Speedometer Build

Overview

I have a ‘73 Corvette that has a Mercedes V12 and a Manual transmission in it. Unfortunately with this, there’s no outputs for a speedometer cable and no gear-driven distributor to drive the Tachometer.

The video below is a overview of the project, and the build process is below.

It’s a good option for people wanting to keep the factory look and function of a mechanical odometer, but have an LS or aftermarket engine.

Parts List

(1) 7806 Transistor

(1) 22k ohm Resistor

(1) 10k ohm Resistor

3D Printed Parts- Link to .STL can be found below

(1) Arduino Nano Every

(1) GPS Module- I used a GT-U7 off eBay

(1) Parallax Feedback 360 High Speed, Continuous Rotation Servo

(1) Generic small metal gear servo- I used these.

(1) GPS Antenna- I used this one from Adafruit

(1) SMA to uFL/u.FL/IPX/IPEX RF Adapter Cable

Generic headers for PCB

(8) #4 x 5/8“ screws

(4) #4 x 3/8” screws

(2) #8 x 3/8” screws

(2) #8 x 3/4” set screws

Step 1

Disassemble the dash until it’s at this point. There are a lot of good guides out there on how to get to the speedometer, so I’m not going to revisit that- But you’ll want to take it down to here. Save all the hardware!

Step 2

Let’s look at these for a second- They are actually Identical castings. There is 1 modification we are making to the Tach and speedometer unit, and then 2 additional one that are Speedometer Specific.

Step 3

Pull the front plate on the speedometer, using a small flathead screwdriver. then pull off the retaining tabs, slide the odometer set carefully away from the worm gear, over and out. repeat for the trip odometer. At this point you can pull the nylon worm gear and shaft carefully out.

Step 4

In this poorly written drawing, you can see the two points that need modified on the speedometer. Point A has a soldered plug and a plastic point that you can drill out with a 1/4” drill bit- the bar and plug should fall out. Point B will be empty and needs to be drilled out to 1/4” as well.

Step 5

Steps 5 through 7 apply to both Tachometer and Speedometer. Do it for both.

Remove the two remaining screws on the unit and pull it off. Holding the removed piece, you can see a tab on the inside of the unit- bend it back with pliers or break it off. There is a spring holding the two pieces together- this is not needed either.

Step 6

Steps 5 through 7 apply to both Tachometer and Speedometer. Do it for both.

Remove the two remaining screws on the unit and pull it off. Holding the removed piece, you can see a tab on the inside of the unit- bend it back with pliers or break it off. There is a spring holding the two pieces together- this is not needed either.

Step 7

Steps 5 through 7 apply to both Tachometer and Speedometer. Do it for both.

Apply superglue to the center point and re-assemble the plate. If done right, it should spin and rotate like normal, but at a 1:1 rate. We want the input shaft and the needle to spin at a 1:1 rate. I like to set on the front plate to align the unit while the superglue dries.

Step 8

Steps 5 through 7 apply to both Tachometer and Speedometer. Do it for both.

Install the two screws and on the tachometer, this is the final step. Feel free to install the front plate and set it aside. Speedometer has 1 more step.

Step 9

Take the velocity-stack shaped small piece from the 3D printed parts pile and superglue it to the white drive shaft. IMPORTANT: Piece MUST be installed before gluing. It cannot pass through the hole by design.

Step 10

Re-assemble the unit, and take care to verify the odometer guides line up with the bar. Install the clips and the front support bar- make sure the drive bar is in the hole.

Verify it worked by spinning the small piece that’s connected to the drive bar that’s now out the back of the odometer clockwise. NOTE: I do not recommend spinning the drive bar counterclockwise. It’s hard on the drive gears and also a felony.

Step 11

Take the tach and speedometer and install them back onto the gauge faces, and install the needles. Remember to include the turn signal lenses.