The Stovebolt Site Forums The Shop Area The HiPo Shop Speedo Adapter or Speedo Gear?

Well I have the 3.38 gears and I have been putting off the speedo calibration for a while. Its know time to look at doing it. What works better. The Speedo adapter witch I can only find at patricks or the new speedo gear that I need to install in the rear. Please let me know what you think works better. Im up for a few good ideas.

Thanks

I can't answer your question. I have used both in different trucks. What transmission are you using? sm420? It might make a difference.

I callibrated the speedo on my 59 gmc by changing the speedo gear and using charts. However I am using a Dodge transmission, so I had to use the dodge speedo pinion. Your experience will likely differ from mine.

It also depends on how far off it is. Compare the Odometer with the mile markers on the freeway. Often you can change the pencil gear by one or 2 teeth & get & get it close.

How easy is it to obtain drive/driven gears that work together and give you reasonable results?

I had an adapter made-to-order by the Speedometer Shop and it reads with almost no error. You give them a mile-marker mileage number - this way it takes into account your transmission, rear-end, and 16" tire diameter.

I'll do this again with the 1954 pickup that I am now restoring: 3.55 rear end, original Hydra-Matic transmission, and radial truck tires.

Jim Carter also sells these adapters and his price is quite reasonable (about $75).

Well thank you for the imput. I am running the SM-420 and stock wheel 235/75 R 15 with 3.38 gears. I will see if I can get a trans gear or a adapter. Ill see what the prices are.

You should calibrate it to the speed you most run in. My speedometer does not track correctly from 0 to 100. I used a set speed (drill motor set to correct rpm, 1000 rpm per mile) to check the speedometer head. The faster the drill went the slower the needle showed. Since the gears drive a magnet, which in turn moves the needle, I had to assume the magnet has gotten weak. There for, I calibrated mine to read correctly at 60 mph. Its the easiest speed to set since all the numbers I found on the internet were for 60 mph. At slower speeds it reads a little fast, anything higher then 60 it starts reading a little slower. Milage is dead on since its gear to gear from transmission to indicating numbers.

So when you call for a adapter, let them know how fast or slow it is by milage and speed. At least 10 miles of steady driving on a marked highway is best.

here is some information for you,

http://www.cadvision.com/blanchas/54pontiac/Speedometer.html


Joe

My Garmin GPS has a very accurate MPH reading included in its functions. If you have access to one, make up a comparison chart every 10 MPH to provide to your speedo shop, and they can make an adapter that will be as accurate as possible.
Jerry