Rear end gear choice
Thread Starter
Intermediate
Joined: Sep 2023
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Rear end gear choice
I have a 74 with a 454 and a 4 speed, mild cam motor with a 308 gear and would like something a little better off the light. I do not do a bunch of highway driving. any suggestions as to what ratio I should go with?.
Intermediate
Joined: Jul 2024
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3.42 or 3.55 I'd believe. (Somewhere in that range). A big block with lots of low end torque doesn't need to rev as high off the line. But the 4-speed would like some lower rear gears.
Last edited by Skunks; Aug 18, 2024 at 06:57 AM.
1967 Pedal Car Champion
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From: US-PNW
Do you happen to know which 4-speed is in there? Is it original to the car as far as you know?
The axle ratio offerings for the 454 in '74 were: 3.08, 3:36, and 3.55. GM made 3 different Muncie 4-speeds: the M22 (not offered in '74), the standard M20 (wide ratio), and the optional, no extra cost M21 (close ratio). The M20 gave better off the line performance while the M21 gave better top end performance due to their gear multiplication ratios.
The axle ratio offerings for the 454 in '74 were: 3.08, 3:36, and 3.55. GM made 3 different Muncie 4-speeds: the M22 (not offered in '74), the standard M20 (wide ratio), and the optional, no extra cost M21 (close ratio). The M20 gave better off the line performance while the M21 gave better top end performance due to their gear multiplication ratios.
1967 Pedal Car Champion
Joined: Mar 2020
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From: US-PNW
Both of my cars are stock low-hp big blocks with 3.08 axles, one with standard M20, the other with an automatic; I've never felt they didn't offer off the line fun. These are really torque-y engines and should provide good get up and go. Have you tuned the engine for top performance: i.e., "all in" timing advance at 3000rpm, matched vacuum advance can, proper carb tuning?
Tech Contributor
Joined: Aug 1999
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From: Connecticut, USA
The factory offered 273,308, 336, 355, 370,411,and 456 ratios through the years. Some were not too common- 273, 456, most automatics came with 308, some 336 or 355. 336 was the typical gear with the M20. 370/373 probably would give you the punch you want, it will add about 400 rpm to your cruising.
More important today is how the diff is built and what brand of gears used. There are no more USA made gears unless you find some NOS US Gear which have been out of production about 3 years now.
Back when USA made gears were sold they were the same price regardless of ratio. Today Chinese made gears are 2-3x the price and priced by the ratio.
More important today is how the diff is built and what brand of gears used. There are no more USA made gears unless you find some NOS US Gear which have been out of production about 3 years now.
Back when USA made gears were sold they were the same price regardless of ratio. Today Chinese made gears are 2-3x the price and priced by the ratio.
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Drifting
Joined: Oct 2005
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From: East London/SW Essex UK
I would have thought the M21 and 3.08 is the perfect combination......any lower a rear end ratio and surely you'll just melt the tires trying to get the torque down to the ground.......perhaps better to consider timing curve (as already said) and invest in some sticky rear rubber?
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Me, I'm thinking a 3.36 or a 3.55 would be the ideal compromise.
Team Owner
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If you rear end is good why don't you install a 5 speed with non OD or even something like a tko500 3.27, 1.97, 1.34, 1, 0.68
The 4 speeds have high first gears. 2.56:1 ; Second Gear. 1.91:1 ; Third Gear. 1.48:1. 4th 1:1
The 4 speeds have high first gears. 2.56:1 ; Second Gear. 1.91:1 ; Third Gear. 1.48:1. 4th 1:1
1967 Pedal Car Champion
Joined: Mar 2020
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From: US-PNW
1st: Build facility (i.e., P = Muncie Transmission Plant)
2nd: Model year code (i.e., 2 = 1972)
3rd: Month code it was assembled (i.e., M = August)
4th & 5th: Day code it was assembled (i.e., 03 = 3rd Day)
6th: Gear ratio set (i.e., A=M20) (This code was added to identify ratios. Some early 1969 transmissions did not have this code. A=M20 B=M21, C=M22)
Team Owner
Joined: Jul 2004
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From: Redondo Beach, California
The ideal tranny for a 3.08:1 diff is a Richmond 5 speed transmission. The Richmond 5 speeds were originally named Doug Nash 4 speeds. The Richmonds have a first gear that is numerically larger than the Muncie first gear, while 5 gear is 1:1 as is the Muncie's 4th gear.
Drifting
Joined: Oct 2005
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From: East London/SW Essex UK
With its stock L36, F41 suspension, 7 leaf rear spring but 'soft dampers'.......traction 'off the line' with the 3.08 rear end and its C/R M21 certainly the first 30 yards or so was perhaps slower than an alternative rear end ratio........but I never really needed to be concerned about loss of and burning-off rear-end rubber. Front to rear weight transfer with the big block torque would help the cars rear-end to 'sit-down', dig-in hard and assist traction. The 'long' 1st gear ratio would simply mean one less gear change than your competitor........
Le Mans Master
Joined: Jan 2001
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chriscovell
I agree with gkull ..... Get one of those new 5-speed TKX, good for 600 lb-ft of torque and you can get Overdrive.
If I remember right my 1968 muncie M21 had a 2.28/1 First gear ratio ...you can get a TKX with a 2.87 first gear and 0.68 OD. The TKX with 2.87 First will give you 8.84 to 1 First Gear multiplication which should give you more than enough pep off the line. AND that 0.68 Overdrive will let that torquey Big BLock 3.08 cruise down the freeway at almost idle.
See below. You could get a 3.27/1 First but your OD would then "only" be 0.72......but your First gear multiplication would be close to the so-called "Optimum" 10:1..... I'd rather have the 0.68 OD myself....but there you are.
https://www.tremec.com/menu.php?m=184
I agree with gkull ..... Get one of those new 5-speed TKX, good for 600 lb-ft of torque and you can get Overdrive.
If I remember right my 1968 muncie M21 had a 2.28/1 First gear ratio ...you can get a TKX with a 2.87 first gear and 0.68 OD. The TKX with 2.87 First will give you 8.84 to 1 First Gear multiplication which should give you more than enough pep off the line. AND that 0.68 Overdrive will let that torquey Big BLock 3.08 cruise down the freeway at almost idle.
See below. You could get a 3.27/1 First but your OD would then "only" be 0.72......but your First gear multiplication would be close to the so-called "Optimum" 10:1..... I'd rather have the 0.68 OD myself....but there you are.
https://www.tremec.com/menu.php?m=184
Melting Slicks
Joined: Jun 2002
Posts: 2,844
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From: Orrtanna Pa.
Gear swap is a lot less costly than a trans swap, so I vote for the gears. Especially if the 4 speed is still working fine.
I had 3.70 gears with the th400 for years behind a 355, and later a 406. Had to upgrade tires to MT 295 55 15 drag radials with the 406 because hitting the gas hard in second would cause the rear to hula dance at 55 mph. Once traction issue was resolved I could hit second as hard as I wanted. I went with 4:11s later with the 406 for better pull off the line. Then when I swapped to a 427,.....The 4.11's seemed too steep. There is so much power now I feel like the 3.70 or maybe even 3.55 would have been a better choice.
I had 3.70 gears with the th400 for years behind a 355, and later a 406. Had to upgrade tires to MT 295 55 15 drag radials with the 406 because hitting the gas hard in second would cause the rear to hula dance at 55 mph. Once traction issue was resolved I could hit second as hard as I wanted. I went with 4:11s later with the 406 for better pull off the line. Then when I swapped to a 427,.....The 4.11's seemed too steep. There is so much power now I feel like the 3.70 or maybe even 3.55 would have been a better choice.