Speedometer Gear In Transmission
 

How do you determine which one you need?

Vendors have them listed as 18 teeth to 25 teeth, each one is a different color.

Just off a restoration and speedometer is not working.

Speedometer was restored by Ricks and cable is a new repop. i had the trans rebuild but not sure if he replaced the speedometer gear, but there was one in there when i hooked the cable up. I havent pulled the current one out yet.

Car is a 65 coupe with a 4 speed, 396. Originally was a 327/300 hp car.

Thanks!

Hi Patrick,

I just went through this and found the calculators get you close but fine tuning is needed so get a set of gears

Phil

https://www.ebay.com/i/132657059883?chn=ps

You need to specify the installed axle ratio and tire size/revs per mile.

Duje

Thanks for the help, i'm going to get the specs today and see what it shows. I went ahead and ordered the gear kit, same price as if i just purchased one from an online corvette parts vendor :ack:..so also thanks for saving me some money!

Update: there was an old gear in there and looked like the opening for the speedo cab was rounded off. Installed a new one and speedo works now.

now I’m off to check the accuracy of the gear.

thanks for everyone’s help.

Revisiting this....I finally got all the major issues fixed on my build and now getting back to the little things.

I started driving the car more and felt like i was driving faster than the speedometer was showing.

Had my wife follow me and it was showing a slower speed.

I downloaded a speedometer app and drove the car and the speedometer is reading about 3-4 mph slower than the app. I have a new gray gear in the trans which is the highest number in the gear kit i bought.

How do i fix this or what is my options.

I live in a neighborhood with school zones all around, so cops will get you for going 5 mph over.

Thanks!

Your concerned because a 60 year old mechanical gauge is reading 3 mph low ?
They were prob that far off when new.

Just drive slower thru the school zones.

I’m sorry but did my post offend you? I thought a new gauge would read more accurate than that.

Just FYI, there are 2 different drive gears available for the transmission output shaft, I believe they are either 8 tooth or 9 tooth. If you really want to get it close, you may have to change that gear, which means tranny comes back out.Personally, I wouldn't worry about it, load a GPS speedometer app on your phone and use if if you want to be more precise.

yep i was thinking the same, thanks jrm

At what speed? Any speedometer error is linear. 3-4 mph low at 100 is insignificant. At 20 mph that's a big deal.

Crusin around 35-40 mph in a 35 mph zone. Thanks

So about 10% low, then.

yep, so if i have a grey 22 in there now drop down a size or go for the blue 20? i have a lift so pretty easy to swap out. thanks

Haven't checked out speedometer gears in years but if the speedometer is reading around 10% slow that would seem a logical next step - next lower number of teeth to speed up the cable rotation. Not sure if speedometer head output is completely linear or not so may be a multiple trial experiment.

To begin with, how do you know that the speedometer on your wife's car is accurate? You really need to check this with a GPS inside your vette. I use a GPS in multiple vehicles that I own 2015 Toyota, 2019 Toyota, 2005 GMC, 1942 Ford GPW, 1959 Vette, and a 1967 vette. None of them are completely accurate at 35MPH, 65MPH 75 MPH, etc. They all vary by 2-3 MPH. The 67 shows 3MPH faster than the actual speed at 35-40, so I shouldn't worry about tickets. It's better to be in error on the lower speed side than the over the reading. If you want it exact, it may require the removal of the speedometer head to have it recalibrated by a competent shop.

i swapped the gear out with a 21 red today, but discovered a coolant leak where the coolant neck meets the intake, ill post an update after i get the leak fixed, thanks everyone.

I have a similar problem in my '63 with a Powerglide, I think it reads about 10% low and I have the smallest gear I could buy when I did the car. I'd like to fix it but for now just live with it. This was 20 years ago, I doubt there's been any change in the gears available. :/