM
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"65 400 m20 4:11 turns 3 grand at 65 mph, will break the tires loose in all four gears but fourth is more of squawk. Third gear is the put you in your seat shift. Like stated above the early 400 will have no problem at 3,000 rpm, although the later 400 didn't like the higher r's.
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Old October 24th, 2018, 10:31 PM
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I had 3:42's in my '71 Supreme,with a turbo 350 trans. Turned 3200 rpms @ 70 mph. No problems at all. Took it to Homecoming a couple of times, 105 miles one way.
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Old October 25th, 2018, 08:40 AM
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Thanks for everyoneís responses I will go with the 3.42.
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Old October 25th, 2018, 12:01 PM
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I have a stock 69 with the G block 400 and M21 with 3:91ís and stock bias ply 14Ē redline tires. I donít drive interstate often but when I do it 3000 + rpm at 55-60. The car Handles it fine but I donít like it. I am not changing it cause itís a totally stock car and is a step back in time which I think is way cool. I just plan my routes I drive accordingly.
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Old October 26th, 2018, 12:37 PM
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With the m21 you have a first gear more able to run 390 and beyond ratios
The m21 m22 also do not have the "oil slinger" on first gear
ratios over 390 with m20 tend to use / lose gear oil when spun that fast RPM
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Old October 26th, 2018, 12:40 PM
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Originally Posted by Tjohn8573 View Post
Getting ready to upgrade me rear end and have a hard time deciding which gears to go with 3:42 or 3:90. I want the car to be fun to drive but don’t want to be to disappointed in the highway cruise ability. Appreciate any opinions on the matter.
Thanks Tim
I have a 68 442 convertible with a 455, M-20 four speed, & 3.42 rear end. It's perfect on local streets and the highway
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