1972 Speedo gear help!
Thread Starter
Veteran
Joined: Oct 2005
Posts: 1,284
Likes: 81
From: Nashville, TN
1972 Speedo gear help!
Ok, So my transformation from a th400 to m-20 is complete. I have tried the red, blue and silver gears and none are correct. I have 26.5" tires and what I believed to be a 3.36 rear end but using my cell for a gps MPH gauge I am pretty sure I have a 3.55 in there. I have tried the red, blue and silver gears and none of them are correct. At 3000 rpm I am going around 68 mph, I have been using the only gear calculator I could find but it is for 63-67 vettes. I have a 72 with a 70-74 cast m-20 4 speed. Please help me!
Last edited by King Lear; Feb 14, 2015 at 12:59 PM.
Former Vendor
Joined: Aug 2006
Posts: 76,656
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From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
How far are you off? A general rule of thumb is 1 tooth per 5 mph. The only issue is if you run past the mesh range of the drive gear inside the transmission.
Someone posted a calculator page here that was for all cars a while back, maybe someone will see this and post it again.
Willcox
Someone posted a calculator page here that was for all cars a while back, maybe someone will see this and post it again.
Willcox
Thread Starter
Veteran
Joined: Oct 2005
Posts: 1,284
Likes: 81
From: Nashville, TN
How far are you off? A general rule of thumb is 1 tooth per 5 mph. The only issue is if you run past the mesh range of the drive gear inside the transmission.
Someone posted a calculator page here that was for all cars a while back, maybe someone will see this and post it again.
Willcox
Someone posted a calculator page here that was for all cars a while back, maybe someone will see this and post it again.
Willcox
Thread Starter
Veteran
Joined: Oct 2005
Posts: 1,284
Likes: 81
From: Nashville, TN
Former Vendor
Joined: Aug 2006
Posts: 76,656
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From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
You'll have to be careful to stay within the range of the drive gear or you will strip the teeth off of it.
I don't think you can go down any lower than a 20 tooth if my guess at the drive gear is correct. You may wish to stay with one of the gears you have and run a reduction ratio adapter. I based my choice on the drive gear solely off the other gears you've tried.
This adapter might be a better choice I'm not sure.
http://willcoxcorvette.com/product_i...ducts_id=24539
I don't think you can go down any lower than a 20 tooth if my guess at the drive gear is correct. You may wish to stay with one of the gears you have and run a reduction ratio adapter. I based my choice on the drive gear solely off the other gears you've tried.
This adapter might be a better choice I'm not sure.
http://willcoxcorvette.com/product_i...ducts_id=24539
Melting Slicks
Joined: Mar 2010
Posts: 2,268
Likes: 374
From: Randolph nj
the gears are still available from GM they are much cheaper too .. the adapter is 35.00-45.00 on ebay with the adapter you will find that it fits better and you wont struggle with the cable
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Former Vendor
Joined: Aug 2006
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From: Jeffersonville Indiana 812-288-7103
St. Jude Donor '08-'09-'10-'11-'12-'13-'14-'15
The calibration scale is linear with 250 rpms being 15 mph, 500 = 30 and 1000 = 60 and so on. The GM/Delco book tolerance is listed at 3-5 mph... But, it is odd to see one that is off 5 mph at say 15 mph and dead on at 60, even when they are wore out.
If your gauge is doing this then it may be the result of a rebuild using the later model GM head. Or someone just didn't calibrate it correctly. The last available GM speedo head for the 1969-1977 cars was junk and had the wrong magnet (flat) in it. When calibrating the speedo using this head we had to replace the tension spring with a lighter spring and then play with the calibration for hours trying to get it in spec. This lighter spring also made for a much jumpier speedometer than when the car was new.
Willcox.
Last edited by Willcox Corvette; Feb 16, 2015 at 08:54 PM.