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Swapping to a TREMEC five- or six-speed almost always changes what your speedometer reads. Answer two questions and we’ll tell you which parts your car needs — then, if it’s a cable setup, size the driven gear that makes it read true.
What Does My Car Need?
It comes down to what your gauge wants and what your transmission puts out.
What you need
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Which Speedometer Gear Do I Buy?
Every transmission we sell uses a 7‑tooth drive gear, so the driven gear is normally the only thing you change. Fill in your tire and axle ratio — the answer updates as you type.
| Driven gear | Color | Part no. | Reads at a true 60 mph | Error | Buy |
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Tooth counts are whole numbers and only certain counts are made, so the nearest available gear is normally the answer — the steps land roughly 6 mph apart. Tires also measure a little differently from their printed size, and tread wear changes rolling diameter, so treat these as very close approximations rather than absolutes.
Speedometer tool v1.1
More detail, if you want it
Drive gear vs. driven gear
There are two gears in a cable-driven hookup. The drive gear rides on the transmission output shaft inside the tailhousing and is turned by the driveshaft. The driven gear is the one that pokes into the side of the tailhousing behind a retaining clip; it is turned by the drive gear, and its shaft is what the speedometer cable (or electronic sender) clips onto.
Every transmission we sell uses a 7‑tooth drive gear, so in almost every case the driven gear is the only thing you change. If you’re unsure, just count the teeth.
Why your gear can’t always be exact
The teeth are like stairs: each gear is exactly one tooth different from the last. To get a speedometer reading dead-on you may need something in between those steps, which land about 6 mph apart. US federal law (and the law in most other countries) allows some room between your actual speed and what the speedometer shows, so being a couple of percent out is normal.
If you want a very accurate reading you can move to an electronic or GPS speedometer, or fit a ratio adapter — a small gearbox that takes the rotation coming out of the transmission and speeds it up or slows it down with a pair of gears inside.
The calculation itself: tire revs/mile is 20168 ÷ tire diameter, and the
driven gear needed is (drive teeth × axle ratio × tire revs/mile) ÷
speedo revs/mile. The original MDL calculator assumed 1,001 revs/mile — enter
1001 above to reproduce its answers exactly.
Cables, adapters and plug kits
The gear sets the ratio; the cable carries it to the gauge. Ford transmissions use a slip-in cable end that pushes into the driven gear housing, while GM gauges normally want a 7/8" threaded cable. Mixing the two is common on swaps, which is what the adapters below are for.
- Speedometer cables by application — 1965-68 Mustang, 1969-73 Mustang/Cougar, 1987-93 Mustang, 1967-69 Camaro/Firebird, Falcon front-shift and more
- Ford slip-in to GM 7/8" threaded cable adapter
- Ford slip-in to GM 7/8" threaded, double O-ring seal
- Right-angle reverse drive adapter — 1964-67 Shelby Cobra Smiths gauges
- GM driven gear insert with seal — T5, non-Magnum T56, TH350/400
- Billet plug kit — seals the port when you go electronic
Going electronic: senders, harnesses and converters
An electronic speedometer counts pulses instead of cable turns, so the gear ratio stops mattering — you calibrate the gauge itself. What you need is a sender that fits your transmission and a harness that speaks to your gauge or ECU.
- Digital 16-pulse sensor with 10 ft harness — TREMEC TKO / TKX
- Analog VSS with cable pass-through — 1987-93 Mustang (8,000 pulses/mile) — use this when you want the electronic signal and a working cable gauge
- Analog VSS, no cable connection — 1994-98 Mustang
- Electronic speedometer VSS harness — TREMEC Magnum OSS and Ford TKX
- GM electronic speedometer harness
Going the other way — an original cable gauge behind an electronic transmission — is what the Abbott Cable-X converter is for. It reads the VSS signal and spins a mechanical cable output to within about ±2%.
Still not sure? Tell us the car, the transmission and what the gauge looks like behind the dash, and we’ll tell you exactly what to order. 208.453.9800, Caldwell, Idaho.
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