Converting the old mechanical Stewart-Warner speedometer to GPS-stepper motor

After viewing this page, you might agree it would be more accurate to title it, “Creating a Digital/Analog Speedometer From Scratch“. The only parts I ended up using from the original Stewart-Warner unit are the chrome bezel, glass window, and the metal case.

This somewhat impractical idea turned into quite a long, drawn out sub-project that I nearly gave up on when I came into some extra funds and went ahead and purchased a new electronic speedo unit from Speedway Motors. After fiddling with the imported Speedway unit for nearly a year trying to make it work, and still not having a reliable speedometer with recorded miles that didn’t self-erase, I concluded this purchased unit had too many problems, so my GPS-stepper-motor project got dusted off and revived.

A little more background

Back in 1976, when I first assembled this Valkyrie, I installed a nice new Stewart-Warner mechanical speedometer in the dash next to a Sun brand tachometer with a matching black and chrome style. They looked great together, but I never got around to getting a long speedometer cable and figuring out how to make the German built ZF transaxle work with the American made Stewart-Warner speedometer. It was one of those details I kept putting off because there were so many more important issues that needed solving. Thirty-five years later, during the second major rebuild, I decided I really needed a working speedometer. I also noticed that electronic speedometers, having been used by OEMs for years in new cars, were becoming popular in the aftermarket. My thinking was, an electronic speedo could pick up pulses from a hall-effect sensor mounted near the output shaft flange of the ZF transaxle and I wouldn’t have to be concerned with adapting long mechanical speedometer cables and figuring out gear ratios, and all that stuff. After seeing how relatively expensive new electronic speedometers were at the time, I got the great idea that I was going to figure out how to convert my existing SW speedo unit to work with a stepper motor. Many years earlier, I had done some programming of microcomputers in the industrial control environment, and this sounded like it could be a fun project, even though I already had other ongoing tasks related the the Valkyrie restoration.

The original face and pointer were not reused.

Chrome bezel with the glass and the case are the only parts used from the original speedometer..

The odometer mechanism was mounted in this cast frame.

speedo-project-CAD-4-views-12-05-2024 — 3D CAD and 3D printing helped make this a success

PXL_20220722_184152912 — 3D printing the stepper motor holder

PXL_20250509_215527541 — Stepper motor holder design was tweaked for more a bit room.

PXL_20250508_191053517 — Single line LCD for odometer

stepper-controller-module — Stepper motor controller

Limit-switch — Limit switch signals at known point for calibration

DC-DC-converter — 12 VDC-to-5 VDC converter

Arduino-UNO-R4-Minima — Arduino microcomputer converts GPS info to stepper motor position and records mileage.

PXL_20250508_191745633.MP — A new face was printed on photo paper

PXL_20250508_191352288 — SMD- LEDs provide backlighting

PXL_20250508_191347143 — the original Stewart-Warner metal case was reused.

PXL_20250508_191304930 — 3D printed cam lobe used for power-on calibration.

PXL_20250430_172857843 — Circuits were first tested on a breadboard

PXL_20250513_023538999.MP — Mobile testing required placing all components ina box.

photo_10_2026-07-31_19-19-56 — Items not fitting in the case are mounted on a lexan panel, which will clamp to a bar behind dash, as demonstrated in image.

photo_1_2026-07-31_19-18-58 — Tesa tape wrapped wires made installing the speedo system smoother compared to zip ties.

photo_2_2026-07-31_19-18-58 — Tesa tape also retains wires connecting to the Arduino expansion board

photo_2_2026-07-31_19-19-56 — This square tube shows how the assembly will mount behind the dash to an existing bar.

photo_7_2026-07-31_19-18-58 — 3D printed wing nuts made speedo gauge installation tool-free.

photo_4_2026-07-31_19-18-58 — 3D printed bracket snaps on to square tube under dash.

Disclaimer: The images and drawings displayed on this website represent how I designed and/or implemented the construction of my car. I am a car hobbyist, not a professional automotive engineer and I am not representing that any images or text on this website are the best or correct way to do anything. The content on this website is only for thought provoking and entertainment purposes. Bottom line: I am not responsible for anything resulting after someone views this content.

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