My '70 Elkay BB came with the TH400 trany and the stock 3:31 rear end. Somewhere along the line, a previous owner switched the stock 14" wheels with 15" rally wheels which has thrown the accuracy of the speedo off.
I found a web site that is supposed to determine what gear ratio I should have but it's still black art to me.

Based upon my rear end ratio and tire diameter (25"), the web site tells me that I need a 9 tooth tranny gear and a 24 tooth speedo gear if I have "small speedo gears" and a 15 tooth tranny gear and a 40 tooth speedo gear if I have "Large speedo gears".

I don't know if I have large, small or super sized gears. C'mon guys... Has anyone (I know you are out there) put 15" tires on their Chevelle or Elkay and had to switch speedo gears. What did you switch and where did you get what you needed?

turbo 400's have large speedo gears. tom

Pull the "bullet" out of the side of the tranny. All you need is a 7/16" wrench and maybe a pair of pliers to remove the speedo cable. The "driven gear will come out with the "bullet" Once the "bullet is out you then have access to the "drive" gear and you can start counting teeth.

After you do that then go back to your calculator and do the math. If there is any way possible to change ONLY the driven gear to get your speedo accurate thats what you want to do. Changing the "drive" gear is a major task compared to changing the "driven" gear. The "driven" gear just slips out of the "bullet".

All of this will become clear when you get under your car and look at where your speedometer cable attaches to your transmission.

I have attached a pic of the "bullet". Forever more you will now know why its called a "bullet". :)

You'll note that there is a rubber "O" ring on it. Inside the "bullet" there is another seal. If you take the "bullet" out thats a great time to put in new seals. GM sells them at all of their dealerships.


Here's another link that will give you a look at what you can expect speedo gears to look like. Look at the 7004R/ TH400 section.

http://members.tripod.com/~grannys/speedo.html


The site is about an RX-7 tranny swap but the info is universal.

I hate to say it but I tried that on my jeep and the tranny gear didn't touch the speedo gear had to put it back but that was a standard trans.