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Diff ratio - compare revs 3:15 v 3:54 |
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farna 
Supporter of TheAMCForum 
Moderator Lost Dealership Project Joined: Jul/08/2007 Location: South Carolina Status: Offline Points: 19692 |
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 Posted: Nov/08/2018 at 7:09am |
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Play with the gearing cal at www.4lo.com first! You can change tire diameters and see that a tire size change doesn't do a lot, but it's good for fine tuning. A 26" tire with 3.54 gear gives 2745 rpm @ 60. A 29" tire drops rpm to 2461 - a difference of only 284 rpm. Worth it, but a 215/65R15 is 26", 225/80R15 would be 29" (29.2")... and there is actually a tire made that size! Only found one and it was out of stock. A 225/75R16 is 29.3", so close, and more available. Note that Jeep XJ wheels have a deep offset, as most 4WD and FWD vehicles have. As noted, a Jeep XJ axle with Jeep wheels fit under a Hornet/Concord, but it's about 3" wider than a Hornet/Concord axle...
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Frank Swygert
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jjwbeach
AMC Apprentice 
Joined: Jun/26/2011 Location: Cocoa Beach, FL Status: Offline Points: 81 |
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Posted: Mar/08/2019 at 2:05pm |
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Do you know of a highway AMC model 20 like a 3.08 or 2.87 that has 7/16 hole instead of 3/8 hole? I am in need of around 3.0 ring and pinion for my Spirit that would fit 7/16th.
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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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401MATCOUPE
AMC Addicted 
Joined: May/20/2010 Location: Salisbury, MD Status: Offline Points: 5367 |
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Posted: Mar/08/2019 at 2:15pm |
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Do you mean a 7/16" thread in the ring gear, vs 3/8" thread? If so, any ring a pinion gear set from 1967 up car uses a 7/16" thread. 3/8" thread was only used on early AMC 20 rear axles.
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Ross K. Peterson
68X,GoPac,343,AT,52A(1stCar) 68X,GoPac,390,4sp,52A 69X,GoPac,390,4sp,64A 70X,GoPac,390,4sp,87A,8 70X,GoPac,390,4sp,BBO,8 70 Jav SST,390,AT,BSO 74MatX,401,AT,Prototype 74MatX,401,AT |
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jjwbeach
AMC Apprentice
Joined: Jun/26/2011 Location: Cocoa Beach, FL Status: Offline Points: 81 |
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Posted: Mar/08/2019 at 2:29pm |
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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...
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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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jjwbeach
AMC Apprentice
Joined: Jun/26/2011 Location: Cocoa Beach, FL Status: Offline Points: 81 |
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Posted: Mar/08/2019 at 2:33pm |
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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?
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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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farna
Supporter of TheAMCForum
Moderator Lost Dealership Project Joined: Jul/08/2007 Location: South Carolina Status: Offline Points: 19692 |
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Posted: Mar/08/2019 at 4:40pm |
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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...
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Frank Swygert
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amxron
AMC Nut
Joined: Nov/18/2011 Status: Offline Points: 295 |
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Posted: Mar/08/2019 at 10:39pm |
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About 90% of the V-8/auto cars built in the 70's had 2.87 gears.
Ron.
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AMXron
Fleet/Jeep Mgr. Orbit AMC/Jeep 50-1787 |
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amxdreamer
AMC Addicted
Joined: Jul/30/2008 Location: Vancouver, BC Status: Offline Points: 8511 |
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Posted: Mar/08/2019 at 10:55pm |
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I like the 3.31's that replaced the 2.87's that came in my car.
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Tony
Vancouver, BC 1970 AMX 1972 Badassador AMO#10333 |
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tomj
AMC Addicted
Joined: Jan/27/2010 Location: earth Status: Offline Points: 7555 |
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Posted: Mar/08/2019 at 11:14pm |
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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. |
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1960 Rambler Super two-door wagon, OHV auto
1961 Roadster American, 195.6 OHV, T5 http://www.ramblerLore.com |
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purple72Gremlin
AMC Addicted
Charter Member Joined: Jul/01/2007 Location: Illinois Status: Offline Points: 16614 |
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Posted: Mar/08/2019 at 11:20pm |
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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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