No matter what make or model of car, two of the most common criteria when choosing a used one are: the number of previous owners, and the vehicle’s mileage. Any car’s registration log will indicate the number of ownership transfers, but there is no legislation requiring a record of the total distance the vehicle has travelled. Every car, however, logs this data through a device called the odometer (or odo for short), which displays the mileage in the main instrument panel.
The odo reading is a good measure of the car’s life on the road, quite literally. Industrial machines use a meter that starts clocking (in hours run) once the equipment is switched on, but the car’s odo begins recording the mileage only when on the move. Its most critical connection is therefore the tyre set, specifically the pair in front.
With every revolution, the tyre travels a distance equal to its full circumference. Assuming this to be two metres (most passenger car tyre circumferences are between 2m and 2.5m, although the exact dimension is dependent on wheel and tyre size), after 50 revolutions the car would have travelled 100 metres or 0.1 kilometre, and one kilometre after 500 revolutions. The odometer is designed to count the number of revolutions made by the tyre and deliver the information to the driver in terms of kilometres.
The original odometer is a precision mechanical device incorporating a set of gears and a series of numbered tumblers. It looks rather like the innards of a timepiece and it’s driven by a coaxial cable (a flexible shaft) connected to the front wheel-hub. Usually turning at the same speed as the wheel-hub, this cable also “powers” the speedometer.
The odometer drives a miniature gearbox connected to a mechanism that turns the tumblers in 1km steps, with the numbers “turned” and displayed in ascending sequence. With every 10 turns, the mechanical links rotate the adjacent tumbler by one digit, and so on. The most basic odometers show up to six digits including one decimal place, which means the readout is reset to zero after 99,999.9 kilometres.
Soon enough, car manufacturers supplemented the odometer with a four-digit tripmeter that could be manually reset, added another digit to the odo display (so it could read up to 999,999.9km) and diverted the coaxial cable drive from the wheel-hub to the transmission.
None of these improvements, however, incorporated any fraud-deterrent feature. Hence, unscrupulous dealers and sometimes even private owners could still “clock the meter”, as the term goes. Technically, it could be clocked back to any mileage figure. Even driving the vehicle in reverse can do the trick.
Most instrument clusters today are fully electronic, so the cable-driven odometer is no longer used. Instead, a sensor and an electronic circuit now do the job. The current norm is a magnetic pick-up placed in close proximity to a toothed wheel in the gearbox (auto or manual) that induces pulses of current. The signal is sent from the sensor to the ECU, which converts the pulses to an appropriate voltage to activate a stepper-motor (for a mechanical odometer) or a printed circuit board (for a digital odometer).
Further development of dashboard instrumentation has paved the way for multiple driving-related displays, including fuel economy information. By using the odometer signals and extracting relevant engine measurements from the ECU, the on-board computer can easily and accurately calculate fuel consumption and send the output to the LCD display. All this is, believe it or not, a lot simpler than it sounds, thanks to microprocessors.
Compared to their analogue equivalents, modern instruments have far fewer parts, are smaller and lighter, rarely malfunction and never wear out. The modern odometer is thus more accurate and more useful than traditional odos, with added features such as forward/backward distance measurement and two tripmeters instead of one.
One last point to note… The odometer’s algorithm relies on a preset tyre diameter when converting the pulses to signals and, eventually, figures on the odo display. So, non-standard tyre-and-wheel combinations that result in a vastly different “rolling circumference” will cause the odometer data to vary from the actual distance done.
