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Diff ratio - compare revs 3:15 v 3:54

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FSJunkie View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FSJunkie Quote  Post ReplyReply Direct Link To This Post Topic: Diff ratio - compare revs 3:15 v 3:54
    Posted: Mar/13/2019 at 12:19pm
A 1:1 transmission high gear doesn't have to suck. It doesn't have to force a compromise axle ratio...if the lower gears in the transmission are a high enough (numerically) ratio. That is where AMC screwed up.

Most AMC manual and automatic transmissions have a 2.5:1 low gear and a 1:1 high gear. That sucks. The ratio spread simply is not wide enough to allow for one axle ratio that produces good low speed performance and good high speed performance. Most cars end up with a compromise gearing that doesn't make them truly good at anything.

...unless you bought Borg-Warner overdrive. Low gear is the same 2.5:1 but the high gear is now 0.7:1. You can run a high axle ratio for good low speed performance but drop into overdrive for high speeds. That 3.54 axle ratio becomes a 2.50 after the overdrive is engaged. It's absolutely perfect.

AMC got smarter in later years too. Starting in about 1975 they switched to 3 speed manual transmissions with low gears around 3.2:1 and 4-speed transmissions with low gears around 4.2:1, all with 1:1 high gears. These transmissions got themselves off the line very well with axle ratios in the 2.5:1-3.08:1 range and produced excellent highway cruising.

They got smarter on their automatic transmissions with the Eagle when they introduced the wide ratio lockup TC A998. A 2.7:1 first gear and a true 1:1 high gear. A 2.35:1-3.08:1 axle works perfectly with these. My Eagle gets itself off the line just as well with it's A998/2.35 as my Hornet does with it's A904/2.73.


Wide ratio 3 and 4 speed transmissions are your friend. AMC engines have a broad torque curve that works well with wide ratios.

1955 Packard
1966 Marlin
1972 Wagoneer
1973 Ambassador
1977 Hornet
1982 Concord D/L
1984 Eagle Limited
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Post Options Post Options   Thanks (0) Thanks(0)   Quote vinny Quote  Post ReplyReply Direct Link To This Post Posted: Mar/13/2019 at 10:59am
I had a few GM vans with I think 3.54 and 700R4 or newer so not bad, couldn't fault them. With 1 to 1 in the Rambler I like the 3.15 as a compromise.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Greyhounds_AMX Quote  Post ReplyReply Direct Link To This Post Posted: Mar/13/2019 at 9:22am
But you can have both if you run 3.54's with a T5.
1968 AMX 390 w/T5
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Post Options Post Options   Thanks (0) Thanks(0)   Quote purple72Gremlin Quote  Post ReplyReply Direct Link To This Post Posted: Mar/08/2019 at 11:20pm
  Just me.    In the old transmissions high gear is 1-1. (Unless you have OD).   I have a 72 Gremlin with the 3.54s. Its noisy at high speeds.    Way I see it...what do you want.
You want a highway criuser, get 2.56 or 2.87s.
You want performance ...3.54s.
Cant have both. 
Its a compromise. 
Stop complaining 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: Mar/08/2019 at 11:14pm
hey, i've got an online transmission axle tire calculator here if you wanna do what-ifs with axles and tires.


basically it calculates MPH in each gear, in the blue columns to the right. sorry it's got too many digits. makes it look intimidating.


1960 Rambler Super two-door wagon, OHV auto
1961 Roadster American, 195.6 OHV, T5
http://www.ramblerLore.com

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Post Options Post Options   Thanks (0) Thanks(0)   Quote amxdreamer Quote  Post ReplyReply Direct Link To This Post Posted: Mar/08/2019 at 10:55pm
I like the 3.31's that replaced the 2.87's that came in my car.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amxron Quote  Post ReplyReply Direct Link To This Post Posted: Mar/08/2019 at 10:39pm
About 90% of the V-8/auto cars built in the 70's had 2.87 gears.
 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: Mar/08/2019 at 4:40pm
Most of the Jeep places only carry 3.31 and lower gears. Looks like you can get 3.08 easy enough:
Your other option is to drill the 7/16" holes in the carrier out to 3/8" and use the bolts that came with the 2.87 gears... or new grade 8 3/8" (fine thread, IIRC) bolts.

I was thinking you should be able to find higher gears as my J-10 had 2.73 gears... but then I remembered it used Dana 44 axles, not AMC 20...
Frank Swygert
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jjwbeach Quote  Post ReplyReply Direct Link To This Post Posted: Mar/08/2019 at 2:33pm
Thank you for helping me too. Do you think I need to call a few Jeep aftermarket places and see if they carry something like the correct threading with highway ratio r&p options?
1970 AMX BBG X-code 390, amc turbo 400 auto, model 20 3:54
1979 Spirit AMX H-code('71 amc blue), 401, A/C, amc turbo 400 auto, model 20 3:54
1970 BBO AMX 4 speed, A/C, Tilt, original needs restoring
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jjwbeach Quote  Post ReplyReply Direct Link To This Post Posted: Mar/08/2019 at 2:29pm
Yes, sure do, I purchased a 2.87 ring and pinion to change out my 3.54, and the threads on the 2.87 are 3/8th instead of 7/16ths, so I wanted to get a highway ratio and use the chunk I have...is my thinking correct there or am I needing some more info, as when I called around, no one has a highway gear with 7/16 threads for my chunk I have now...
1970 AMX BBG X-code 390, amc turbo 400 auto, model 20 3:54
1979 Spirit AMX H-code('71 amc blue), 401, A/C, amc turbo 400 auto, model 20 3:54
1970 BBO AMX 4 speed, A/C, Tilt, original needs restoring
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