Speedometer gears

[Donald M.]

I'm having a devil of a time locating a speedo gear for my 54 Vette. I think the problem is I just don't know what I'm doing. Someone told me I need a 14 tooth gear, which seems strange since gears I can locate that I think would fit an early power glide have from 17 to 24 teeth. Can anyone help me?

Don

[Richard G.]

Re: Speedometer gears

Don
input at the speedometer head should be 1,000 RPM at 60 MPH.
I have an excel sheet that can do the calculations.

Here is what is needed to be precise;
Drive and driven speedometer gears tooth counts.
Rear end ratio.
Tire size, preferably revolutions per mile.

The basic formula is: revs @ 60 = (tire revs/mile) (axle ratio)
As far as revolutions per mile 6.70-15s are 760 revolutions and 7.75-15s are 775.
Knowing the tire's outside diameter will get you close but because of the flat spot, when the tire is on the road, the calculations will not match the diameter.
If your tires are new typically you can get the rev per mile from the manufacture's specifications.

The difficult part is knowing the gearing selection.
The typical combination has to be one that was manufactured.

Another method is with a know gear set you figure out the % error and change the gears that same percentage.
Back when our cars were built close was good enough.
I know a 63 Corvette had a 3.5% error in the speedometer reading when it was new.
A 3.5% error is 2-mph at 60. It was on the slow side so GM was good with it as is.

[Chris S.]

Re: Speedometer gears

Mary Jo has one for sale

[Ray K.]

Re: Speedometer gears

The speedo gear set part # 3704230 listed in my '60 P & A catalog has a 13 tooth driven gear. This part # has application for '50 -54 Chev pass cars with P. G. and also 53 - 55 Corvette ( 6cyl). For use with a 3.55 rear axle ratio.

Ray

[Donald M.]

Re: Speedometer gears

Mary Jo has one for sale

Thanks, but I think what she has is a drive gear. I need the driven gear. I wasn't clear about that. In any case I emailed her about it.

Don

[Donald M.]

Re: Speedometer gears

Ray, do you know where I can get this part?

Don

The speedo gear set part # 3704230 listed in my '60 P & A catalog has a 13 tooth driven gear. This part # has application for '50 -54 Chev pass cars with P. G. and also 53 - 55 Corvette ( 6cyl). For use with a 3.55 rear axle ratio.

Ray

[Ray K.]

Re: Speedometer gears

Don,

Maybe a transmission shop that has been around for a long time that worked on the early '50's Power Glides. Also, a wrecking yard that might still have some old Chev's with the early P. G. . Definitely a long shot but that is best I have to offer.

Ray

[Donald M.]

Re: Speedometer gears

Ray, I may try searching for a gear as you've suggested. I found one online, NOS, for $285. Not gonna pay it. One option that looks really good is the SpeedHut Speedbox. It uses an electric motor to drive your speedometer and gets its signal from a GPS unit. I'm impressed.

Don

[Chris S.]

Re: Speedometer gears

I might have one
I have a couple PG's
Let me look

[Donald M.]

Re: Speedometer gears

I'm interested.

Don

I might have one
I have a couple PG's
Let me look