What a speed sensor actually does
At its simplest, a speed sensor watches something rotate and sends a pulse every time a tooth, bolt or magnet passes. The ECU counts those pulses. More pulses per second means faster rotation, and once the ECU knows how many pulses equal a given road speed, it can report speed at any moment.
Before wiring anything, you need two answers: where the sensor is, and what type it is.
Where to find one
- On the transmission. The most common factory location. It reads the output shaft, which turns in direct proportion to road speed.
- At one wheel hub. Some cars read a toothed ring on a single hub instead.
- At all four hubs. That almost always means ABS. These sensors exist so the ABS computer can spot a locking wheel under braking.
The four sensor types
| Type | Signal | How easy to use |
|---|---|---|
| Hall effect | Digital square wave | Easy. Share the signal straight to a digital input |
| Interrupt style | Digital | Easy, wired like a Hall sensor |
| Magnetic inductive (reluctor) | Passive AC signal that grows with speed | Fine on its own, tricky when shared with ABS |
| Magneto resistive | Active, current-based | The hardest to share with an ABS module |
Getting the type right matters because each one is wired differently. A digital sensor on the gearbox is the dream: tap the factory signal into a spare digital input and you're done.
Sharing ABS sensors (or not)
If the car still runs ABS and the wheel sensors are inductive or magneto resistive, sharing them gets complicated. The ABS computer and your ECU both need a clean signal, and connecting both directly can upset one or the other. Doing it properly takes extra interface circuitry to keep the two isolated. Our ABS wheel speed sensor guide goes into the options, but a lot of the time we simply recommend fitting a dedicated sensor.
Fitting your own sensor
It's less work than it sounds. A Hall effect sensor on a small bracket can read anything steel that passes at a steady spacing: wheel stud heads behind the hub, the bolts on a CV joint, or the bolts on a driveshaft flange. For driveshafts, a magnet collar such as the Haltech split collar gives you evenly spaced targets without machining.
Driven or undriven wheel?
A driven wheel shows how fast the tires are turning, including wheelspin. An undriven wheel shows true road speed. We usually fit the driven sensor first, then add an undriven one. With both, the ECU can compare them and calculate slip, which is the foundation of any traction control strategy.
GPS instead of a wheel sensor
A GPS speed input module takes its speed from satellites and outputs a speed signal the ECU treats like an undriven wheel. It's a tidy option when mounting a sensor at the wheel is awkward.
Calibrating road speed
There's no need to work out gear ratios, diff ratio and tire size by hand. Drive at a steady, known speed, either on a dyno or checked against a GPS app on your phone, enter that speed in the software and hit calibrate. The ECU measures how many pulses it's seeing at that speed and works everything else out from there. Recheck it if you change tire size or final drive.
What you can do with road speed
- Send speed to a digital dash.
- Speed limits for valet mode or a pit lane limiter.
- Smoother idle return: anti-stall strategies that bring the engine back to idle gently based on how fast the car is rolling.
- Boost control that changes with road speed.
- Traction control built on the difference between driven and undriven wheels.
For setup details, see our article on vehicle speed sensors. Not sure which sensor suits your car? Ask us.
