66' speedo issue
Thread Starter
Instructor
Joined: Jun 2012
Posts: 228
Likes: 3
66' speedo issue
Hi,
My 66' supposidely has a 3:55 rr end and the RED nylon trans gear. When I drive say at 40mph my speedometor reads around 25mph. Please advise whether I have a speedo problem or using the wrong gear???
Thanks, Jerry
My 66' supposidely has a 3:55 rr end and the RED nylon trans gear. When I drive say at 40mph my speedometor reads around 25mph. Please advise whether I have a speedo problem or using the wrong gear???
Thanks, Jerry
Race Director
Joined: Jan 2002
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From: Close to DC
C2 of the Year Finalist - Modified 2020
Assuming you have a Muncie 4-speed coupled with the 3:55 rear, it looks like you should have a green gear with 22 teeth. Red would have 21 teeth. How sure are you about the rear gears? If you see "AA" stamped on the underside of the rear, you have 3:55. If it's "AM" it's 3:36. 
Dennis
Dennis
Last edited by Bluestripe67; Feb 19, 2013 at 05:09 PM.
Safety Car
Joined: Nov 2004
Posts: 4,182
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From: going faster miles an hour...with the radio on in browns mills new jersey
You mentioned what gears your car "supposedly" has.
The first step toward solving your problem would be to find out what gears your car "actually" has.
P.S. After all these years, any codes showing what ratio was in the rear when it left the factory don't mean squat.
The first step toward solving your problem would be to find out what gears your car "actually" has.
P.S. After all these years, any codes showing what ratio was in the rear when it left the factory don't mean squat.
Team Owner
Joined: Oct 2000
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From: Washington Michigan
A 3.55 axle with stock tires takes a 21-tooth red speedo driven gear. Check the number of driveshaft revolutions it takes to turn the rear wheels one turn and verify the axle ratio. 
Team Owner
Joined: Oct 2000
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From: Washington Michigan
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From: Inverness FL
St. Jude Donor '07
Bill
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From: Inverness FL
St. Jude Donor '07
welllll.... there is no doubt that the speedometer assembly has a problem... now, what do we do.... if the speedometer is otherwise functioning, (no needle bounce or inconsistent speed readings), then Bubba says you try a different gear. how many teeth on that red gear? 21? you can get a 17, 18, 19 & 20 tooth gear; the less teeth the higher the speedo will read; a reasonable rule of thumb might be that for every tooth less you will pick up 5% in speedo speed... but the math seems to indicate that might only be worth 5mph max.
if that is the case, then an inline ratio adapter will make bigger adjustments... if you don't care about the odometer not working; here is my suggestion: call these people and tell them you want to pick up 15mph and see what they suggest in a ratio adapter (and you can always fine tune it by changing driven gears.
http://www.transmissioncenter.org/GM...eter_Gears.htm
Bill
if that is the case, then an inline ratio adapter will make bigger adjustments... if you don't care about the odometer not working; here is my suggestion: call these people and tell them you want to pick up 15mph and see what they suggest in a ratio adapter (and you can always fine tune it by changing driven gears.
http://www.transmissioncenter.org/GM...eter_Gears.htm
Bill