Passengers see a tariff and a display; regulators see a measuring instrument that must stay within fractions of a percent. Between the two sits the stream of pulses, or the digital distance value, that tells the taximeter how far the vehicle has moved. Get that signal right and the instrument does its job for years. Get it wrong and the error is billed to every passenger, on every trip.
The legal frame: MID Annex IX and OIML R 21
In the European Union, taximeters placed on the market fall under the Measuring Instruments Directive, MID 2014/32/EU. Its instrument-specific Annex IX, known as MI-007, defines the taximeter as a device that “works together with a signal generator to make a measuring instrument”: it measures duration, calculates distance from the signal delivered by the distance signal generator, and calculates the fare. A footnote then draws the line that shapes every installation: the distance signal generator is outside the scope of the Directive.
Internationally, the reference is OIML R 21 on taximeters. Its 2007 edition is the one most national rules still cite, and revision drafts have circulated within OIML. Where MI-007 sets essential requirements, R 21 adds the test detail: signal levels, pulse frequency ranges and the errors allowed once the instrument is installed in a vehicle.
The k-constant and the vehicle's coefficient
OIML R 21 defines the taximeter constant k as the number of pulses the taximeter must receive to indicate one kilometre correctly. The vehicle has its own number, usually written w: the pulses it actually delivers over a measured kilometre. Calibration means making k equal to w, within the instrument's resolution, and securing that setting. R 21 requires k to be displayable as a readily accessible decimal number, with every change secured and counted by a non-resettable event counter.
The k-constant is therefore not a property of the taximeter model. It belongs to one vehicle, on one set of tyres, with one drivetrain. Change any of those and the constant is wrong.
w = p × i × (1 000 000 ÷ l)Worked example: from tyre to pulses per kilometre
- 01Establish the rolling circumference
A 205/55 R16 tyre has a nominal diameter of about 632 mm, or roughly 1,985 mm around. Under load it rolls shorter; take a measured effective circumference of l = 1,935 mm.
- 02Convert to wheel revolutions
1,000,000 ÷ 1,935 = 516.8 wheel revolutions per kilometre.
- 03Apply the sensor geometry
A transmission output sensor with p = 8 pulses per shaft revolution and a final drive of i = 4.06 gives 516.8 × 4.06 × 8 ≈ 16,786 pulses/km. Each pulse represents about 6 cm of travel.
- 04Check the frequency range
Input frequency is f = w × v ÷ 3,600. At 5 km/h the taximeter sees 23 Hz, at 50 km/h 233 Hz and at 200 km/h 933 Hz. R 21 type testing spans 5 km/h to at least 200 km/h, so the input must count cleanly across a 40:1 range.
Why tyres move the number
Tread depth is part of the rolling radius. A passenger-car tyre starts with roughly 8 mm of tread; the EU legal minimum is 1.6 mm. Losing 6.4 mm of radius shortens the circumference by 2π × 6.4 ≈ 40 mm. On our 1,935 mm tyre that means 2.1 % more revolutions, and therefore 2.1 % more pulses, per kilometre. If k was set on new tyres, a 20 km trip is billed as about 20.4 km. That alone consumes the full 2 % that R 21 allows for an installed taximeter, which is why R 21 asks for k to be set as close as possible to zero error, compensating for tyre wear where applicable.
Where the distance signal comes from
Older taxis offered an easy answer: a gearbox sensor or a pulse wire behind the speedometer. Modern vehicles are networked machines. Wheel-speed sensors feed the braking and stability ECU, which shares speed and distance with other controllers over classic CAN or CAN FD, often through a central gateway. A dedicated pulse output may exist, may need enabling by the vehicle manufacturer, or may be absent, particularly on electric drivetrains.
R 21 anticipated this. It states that a taximeter connected to an in-vehicle network such as CAN shall offer an input and output for distance information, in which case the taximeter “does not work with pulses but with digital distance information”. In practice installers meet three architectures: a native pulse input, a native digital input, and a dedicated interface that turns network information into the pulse train the taximeter expects. For the network side, see vehicle network architecture and OBD-II and secure gateways.
Open collector and pull-up: the electrical contract
Most pulse outputs, from vehicle speed outputs to many sensors and interfaces, are open collector: an NPN transistor (or a MOSFET, as open drain) that can only pull the line to ground. Nothing drives the line high. A pull-up resistor to a reference voltage, inside the receiving device or added in the harness, defines the high level. Without it the line floats, picks up noise from every nearby wire and produces either no counts or random ones.
Sizing the pull-up
The pull-up value is a trade-off between sink current, rise time and noise immunity. Two relations cover most decisions:
I_sink = (V_pull-up − V_CE(sat)) ÷ R_pu t_rise(10–90 %) ≈ 2.2 × R_pu × C_totalTake the 16,786 pulses/km vehicle above at 200 km/h: 933 Hz, a period of 1.07 ms and a high time of about 536 µs at 50 % duty cycle. Assume a 12 V pull-up, a saturation voltage of 0.3 V, 3 m of cable at roughly 100 pF/m and 1 nF of input filtering, so C_total ≈ 1.3 nF.
R 21 gives a useful benchmark for its test connector: rise and fall times of at most 20 % of the pulse width, a minimum pulse width of 25 µs, logic low below 0.8 V, logic high between 3 V and 12 V, and an input resistance above 4.7 kΩ. They define a test interface, not every vehicle input, but a signal that meets them with margin will be counted by any well-designed taximeter.
Ground reference, supply and the second pull-up
- Ground offset adds to the low level. An output saturating at 0.3 V, referenced to a chassis point that sits 0.6 V away from the taximeter's ground during cranking or heavy load, arrives as 0.9 V: above a 0.8 V threshold. Run a signal ground alongside the signal as a twisted pair, and avoid a ground loop through shields bonded at both ends.
- One pull-up, not two. If the source already has an internal pull-up and the taximeter adds its own, the two appear in parallel. Lower resistance means more sink current, and a 12 V pull-up into a 5 V-only input can damage it. Read both data sheets before adding a resistor.
- Supply class E3. MI-007 places taximeters in electromagnetic class E3, for instruments supplied by a vehicle battery: starter-motor voltage dips and load-dump transients are expected events. R 21 refers to ISO 16750-2 for supply variations and ISO 7637-2 for transients. A signal interface on the same supply has to ride through the same events.
- Ignition logic. Taximeters usually combine a permanent terminal 30 feed for memory and clock with an terminal 15 wake signal. MI-007 requires them to survive a temporary drop, such as an engine restart, without losing data; R 21 cites 20 seconds as an example boundary for a temporary drop.
What a clean signal looks like
A good distance signal is boring: a square wave whose frequency is exactly proportional to speed, with sharp edges, solid levels and nothing at all at standstill. Everything else is a defect with a direction: extra pulses overcharge, missing pulses undercharge. A multimeter cannot tell the difference; an oscilloscope at the taximeter end of the cable can.
A practical check sequence
- 01Inspect before measuring
Check seals, connectors, cable routing and the installation record. Compare the k value on the display with the documented value and note the event counter.
- 02Measure the supply
Log the taximeter supply during engine start. On a stop-start or hybrid vehicle, repeat for an automatic restart.
- 03Probe at the receiving end
Connect the oscilloscope at the taximeter input, referenced to its own ground, not at the source. Check standstill, a slow roll and a steady speed.
- 04Run a measured distance
Drive a known, measured course or use a suitable roller test, and compare counted pulses with k × distance. National procedures define the course, the speeds and the acceptance limits.
- 05Correct, then secure
If k needs adjustment, change it through the secured procedure, record the new value and event count, and have the instrument sealed and verified as national rules require.
For a structured approach to faults beyond the taximeter itself, see field diagnostics and installation best practices.
Calibration, sealing and verification
Two regimes meet in a taxi. Placing on the market is harmonised: a taximeter manufacturer demonstrates conformity with MI-007 through module B with F, B with D, or H1. Putting into use and keeping in use is national: who verifies a taximeter after installation, how often it is re-verified, which seals and marks are used and which repairs trigger a new verification are set by national legal metrology. Türkiye, for example, transposed the Directive as the Ölçü Aletleri Yönetmeliği (2014/32/AB), while in-service procedures follow national rules there as elsewhere.
MI-007 still shapes the installation itself. It requires that:
- the taximeter can be adjusted to the constant of the distance signal generator it is connected to, and the adjustment can be secured;
- where properties of the taxi matter for correctness, the connection between taximeter and taxi can be secured;
- the installation instructions, if followed, sufficiently exclude fraudulent alteration of the measurement signal representing distance;
- after installation, time measurement, distance measurement and fare calculation can each be tested separately;
- non-resettable totalisers record total distance, distance hired, number of hirings, supplements and fares, and keep their values for one year without power.
National law may also require devices such as printers or fiscal equipment on the taximeter's interfaces, and Annex I adds the general rule: a connected device must not influence the metrological characteristics in any inadmissible way.
Electric and hybrid taxis
Electric and hybrid taxis change three things. There may be no transmission output shaft to sense, so distance comes from the vehicle's own outputs or its network. The drivetrain is live in READY rather than simply with the ignition on, and the 12 V system may be supported by a DC-DC converter only in some states. And inverter switching adds fast edges, which makes routing and pull-up choice more critical. Anything that keeps drawing current after shutdown must also respect the vehicle's sleep current budget. The details are in CAN in electric and hybrid vehicles.
Frequently asked questions
Is the distance signal generator covered by the MID?
No. MI-007 states explicitly that the distance signal generator is outside the scope of the Directive. The taximeter manufacturer specifies the conditions for compatibility with the generator, and national rules govern installation and verification.
What is the difference between k and w?
w is what the vehicle delivers: pulses over a measured kilometre. k is what the taximeter is set to expect for one kilometre. Calibration makes k equal to w and secures the setting.
Can a taximeter take distance from the CAN bus?
OIML R 21 foresees it: a taximeter connected to an in-vehicle network such as CAN may work with digital distance information instead of pulses. Otherwise a dedicated interface provides the pulse signal. Either way, the installation is verified as a whole.
How much error is allowed once the taximeter is installed?
R 21 sets 2 % for distance and 0.2 % for time on an installed taximeter, at initial and in-service verification. National legislation may set its own limits and procedures, and the local rule always applies.
Why does a taximeter count while the car is parked?
Usually a floating or noisy input: a missing pull-up, a coupled spike or a vibrating sensor. At standstill with the engine running, a clean input shows no edges at all.
