1957 4.56 speedometer driven gear

[Jeff B.]

Have a 1957 w/3 spd trans, that was born with a 3.70 rear end. Sometime along the way someone installed 4.56 gears. The speedometer reads way too fast. It currently has a green, 22 tooth driven gear. How can I make the speedometer read correctly?

[Joe L.]

Re: 1957 4.56 speedometer driven gear

Have a 1957 w/3 spd trans, that was born with a 3.70 rear end. Sometime along the way someone installed 4.56 gears. The speedometer reads way too fast. It currently has a green, 22 tooth driven gear. How can I make the speedometer read correctly?

Jeff-------


Assuming you are using tires that are at the original OD, the parts that you would need to correct your speedometer are long-since GM-discontinued and I know of no reproductions or replacements although it's possible some could exist. Assuming you could get the parts, you would need to partially disassemble the transmission since you would have to replace the speedometer DRIVE gear. You would also need to change the speedometer driven gear, but that's a simple matter. The parts you would need are as follows:

speedometer DRIVE gear-----GM #3845079------20 tooth steel gear

speedometer DRIVEN gear----GM #3845071------6 tooth steel gear

There is no driven gear that you can install to replace your current gear that will be compatible with your existing DRIVE gear AND which will give you a correct speedometer reading.

My suggestion in a case like yours would be to use a speedometer adapter to correct your speedometer reading. An adapter is a small "gearbox" that attaches to the speedometer fitting on the transmission and to which the speedometer cable is attached. A speedometer shop would be the best way to determine what adapter you need and prepare/sell you the proper adapter.

If you want to "chance it" based upon my estimation of what you need, I would recommend a GM #3890575 adapter which has a ratio of 0.8319. Unfortunately, this adapter is GM-discontinued but you might be able to find one.

[Mike M.]

Re: 1957 4.56 speedometer driven gear

the DRIVE gear you need was common to 55 up pass car chevys with 4.11 and 4.56 rear end ratios. the driven gear you'll need was common only to the 4.56 rear pass car and corvette (55 up). both drive and driven gears you'll need are STEEL gears. i went thru all this about 30 years ago when i put my 57 283 HP t-10 4.56 rear corvette thru the duntov process. i visited many tranny shops and junk yards and finally came up with the gear set that gave accurate speedo readings. nothing like the feel of a well tuned "bone stock" 57 283 hp vet with 4.56 rear gears. mike

[Joe L.]

Re: 1957 4.56 speedometer driven gear

the DRIVE gear you need was common to 55 up pass car chevys with 4.11 and 4.56 rear end ratios. the driven gear you'll need was common only to the 4.56 rear pass car and corvette (55 up). both drive and driven gears you'll need are STEEL gears. i went thru all this about 30 years ago when i put my 57 283 HP t-10 4.56 rear corvette thru the duntov process. i visited many tranny shops and junk yards and finally came up with the gear set that gave accurate speedo readings. nothing like the feel of a well tuned "bone stock" 57 283 hp vet with 4.56 rear gears. mike

Mike------

I'll bet one would have to visit a whole lot more tranny shops and junkyards these days to come up with those speedometer gears.

[Jeff B.]

Re: 1957 4.56 speedometer driven gear

Joe:
Since the current drive gear and driven gear are apparently compatible, probably the easiest fix would be an adapter. Assuming the speedometer would be correct if the rear end was a 3.70, dividing 3.70 by 4.56 is about .8112. I found a transmission parts supplier that can custom make adapters to your specs. Does my calculation make sense to you?

[Joe L.]

Re: 1957 4.56 speedometer driven gear

Joe:
Since the current drive gear and driven gear are apparently compatible, probably the easiest fix would be an adapter. Assuming the speedometer would be correct if the rear end was a 3.70, dividing 3.70 by 4.56 is about .8112. I found a transmission parts supplier that can custom make adapters to your specs. Does my calculation make sense to you?

Jeff-----


Yes, it does. However, you need to also consider tire size. The green 22 tooth gear you have installed is the correct gear for 3.70:1 and would give an accurate speedometer reading were that rear ratio currently installed in the car but only if the tire size is the same as original . So, the adapter ratio you mention will only result in accurate speedometer reading if your tire size is the same as original.

By the way, GM actually used speedometer adapters in PRODUCTION for some applications. After 1970, the highest tooth count driven gear that could be installed in a manual transmission was a 22 tooth silver gear. So, rear ratios higher numerically than 3:70:1 required the use of an adapter.

[Duke W.]

Re: 1957 4.56 speedometer driven gear

What tire size is currently on the car?

Duke

[Loren L.]

Re: 1957 4.56 speedometer driven gear

In addition to those cars mentioned by MM, #3845071 was also used by a number of 55-7 trucks, 3/4 ton and up. I'd google and see what comes up.

[Wayne M.]

Re: 1957 4.56 speedometer driven gear

Here's what Mike describes ( I think ). I've marked the 6 teeth on the drive gear 3845079. The other pic is the same drive with an NOS driven # 3860329 (20 teeth, steel with a yellow paint dab). But my 1965 P&A30 shows another 20-tooth steel driven gear, for '57 thru '60 cars; # 3845071, which Joe L. mentions. I think I have one of those as well, and it is about 1/4" shorter and the cable end has slots. Could take a comparison pic. Maybe for a shorter early style "bullet" ?

4.56-6 tooth drive.JPG456speedo.jpg

[Jeff B.]

Re: 1957 4.56 speedometer driven gear

This car currently has "reproduction" 6.70x15 wide white tires, similar to the original type tires. Once the adapter is installed, I figure the speedo should be right, if not, real close.
Thanks for everyone's info.
Jeff

[Joe L.]

Re: 1957 4.56 speedometer driven gear

Here's what Mike describes ( I think ). I've marked the 6 teeth on the drive gear 3845079. The other pic is the same drive with an NOS driven # 3860329 (20 teeth, steel with a yellow paint dab). But my 1965 P&A30 shows another 20-tooth steel driven gear, for '57 thru '60 cars; # 3845071, which Joe L. mentions. I think I have one of those as well, and it is about 1/4" shorter and the cable end has slots. Could take a comparison pic. Maybe for a shorter early style "bullet" ?

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Wayne------


First of all, I incorrectly reversed the tooth counts of the 1957 gears. The GM #3845071 DRIVEN gear is 20 tooth count. The GM #3845079 DRIVE gear is 6 tooth count.

I believe the early style fitting ("bullet") was longer and, thus, the driven gear was longer. The GM #3845071 is 3.165" overall length (end-to-end).

[Steven B.]

Re: 1957 4.56 speedometer driven gear

the DRIVE gear you need was common to 55 up pass car chevys with 4.11 and 4.56 rear end ratios. the driven gear you'll need was common only to the 4.56 rear pass car and corvette (55 up). both drive and driven gears you'll need are STEEL gears. i went thru all this about 30 years ago when i put my 57 283 HP t-10 4.56 rear corvette thru the duntov process. i visited many tranny shops and junk yards and finally came up with the gear set that gave accurate speedo readings. nothing like the feel of a well tuned "bone stock" 57 283 hp vet with 4.56 rear gears. mike

Where could you find a "bone stock", "283" CI, "283" HP '57??