Checking speed on a car

Turn on the speedometer
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The return spring is calibrated such that a given revolution speed of the cable corresponds to a specific speed indication on the speedometer. This calibration must take into account several factors, including ratios of the tailshaft gears that drive the flexible cable, the final drive ratio in the differential , and the diameter of the driven tires.

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One of the key disadvantages of the eddy current speedometer is that it cannot show the vehicle speed when running in reverse gear since the cup would turn in the opposite direction - in this scenario the needle would be driven against its mechanical stop pin on the zero position. Many modern speedometers are electronic. In designs derived from earlier eddy-current models, a rotation sensor mounted in the transmission delivers a series of electronic pulses whose frequency corresponds to the average rotational speed of the driveshaft , and therefore the vehicle's speed, assuming the wheels have full traction.

The sensor is typically a set of one or more magnets mounted on the output shaft or in transaxles differential crownwheel, or a toothed metal disk positioned between a magnet and a magnetic field sensor. As the part in question turns, the magnets or teeth pass beneath the sensor, each time producing a pulse in the sensor as they affect the strength of the magnetic field it is measuring. Most modern electronic speedometers have the additional ability over the eddy current type to show the vehicle speed when moving in reverse gear.

How does GPS tracking report on speeding? | Verizon Connect

A computer converts the pulses to a speed and displays this speed on an electronically controlled, analog-style needle or a digital display. Pulse information is also used for a variety of other purposes by the ECU or full-vehicle control system, e.

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Another early form of electronic speedometer relies upon the interaction between a precision watch mechanism and a mechanical pulsator driven by the car's wheel or transmission. The watch mechanism endeavors to push the speedometer pointer toward zero, while the vehicle-driven pulsator tries to push it toward infinity.

The position of the speedometer pointer reflects the relative magnitudes of the outputs of the two mechanisms. Typical bicycle speedometers measure the time between each wheel revolution, and give a readout on a small, handlebar-mounted digital display. The sensor is mounted on the bike at a fixed location, pulsing when the spoke-mounted magnet passes by.

However, this is rarely a critical problem, and the system provides frequent updates at higher road speeds where the information is of more importance. The low pulse frequency also has little impact on measurement accuracy, as these digital devices can be programmed by wheel size, or additionally by wheel or tire circumference in order to make distance measurements more accurate and precise than a typical motor vehicle gauge.

However these devices carry some minor disadvantage in requiring power from batteries that must be replaced every so often in the receiver AND sensor, for wireless models , and, in wired models, the signal being carried by a thin cable that is much less robust than that used for brakes, gears, or cabled speedometers. Other, usually older bicycle speedometers are cable driven from one or other wheel, as in the motorcycle speedometers described above. These do not require battery power, but can be relatively bulky and heavy, and may be less accurate.

The turning force at the wheel may be provided either from a gearing system at the hub making use of the presence of e. The former type are quite reliable and low maintenance but need a gauge and hub gearing properly matched to the rim and tyre size, whereas the latter require little or no calibration for a moderately accurate readout with standard tyres, the "distance" covered in each wheel rotation by a friction wheel set against the rim should scale fairly linearly with wheel size, almost as if it were rolling along the ground itself but are unsuitable for off-road use, and must be kept properly tensioned and clean of road dirt to avoid slipping or jamming.

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Vehicle manufacturers usually calibrate speedometers to read high by an amount equal to the average error, to ensure that their speedometers never indicate a lower speed than the actual speed of the vehicle, to ensure they are not liable for drivers violating speed limits. Excessive speedometer errors after manufacture, can come from several causes but most commonly is due to nonstandard tire diameter, in which case the error is:. In many countries the legislated error in speedometer readings is ultimately governed by the United Nations Economic Commission for Europe UNECE Regulation 39, [6] which covers those aspects of vehicle type approval that relate to speedometers.

The main purpose of the UNECE regulations is to facilitate trade in motor vehicles by agreeing uniform type approval standards rather than requiring a vehicle model to undergo different approval processes in each country where it is sold. European Union member states must also grant type approval to vehicles meeting similar EU standards.

Reasons Why Your Car's Idle Speed Might Be Too High

The ones covering speedometers [7] [8] [9] are similar to the UNECE regulation in that they specify that:. There are slight differences between the different standards, for example in the minimum accuracy of the equipment measuring the true speed of the vehicle. They had to be introduced when speed cameras were first used. This means there are no legally accurate speedometers for these older vehicles.

The speedometers in vehicles manufactured before these dates but after 1 July or 1 January for forward control passenger vehicles and off-road passenger vehicles must conform to the previous Australian design rule. All vehicles manufactured in Australia or imported for supply to the Australian market must comply with the Australian Design Rules. The Motor Vehicles Approval Regulations [15] permits single vehicles to be approved.

On 25 March the NHTSA revoked the rule because no "significant safety benefits" could come from maintaining the standard. GPS devices are positional speedometers, based on how far the receiver has moved since the last measurement. Instead, the GPS's positional accuracy, and therefore the accuracy of its calculated speed, is dependent on the satellite signal quality at the time. Speed calculations will be more accurate at higher speeds, when the ratio of positional error to positional change is lower.

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The GPS software may also use a moving average calculation to reduce error. Some GPS devices do not take into account the vertical position of the car so will under report the speed by the road's gradient. As mentioned in the satnav article, GPS data has been used to overturn a speeding ticket; the GPS logs showed the defendant traveling below the speed limit when they were ticketed.

That the data came from a GPS device was likely less important than the fact that it was logged; logs from the vehicle's speedometer could likely have been used instead, had they existed.


From Wikipedia, the free encyclopedia. See also: Vehicle speed sensor. Main article: Cyclocomputer. This section's factual accuracy may be compromised due to out-of-date information. Please update this article to reflect recent events or newly available information. June This section's factual accuracy is disputed. Relevant discussion may be found on Talk:Speedometer. Please help to ensure that disputed statements are reliably sourced.

September Learn how and when to remove this template message. Main article: Automotive navigation system. When you find these codes, you will be able then to follow them to possible causes, or consult with a mechanic for further interpretation. You can try cleaning it and see if that improves your idle speed. There are many possibilities when your engine is idling too fast. Here are some common ones to help guide you towards the root of the problem. One tip is to make sure you are checking the engine idle with the air conditioning and defroster in the OFF setting. With some cars, the onboard computer automatically revs the idle speed slightly when accessories are running, and you won't get a true idle speed if they are at work.

Vincent Ciulla is a certified master automotive technician who has diagnosed and repaired light trucks, domestic and foreign cars, and diesel engines, for more than three decades. Updated January 19, In vehicles that have carburetors, a bad accelerator pump or power circuit may be to blame. The engine may be overheating, and the cooling system might need repair. The fuel pressure regulator may be operating at too low of a pressure and might need to be replaced. The Ignition timing might be in need of adjusting. Ignition problems stemming from the distributor cap, rotor, ignition wires, or spark plugs can cause a high idling issue—each of these components should be examined.