Regardless of make or model, all vehicles contain dashboard gauges that allow you to monitor various vehicle systems and information. While the speedometer and the fuel gauge are the most commonly-referenced gauges, many cars also have a temperature gauge, oil pressure gauge, voltmeter, and tachometer on their dash that can provide important information on your vehicle’s performance and health. Knowing how to read the information provided by these gauges and what each gauge represents is crucial to operating your vehicle safely and properly. The various dials and gauges on your dash can alert you to issues with important vehicle systems and should be monitored closely when you’re behind the wheel. To learn more about these instruments, the information they provide, how to read them, and what to do if they stop working, read on.
There are six common types of dashboard gauges in most vehicles. Each of these gauges can be either analog or digital, depending on the year of the vehicle and what types of aftermarket accessories may have been installed. Analog gauges, or analog dials, use a more traditional measuring design with a mechanical dial. This dial shows the output of information collected by the deflection of a pointer on a scale. This gauge dial or pointer is actuated and moves based on the current state of the system which it monitors. Digital gauges, which are most common in modern vehicles and custom applications, show the system output in the form of numbers or text on a digital display screen. It is not uncommon for vehicles to have a mix of both digital and analog dashboard gauges. If you are unsure what type of gauges your vehicle has, refer to your owners manual for more information on your instrument cluster.
Speedometer
The most commonly used, and arguably the most important, gauge on your vehicle is your speedometer. Speedometer function and type can vary depending on the vehicle. Vehicles with digital speedometers rely on information from speed sensors to provide an accurate speed reading. Speed sensors are plugged into an electrical connector and transmit a signal to your vehicle's computer, providing the information needed to accurately calculate your car's speed based on the relative rotation of the transmission, differential, crankshaft, wheels, or a mix of these signals. Speed sensors also provide the information necessary to calculate travel distance, and relay this information to the odometer on your dashboard as well.
In analog or mechanical speedometers, your vehicle’s speed is measured by monitoring the rotation of your vehicle’s crankshaft. The drive cable, or speedometer cable, is attached through a spiral gear to a permanent magnet. The magnet sits inside a cup-shaped metal piece known as the speed cup. The speed cup is attached to the needle on your speedometer, which is held in place by a hairspring. The needle moves as the magnet is affected by the spinning of the speed cup, allowing the speedometer to display the speed based on a range of numbers from zero to an upper limit that can vary by make and model.
Depending on your vehicle, there can be a variety of reasons why your speedometer stops working or displays erratic or inaccurate readings. The most common causes of a speedometer that has stopped working entirely include a blown fuse, faulty speed sensor, a broken gear on the speedometer cable, damaged wiring, or a faulty engine control unit. If your speedometer is acting erratically or providing inaccurate readings, this can also be due to a damaged sensor or speedometer cable. This may cause your speedometer to bounce or rise and fall erratically, and in some cases a speedometer cable can be lubricated to allow it to spin more consistently to fix the problem. Regardless of the source of the problem, it’s important to locate the source of the problem and complete any repairs as needed. Having a working speedometer is required by law in most states, and if your speedometer is not working properly, it may make driving the speed limit more difficult – even leading to a warning or a ticket. Depending on the design of your vehicle, a failing speedometer may also prevent you from using certain features like cruise control, and may have other effects on the performance of your engine and drivetrain.
Fuel Gauge
Another of the most important dashboard instruments is your fuel gauge. Reading a fuel gauge is simple – the “F” in this gauge represents a full fuel tank, while the “E” stands for empty. However, manufacturers actually design fuel gauges to provide slightly inaccurate readings. If your tank is full, it will remain on “F” and gradually drop as fuel is used. But when your fuel gauge reads “E”, or empty, you may actually have between 1 and 2 gallons of fuel left in your tank. This design provides a reserve fuel source, giving the driver time to fill up before the tank is completely out of fuel. Most dashboards are also equipped with a fuel warning light that indicates when you fuel reserve is running dangerously low. If this light is on, fill your fuel tank as soon as possible.
If your fuel gauge isn’t providing accurate readings, or if the needle moves erratically, there are several possible causes. A mechanical float is used in most vehicle fuel tanks that adjusts the position of a resistor to send level readings to the instrument cluster on your dashboard, providing your fuel gauge with accurate information. If this float is leaking or has been damaged, it may fill with fuel and sink, and your fuel readings might be inaccurate. The float is a part of your fuel systems sending unit, which may also include a fuel pump, strainer, and filter. If the sending unit relay wears out, your fuel gauge could be interpreting inaccurate voltage readings from the sending unit, and may be stuck on “F” or “E”. It is also possible that the fuel gauge itself is faulty, and it may be a good idea to check for blown fuses or damaged wiring. To properly diagnose the problem, it may be necessary to use a voltage tester or OBD scanner to investigate the cause of the inaccurate readings. For more information about your vvehicle's fuel system, visit our How To Hub for DIY and informational articles and videos.
Temperature Gauge
The temperature gauge in your vehicle monitors the temperature of your engine coolant. This allows drivers to remain aware of changes in the temperature and status of the coolant circulating through your engine’s cooling system. The temperature gauge may indicate “C” for cold and “H” for hot or may have numerical temperatures along its face. The gauge needle should be pointed to “C” when you start your engine, and will gradually rise to the center as the engine warms up. If the needle doesn’t rise slowly the longer your engine is running, or if the gauge remains on “C” consistently, your temperature gauge or temperature sensor may be damaged, or the thermostat in the vehicle may be stuck open, preventing the engine from reaching operating temperature. In most cases, a “C” reading on a temperature gauge is not as concerning or severe as an “H” reading. If your temperature gauge indicates that your vehicle’s coolant is hot, this is one of the first signs of engine overheating. Coolant leaks can also cause low coolant levels and a rise in coolant temperature, which could affect the temperature gauge readings as well as cause overheating. In some cases, damaged or worn water pumps can also increase coolant temperature and cause your temperature gauge needle to be stuck on “H” until the cooling system issue is repaired.
In newer models, temperature gauges may be accompanied by a dashboard warning light to alert you if your coolant's temperature is too high. Some vehicles may not have temperature gauges at all, as manufacturers began relying more on warning lights to indicate when your engine is overheating or at risk of overheating. To better understand your vehicle's warning lights and gauges, take a look at your owners manual. For more information on your vehicle's dashboard warning lights, including your low coolant light or temperature warning lights and what they may mean, visit our helpful article.
Oil Pressure Gauge
The oil pressure gauge on your dash tells you how much oil pressure is generated in your vehicle’s engine, and can give you a general idea of your vehicle’s overall health. Motor oil lubricates a number of important vehicle components and systems, and proper oil pressure is crucial to your engine’s performance and health. Knowing how to read your oil pressure gauge can help you identify a number of potential problems early on.
Oil gauges are typically labeled with “L” and “H” to indicate low and high oil pressure, and may also have a “normal” section near the center that indicates typical operating pressures. The gauge is typically labeled “OIL”, or may have an image of an oil can to identify it. Numerical oil pressure gauges may also include a range of reading increments depending on the vehicle. The needle on the gauge will move as engine oil pressure rises and falls, and should typically settle around the midpoint after the vehicle has had time to run and the oil pressure stabilizes. If your oil pressure gauge has a consistently low reading, it might indicate low engine oil levels, oil pump issues, or other internal engine problems that may be accompanied by ticking or metallic noises during operation. Consistently high oil pressure readings may instead indicate a broken or stuck pressure relief valve or a blockage in the oil delivery line. High oil pressure could damage your oil filter or other parts of the oil delivery system, and should be addressed as soon as possible. If you notice your oil pressure gauge is pointing towards either “L” or “H”, shut your car off until the cause can be identified and remedied to prevent engine damage. For a guide to reading your vehicle’s oil pressure gauge, or information on how to check and fill motor oil, take a look at your vehicle’s owners manual. Some vehicles may not have oil pressure gauges but rely on oil pressure lights to indicate a problem with your vehicle's oil pressure. If your oil pressure warning light is activated, it can be a sign of low oil levels or failure of an oil system component.
There are two types of oil pressure gauges, electrical and mechanical. An electric oil pressure gauge is found in most modern vehicles and reads your oil level when the oil pressure sensor transmits current through an electrical coil, allowing the gauge to measure resistance and display the appropriate reading. Mechanical oil pressure gauges receive direct oil pressure through small pipes or tubing that are installed in the same place you would typically find an oil pressure sensor. Oil is fed into a diaphragm on the gauge at the same pressure that it’s flowing through the engine, typically providing the most accurate oil pressure readings for your vehicle's oil pressure gauge. This is why many aftermarket oil pressure gauges are mechanical, as they can provide the most responsive and accurate oil pressure readings for high-performance and custom engine applications.
Voltmeter
A voltmeter provides information on your vehicle’s battery and charging system health. Voltmeters directly measure the charging systems' performance, and monitor the voltage in your vehicle’s electrical system and car battery. This gauge is usually indicated by a battery symbol and is numbered to indicate the voltage flowing through the battery of your vehicle, or may be labeled “D” for “discharge” or “C” for charge. Typically, the needle should remain in the center of the gauge, usually around 14 volts. If the needle remains around 12 volts after the car has been running for any period of time, there might be a charging system problem.
Some vehicles may not have voltmeters on the dash but are equipped with a low battery or charging alert warning light. If this light is on, it indicates a problem with your vehicle’s charging system. Low voltage readings on your voltmeter or an illuminated low battery warning light could be signs of a dead or dying car battery, or in some cases may be signs of bad alternator, damaged battery cables, or other charging component problems. For more information on your vehicle's battery or charging system, visit our How-To-Hub. If your battery warning light is on, you can also stop by your local O’Reilly Auto Parts for Free Battery Testing and Free Alternator Testing.
Tachometer
The tachometer measures how fast the engine turns in RPM (revolutions per minute). The tachometer gauge usually features single digits, and is marked with “x1000”. In order to calculate your RPM, you will multiply the single digit the needle is pointed towards by 1,000. So if your gauge is pointed at 4, your RPM total is 4,000 revolutions per minute. The higher the RPM reading, the faster the engine is running. Maintaining lower RPM readings often provides the best fuel mileage. The tachometer is especially important in vehicles equipped with a manual transmission because the tachometer can help indicate when it’s time to shift gears.
While you were likely familiar with some of the more common vehicle dashboard gauges, there are a few less common gauges that you may come across, especially in performance vehicles or custom applications. While most vehicles are not originally produced with these gauges, they can be added on if your vehicle has undergone performance upgrades that makes monitoring these systems valuable. Read on to learn more about these less common dashboard gauges.
Boost Gauge
While boost gauges may not be common on most vehicles, they are important instruments on turbocharged or supercharged vehicles. Boost gauges are pressure gauges that monitor the pressure of air as it enters the combustion chamber. The engines in turboed and supercharged vehicles require steady and measured injections of compressed air mixed with fuel to produce increased power. Boost gauges monitor the levels of compressed air to alert the driver of any inconsistencies in the amount of air being delivered to the engine.
Vacuum Gauge
Vacuum gauges are especially helpful when trying to diagnose engine performance issues due to a variety of problems, including component wear, blockage, and parts that are out of specification. These gauges can help you pinpoint the cause of poor engine performance without you having to inspect all of the possible parts and systems that could be at fault by accurately monitoring the air pressure in your engine’s vacuum system. This helps ensure that your engine is free of air leaks, which could impact the air-to-fuel ratio and overall performance. If the vacuum inside the engine is not regulated, rough idle, poor fuel economy, increased emissions, and poor engine performance can occur. Vacuum gauges monitor this parameter when your vehicle’s engine is running, alerting you of any engine performance issues caused by vacuum leaks from hoses or gaskets.
Air/Fuel Ratio Gauges
An air/fuel ratio gauge can help you diagnose and more efficiently tune high-performance vehicles. These gauges are responsible for monitoring the air-to-fuel ratio of high-performance applications and can help you determine your vehicle's fuel mixture while the engine is running. Most air/fuel ratio gauges indicate whether the mixture is either too “lean” or too “rich” with increments in between. These gauges can alert you if your air-to-fuel ratio is too lean, which can lead to engine damage, or too rich, which can reduce engine performance and fuel economy.
Oil and Transmission Temperature Gauge
Oil and transmission temperature gauges help drivers keep track of the operating temperature of their vehicle’s transmission fluid and engine oil, preventing your transmission and engine oil from burning or breaking down. The transmission fluid in your vehicle can wear down over time due to exposure to excessive heat and age. While the specific intervals to change your vehicle's transmission fluid will be recommended in your vehicle’s owners manual, the transmission temperature gauge is another resource that can let you know when it might be time to change your fluid. The ideal temperature for transmission fluid is between 175 degrees to 225 degrees. If the dial on your transmission temperature gauge exceeds 225 degrees, your transmission is at risk of overheating and damage, and you should avoid driving until you have had a chance to diagnose the issue.
Oil temperature gauges monitor the temperature of the oil in your engine. Engine oil lubricates your engine's moving parts and components, reducing friction to ensure consistent engine performance and prevent damage to your engine. If your engine oil is overheating, it is likely contaminated and needs to be changed as soon as possible to limit the risk of engine damage or failure. The oil temperature gauge can alert you of increased oil temperatures before this situation becomes critical or dangerous, allowing you to turn off your engine until you can determine the source of the problem. To learn more about your vehicle’s engine and transmission oil and its importance, check out our How To Hub resources.
Fuel Pressure Gauge
Fuel pressure gauges help drivers monitor fuel pressure levels in their vehicle’s fuel system while also detecting potential issues. By monitoring the fuel system’s pressure levels, these gauges can help optimize engine performance by improving fuel delivery and combustion processes. Keeping an eye on the fuel pressure gauge can also help determine if the fuel pump is working as it should and diagnose any clogs in the fuel filter or other parts of the system.
Dashboard gauges and the information they provide are crucial to operating your vehicle safely and keeping your car running at its best, and are controlled by your vehicle's dashboard instrument cluster. This instrument cluster, or control panel, is crucial to providing you with the information you need about your vehicle’s systems and components. If more than one of your dashboard gauges is not working or is providing sporadic and irregular readings, your dash cluster could be the problem. If this is the case, it is important to replace your dash cluster as soon as possible to ensure your dashboard gauges are working properly. Knowing what these gauges represent and how to read them, as well as understanding your dashboard warning lights, are important to make sure your vehicle is safe to drive on the road and can help you recognize potential problems across a variety of vehicle systems. If you’re unsure why a dashboard warning light is illuminated, or if you need help reading and understanding your vehicle's dashboard gauges, take a look at your vehicle’s owners manual for step-by-step instructions on reading the information on your dash.
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