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rear end ratio

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tomj View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote tomj Quote  Post ReplyReply Direct Link To This Post Posted: Sep/01/2016 at 11:16pm
somethings off here... assuming you have 25" diameter tires, common enough, your axle would have to be about 4.5:1 to do 47 mph at 3000 rpm. 

do what rockAMX did, assuming you have a smartphone, get a speedo app, i use digiHUD for android, works great, no ads or crap. they're surprisingly accurate and easy to use. 

when things don't make sense then its time to check your assumptions and maybe some of your tools...

and fix your tach, or get a cheapie to wire in for test purposes. with tire size you can calculate diameter online, that plus RPM and you can calc axle ratio close enough to pick from the list.

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 ccowx Quote  Post ReplyReply Direct Link To This Post Posted: Sep/01/2016 at 11:36pm
71 Javelin AMX OO,

If you have gone to the 23 tooth driven gear and are still out then you are tapped out for that gear set. The standard 8 tooth drive gear(the one inside the transmission, NOT the one on the end of the cable) needs a 23 tooth gear with 3.54's and 14" tires. Your 15" tires are what are making it read too high. There are no driven gears with more than 23 teeth, so you are maxxed out. 

What you need to do is to change the drive gear inside the transmission to a 6 or 7 tooth gear and then you can find a driven gear between 17 and 23 teeth that will make it read correctly. 

Thanks!

Chris 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rockAMX Quote  Post ReplyReply Direct Link To This Post Posted: Sep/02/2016 at 11:30am
Originally posted by ccowx ccowx wrote:

71 Javelin AMX OO,

If you have gone to the 23 tooth driven gear and are still out then you are tapped out for that gear set. The standard 8 tooth drive gear(the one inside the transmission, NOT the one on the end of the cable) needs a 23 tooth gear with 3.54's and 14" tires. Your 15" tires are what are making it read too high. There are no driven gears with more than 23 teeth, so you are maxxed out. 

What you need to do is to change the drive gear inside the transmission to a 6 or 7 tooth gear and then you can find a driven gear between 17 and 23 teeth that will make it read correctly. 

Thanks!

Chris 


15 inch wheels would make the speedometer slower, not faster. With 15 inch wheels and 70 series tires (standard size for 1971), that would give about a 27 inch tire height (http://www.gtsparkplugs.com/TireCalculator.html). Assuming the 8 tooth drive gear is correct, that would give a driven gear of 21-22 teeth (http://www.gtsparkplugs.com/SpeedoGearCalc.html) assuming a speedo reading of 1000 RPM at 60 MPH, which is quite standard.

My guess is that your transmission came off a different car and has a different number of teeth on the drive gear. Even with 60 series tires and a 26 inch tire height, that gives 23 teeth.

A good place to order the parts is http://www.laspeedometergear.com

DWR
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sonic Silver Quote  Post ReplyReply Direct Link To This Post Posted: Sep/02/2016 at 4:51pm
Originally posted by rockAMX rockAMX wrote:

Originally posted by ccowx ccowx wrote:

71 Javelin AMX OO,

If you have gone to the 23 tooth driven gear and are still out then you are tapped out for that gear set. The standard 8 tooth drive gear(the one inside the transmission, NOT the one on the end of the cable) needs a 23 tooth gear with 3.54's and 14" tires. Your 15" tires are what are making it read too high. There are no driven gears with more than 23 teeth, so you are maxxed out. 

What you need to do is to change the drive gear inside the transmission to a 6 or 7 tooth gear and then you can find a driven gear between 17 and 23 teeth that will make it read correctly. 

Thanks!

Chris 


15 inch wheels would make the speedometer slower, not faster. With 15 inch wheels and 70 series tires (standard size for 1971), that would give about a 27 inch tire height (http://www.gtsparkplugs.com/TireCalculator.html). Assuming the 8 tooth drive gear is correct, that would give a driven gear of 21-22 teeth (http://www.gtsparkplugs.com/SpeedoGearCalc.html) assuming a speedo reading of 1000 RPM at 60 MPH, which is quite standard.

My guess is that your transmission came off a different car and has a different number of teeth on the drive gear. Even with 60 series tires and a 26 inch tire height, that gives 23 teeth.

A good place to order the parts is http://www.laspeedometergear.com

There were no 15 inch, 70 series tires available on his car in 1971.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote rockAMX Quote  Post ReplyReply Direct Link To This Post Posted: Sep/03/2016 at 7:07pm
Well he says he has 15 inch wheels, so he would need 15 tires at the rim.Doesn't matter much if 70 or 60 series.
DWR
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sonic Silver Quote  Post ReplyReply Direct Link To This Post Posted: Sep/03/2016 at 8:19pm
Originally posted by rockAMX rockAMX wrote:

Well he says he has 15 inch wheels, so he would need 15 tires at the rim.Doesn't matter much if 70 or 60 series.
You said "with 15 inch wheels and 70 series tires (standard size for 1971)." And I said there were no 15 inch 70 series tires available for his car from AMC in 1971. I was just clarifying what was available. His 2nd post says he has "14 inch tires."

Edited by Sonic Silver - Sep/04/2016 at 9:04am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote one bad rambler Quote  Post ReplyReply Direct Link To This Post Posted: Sep/03/2016 at 8:53pm
Just go onto Richmond gear web site and use the calculator...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Gremlin73 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/04/2016 at 8:25am
I have 28 inch tires with 4.88 rear gear,at 3000 rpm the speed is 50 mph.you have slightly lower gears than mine. Good luck not very streetable.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sonic Silver Quote  Post ReplyReply Direct Link To This Post Posted: Sep/04/2016 at 9:03am
Originally posted by Gremlin73 Gremlin73 wrote:

I have 28 inch tires with 4.88 rear gear,at 3000 rpm the speed is 50 mph.you have slightly lower gears than mine. Good luck not very streetable.
His 2nd post says that he has 14 inch tires(probably 25 to 26 inches tall depending on size), so that would account for the difference, because his tires are shorter than yours.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote newat58 Quote  Post ReplyReply Direct Link To This Post Posted: Sep/04/2016 at 1:42pm
my wheels are 14 inch spedo exact with gps. I may leave the car alone as i probably wont be on freeway much only town driving. I can be cruising along at 30 hit the gas and get pretty good scratch as it shifts into third. 
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