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

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Craigie View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Craigie Quote  Post ReplyReply Direct Link To This Post Topic: Diff ratio - compare revs 3:15 v 3:54
    Posted: Nov/01/2018 at 2:32am


I currently have 3:54 gearing in my AMC 20 bolted into my V8 Hornet.
At highway speeds of 60/65 mph I'm sitting on 3000 revs

I am contemplating switching to 3:15 gearing to save a bit on gas costs and not be pushing my motor so hard. I can see myself doing a lot of highway driving in the future.

Can anybody comment on what revs I would be sitting on with 3:15 gearing doing those same 60/65 mph highway speeds ? or should I be selecting something else ? From what I have read, my Auto SC/360 came standard with 3:15 gearing.

Many Thanks
Craigie.........

1971 SC/360 Hornet
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Post Options Post Options   Thanks (0) Thanks(0)   Quote ambassador401 Quote  Post ReplyReply Direct Link To This Post Posted: Nov/01/2018 at 2:51am
2670rpm
68 American
68 Rebel
70 Javelin
71 Ambassador
A bunch of Jeeps
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FSJunkie Quote  Post ReplyReply Direct Link To This Post Posted: Nov/01/2018 at 3:42am
I have two cars, both with 327 V8s, both that I have driven on long (1500+ mile) cross-country trips, and both with the same tires:

1965 Ambassador 3.15 axle ratio.
1966 Marlin 3.54 axle ratio.

The Ambassador is noticeably less stressed at speeds over 70 MPH. The Marlin will cruise 75 MPH all day long if I ask it to, but I can tell it would rather not go that fast. Meanwhile the Ambassador is perfectly happy blasting 80 MPH all day. 

You won't notice much of a difference in fuel economy. The Marlin still gets close to 20 MPG at 70 MPH (3300 RPM). The main thing you notice is less noise, vibration, lower engine temperature, and less oil consumption. 

I really, really, really want a 3.54 axle with a 3-speed overdrive. Or a 2.87 axle with a really wide ratio 4-speed.
'66 Marlin: 327/T10/3.54 Twin Grip
'72 Wagoneer: 360/TH400/3.31
'73 Ambassador: 360/TF727/3.15
'77 Hornet: 232/TF904/2.73
'84 Eagle: 258/TF998/2.35
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sonic Silver Quote  Post ReplyReply Direct Link To This Post Posted: Nov/01/2018 at 8:32am
Originally posted by Craigie Craigie wrote:



I currently have 3:54 gearing in my AMC 20 bolted into my V8 Hornet.
At highway speeds of 60/65 mph I'm sitting on 3000 revs

I am contemplating switching to 3:15 gearing to save a bit on gas costs and not be pushing my motor so hard. I can see myself doing a lot of highway driving in the future.

Can anybody comment on what revs I would be sitting on with 3:15 gearing doing those same 60/65 mph highway speeds ? or should I be selecting something else ? From what I have read, my Auto SC/360 came standard with 3:15 gearing.

Many Thanks
11% fewer revs. So, about 330 fewer rpm.
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Have2SC's View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Have2SC's Quote  Post ReplyReply Direct Link To This Post Posted: Nov/01/2018 at 11:50am
 Craigie, IMO, it wouldnt be worthwhile changing from 3:54 to 3:15s. Been there done that. You may want to consider 2:87s if you are looking to make a bigger change to your highway driving rpm's.  Lee
1971 SC360 Hornet 4 speed 1969 BBG AMX 390 Auto 1970 Javelin 390 4 speed "Base Model"     
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Post Options Post Options   Thanks (0) Thanks(0)   Quote raysinvegas Quote  Post ReplyReply Direct Link To This Post Posted: Nov/01/2018 at 12:47pm
I installed a 3.08 from an early 70's CJ and it made a huge difference for me. They are not easy to find according to the shop.
Andy Ray
64 440H
64 440 Convertible
68 Javelin SST 343
69 Javelin SST 343
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Post Options Post Options   Thanks (0) Thanks(0)   Quote LakesideRamblin Quote  Post ReplyReply Direct Link To This Post Posted: Nov/01/2018 at 1:44pm
I have a 2.73 in my 69 Rambler with an AMC 360 and a Ford 8.8 rear end.  Love it.  So the AMC 20 2.87, per Lee above, would be a great highway cruiser gear.
LakesideRamblin
69 Rambler 220 360
68 American 220 199
"If you could kick the person in the pants responsible for most of your trouble, you wouldn't sit for a month." T. Roosevelt
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sonic Silver Quote  Post ReplyReply Direct Link To This Post Posted: Nov/01/2018 at 2:07pm
Originally posted by Craigie Craigie wrote:



I currently have 3:54 gearing in my AMC 20 bolted into my V8 Hornet.
At highway speeds of 60/65 mph I'm sitting on 3000 revs

I am contemplating switching to 3:15 gearing to save a bit on gas costs and not be pushing my motor so hard. I can see myself doing a lot of highway driving in the future.

Can anybody comment on what revs I would be sitting on with 3:15 gearing doing those same 60/65 mph highway speeds ? or should I be selecting something else ? From what I have read, my Auto SC/360 came standard with 3:15 gearing.

Many Thanks
The auto SC/360 with a 2 barrel carb came with 2.87 std and 3.15 optional. The 4 barrel automatic (like yours) came with 3.15 std and 3.54 optional.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote amxdreamer Quote  Post ReplyReply Direct Link To This Post Posted: Nov/01/2018 at 9:41pm
I had a 2.87 in my AMX and it was fabulous on the highway....but was boring in town. I went to a 3.31 and now it is way more fun to drive but sucks on the highway! lol That being said I'm gald I made the change.

IMO the 3.15 will be a big improvement over what you have for the highway but will still leave a little bit of fun compared to going 2.87
Tony
Vancouver, BC
1970 AMX
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FSJunkie Quote  Post ReplyReply Direct Link To This Post Posted: Nov/02/2018 at 2:59am
It depends on what gear ratios you have in the transmission and the torque converter stall if it's an automatic. A wider ratio spread or higher stall allows the use of a lower axle ratio without making it a slug off the line. 

Or overdrive. Just gives you a wider gear spread in the opposite direction. 

The problem is that most of us are not lucky enough to have a low gear of more than 2.5 or a high gear of less than 1. Some of us are lucky and have the later wide ratio A998 or an overdrive hanging on the back of our 3-speed manual. Those lucky people make fewer compromises in their axle ratio. Do you know who compromises the most on their axle ratio? People with 4-speed manuals. 

'66 Marlin: 327/T10/3.54 Twin Grip
'72 Wagoneer: 360/TH400/3.31
'73 Ambassador: 360/TF727/3.15
'77 Hornet: 232/TF904/2.73
'84 Eagle: 258/TF998/2.35
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