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Southwest Fabrication
Caddo Mills, Texas7am–3pm Central, Mon–Fri469-688-6985

Tech explainer

Mechanical, sensor, and the kit

One simplified automatic, three ways the output shaft can report speed: a cable, a sensor, or both at once.

By Southwest Fabrication, Caddo Mills, Texas ·

Short answer

A mechanical drive uses gears and a cable. An electronic drive uses a toothed ring and a sensor. The SW Fab kit puts both jobs on one assembly on the output shaft, so a mechanical gauge can stay and the computer still gets a speed signal.

Look at all three

Drag to look around. Pinch or scroll to zoom. Hover or tap a part to name it. The slider only speeds the picture up. It is not a miles-per-hour reading.

Still drawing of a GM-style automatic with a mechanical speedometer drive in the tail housing.

Mechanical

Inside the tail housing, a drive gear on the output shaft turns a driven gear. That gear sits in a retainer that enters the side of the housing and spins the cable out to the gauge.

Still drawing of the same automatic with a tone ring, a vehicle speed sensor in the tail housing, and a wire to the computer.

Electronic

Same transmission. A toothed ring rides the output shaft, and a vehicle speed sensor threads into the tail housing. Each tooth makes a pulse on the wire to the computer, and the gauge follows.

Still drawing of the SW Fab kit: one shaft assembly driving a mechanical cable and a vehicle speed sensor.

The SW Fab kit

Mechanical and electronic

One assembly on the output shaft is both the speedometer drive gear and the toothed reluctor. It turns the cable for a mechanical gauge and still sends pulses to the computer.

A simplified 4L60E/4L80E-style automatic, not a measured drawing. The tail housing stays see-through so the shaft shows. X-ray fades the rest of the case. Full transmission and Close-up move the camera. Explode pulls the pieces apart.

Why an LS swap customer picks a mechanical kit

The electronic overdrive in a typical LS swap was built without a cable. The original speedometer in an older car was built with one. Leave it like that and the needle sits on the peg.

The third picture is that kit. One assembly on the output shaft drives the mechanical cable and the toothed ring the sensor reads, so the factory gauge can stay in the dash and the computer still gets a speed signal. The gears are cut for your tire and rear gear. If the combination is odd, call 469-688-6985 and the shop will cut it to suit.

The 1998-and-newer 4L60E speed kit and the 4L80E speedometer kit are the bolt-on versions. A full tail housing is the other way to get the same cable drive. The gear calculator is the tooth-count math, and why these transmissions have no cable is the longer written version.

Talk to the shop

Southwest Fabrication, 4540 FM1565, Caddo Mills, Texas. 469-688-6985. Monday to Friday, 7am–3pm Central. Quality Custom Craftsmanship.

4L60E speed kitContact

More from the shop

Tail housing kit parts laid out, including housings, adapter, gears and hardware

Speed kit

4L60E / 4L65E / 4L70E speed kit

A bolt-on cable drive for a 1998-and-newer 4L60E-family transmission, with the vehicle speed sensor still working.

View part
4L80E tail housing used with the speedometer kit

Speed kit

4L80E speedometer kit

A factory-style mechanical hook-up for a 4L80E that leaves the stock electrical setup alone.

View part
Tail housing with the speedometer gear index visible

Tech explainer

Why these transmissions have no cable

What the vehicle speed sensor does, and the ways to get a working speedometer in an older car.

Read more