- Speedometer gear change (up teeth or down)?
- 1965 speedometer reading too fast
- Speedometer is 10 mph too slow
- Symptoms of a Bad or Failing Speedometer Sensor
- Mechanical Speedometer Calibration - Speedo Demons
Speedometer gear change (up teeth or down)?All speedometers must be calibrated to make sure the torque created by the magnetic field accurately reflects the speed of the car. This calibration must take into account several factors, including the ratios of the gears in the drive cable, the final drive ratio in the differential and the diameter of the tires. All of these factors affect the overall speed of the vehicle. Take tire size, for example. When an axle makes one complete turn, the tire it's connected to makes one complete revolution. But a tire with a larger diameter will travel farther than a wheel with a smaller diameter. That's because the distance a tire covers in one revolution is equal to its circumference. So a tire with a diameter of 20 inches will cover about A tire with a diameter of 30 inches will cover more ground -- about Calibration adjusts for these variances and is done by the manufacturer, which sets up the speedometer gear to correspond with the factory-installed ring and pinion ratio and tire size. A car owner may have to recalibrate his speedometer if he makes changes that make his vehicle fall out of factory specifications see the sidebar below. Recalibrating a speedometer can be done by manipulating the hairspring, the permanent magnet or both. Generally, the strength of the magnetic field is the easiest variable to change. This requires a powerful electromagnetwhich can be used to adjust the strength of the permanent magnet in the speedometer until the needle matches the input from the rotating drive cable. No speedometer can be percent accurate. In fact, most manufacturers build speedometers so they fall within a fairly narrow tolerance range, no more than 1 percent to 5 percent too slow or too fast. As long as a car is maintained at factory specs, its speedometer should continue to register vehicle speed within this range. But, if a car is modified, its speedometer may need to be recalibrated. Changing tire size is one of the most common things car owners do that can affect speedometer accuracy. Consider the example below. Your car comes with factory-installed tires that are That means the circumference of each tire is Each new tire has a circumference of
1965 speedometer reading too fast
If you drive your hot rod, even once in a while, then you depend on all of your gauges to be reliable and accurate. Just a small percentage of miscalibration can result in engine damage or failure, overheating, or just running out of gas. Driving around with an incorrect speedometer can be dangerous and could earn you a ticket-not to mention the constant reminder that an important part of your rod or custom just isn't operating the way it should. We don't know how many of you are driving around out there with inaccurate speedometers, but considering that almost every one of you has probably changed your tire and wheel combination or your rearend gears sometime in the past, it's probably a pretty high percentage. We take it for granted that a speedometer measures vehicle speed, but technically, the speedometer measures the number of driveshaft revolutions per mile, which is displayed on the gauge face as speed in miles per hour. When you change the tire height or the rearend ratio, you change the ratio between the number of driveshaft rotations during one complete tire rotation, which changes the number of driveshaft revolutions per mile. The speedometer reading has to be corrected to reflect the change. Taller tires will cause the speedometer to indicate too slow a speed, and shorter tires will cause it to indicate too fast. Similarly, taller gears lower numerically will have the same effect as taller tires, and the speedometer reading will be slower than actual. In either case, you need to figure out the amount of error in your speedometer and how to correct it. How Off Are You? A simple seat-of-the-pants method for figuring out the accuracy of your speedometer is to find an open stretch of highway, preferably one with mile markers. If there are no mile markers, you can measure the distance of one mile with a vehicle that you know has an accurate speedo. Maintaining a constant speed, get a passenger to calculate the time between mile markers using a stopwatch. Divide 3, by the elapsed time in seconds that it took to go one mile and that'll give you your actual speed. For example, say you made your one-mile run at a speedometer reading of 60 mph, and your one-mile elapsed time was 50 seconds. Dividing 3, by 50 equals your actual speed of 72 mph. You already know that going 72 mph when you think you're going 60 could earn you a speeding ticket. Dividing the actual speed by the speedometer reading will tell you the percentage of error. In this case, 72 divided by 60 equals 1. As you may have figured, this method, even with all the arithmetic involved, is far from precise. Stopping and starting a stopwatch at exactly the right moment, and maintaining a perfectly constant speed for a mile, is difficult. But this will put you within the margin of error for mechanical speedometers. Mechanical Speedo Calibration On a typical mechanical speedometer, the gauge is operated by a flexible cable running from the transmission. The transmission output shaft drives the cable via a pair of nylon gears. The drive gear is a worm gear located in the transmission, running off the output shaft. It turns the driven gear, a pinion gear that is connected to the end of the speedometer cable.
Speedometer is 10 mph too slow
The electronics in today's cars and trucks are extremely complex. However, one of the simplest devices we rely on to deliver us factual data is the speedometer. Essentially, the speedometer is powered by three independent components; the speedometer on the dashboard, the cable attached from the speedometer to the driveshaft and the speedometer sensors attached to the driveshaft to measure speed. Some speedometer issues are due to dirty or faulty sensors that relay false data through the cable and eventually show up on the dashboard. Eventually that number is displayed on the speedometer and tells you precisely how fast you are driving. If the speedometer sensor is not working properly, the speedometer will be inaccurate; potentially leading to speeding tickets or unsafe driving situations. The symptoms noted below will give you an indication that a problem exists with your speedometer sensor:. Whether you have a digital or manual speedometer, if it's not displaying accurate information, it doesn't really help you. One of the biggest signs your speedometer sensor is not working properly is the speedometer will display inaccurate readings or the needle or digital numbers will not move at all on the speedometer display. A malfunctioning speedometer sensor can also cause a speedometer to act erratically, fluctuating the reading from one speed to another; even though in reality you're not speeding up or slowing down. If you feel you are driving faster or slower than what the speedometer reads, it is a good idea to have your vehicle inspected by Your Mechanic, as this might be a problem with the sensor that can be repaired or an indication that the speedometer sensor needs replacement. Being proactive about repairing your speedometer issue can improve safety, not only for you, but others on the road. It can also reduce the potential of getting two citations; one for speeding and another for a damaged speedometer. As a safety measure, automotive manufacturers engineer the cruise control in a way that if the speedometer sensor is acting up, it won't engage the cruise control at all. On most cars sold today, this will be indicated by the cruise control light being illuminated on the dashboard - even if it's not engaged. If you experience this problem, it's possible that the speedometer sensor needs to be replaced. The speedometer sensor is located inside the transmission and attached to a driveshaft, which impacts multiple functions of the car. Due to this fact, it's possible that the Check Engine Light will light up on the dashboard if a sensor problem is discovered by the vehicle's CPU. Regardless of why the check engine light is illuminated, it's never a good thing, nor should it be avoided. If the check engine light shows up anytime, contact a mechanic as soon as possible so they can diagnose the problem with the correct diagnostic tools. Sometimes the issue is small and the CPU needs to be reset; but don't take that risk. Today's cars and trucks have electronic fuel injection, which is controlled by the onboard computer. The EFI supplies fuel to the engine based on input from multiple sensors; one of which is the speedometer sensor. If you notice that your car starts to lose power ; especially when you apply pressure to the throttle, this might be caused by a damaged or malfunctioning speedometer sensor. This problem could be potentially harmful to other mechanical components on your vehicle, so don't ignore the issue. Contact a certified mechanic as soon as possible to determine what is causing this issue. As you can see, the speedometer sensor impacts the operation of multiple components that allows you to efficiently operate your car or truck. Anytime you experience any of the above warning signs, contact a professional mechanic to properly diagnose and repair the issue. The most popular service booked by readers of this article is Vehicle Engine Electrical Inspection. Our certified mobile mechanics make house calls in over 2, U. Fast, free online quotes for your car repair. Schedule Vehicle Engine Electrical Inspection. Service Area. Average rating fromcustomers who received a Vehicle Engine Electrical Inspection. Where is the speedometer sensor located? The symptoms noted below will give you an indication that a problem exists with your speedometer sensor: 1. Speedometer does not work Whether you have a digital or manual speedometer, if it's not displaying accurate information, it doesn't really help you.
Symptoms of a Bad or Failing Speedometer Sensor
There can be multiple reasons for the speedometer to malfunction, and replacement need not be the only solution. Here, we have tried to provide likely issues that can occur with the speedometer, along with troubleshooting tips for the same. Before the invention of the speed gun, cars in the early 20th century were required to have two speedometers, one on the dashboard and one on the front fender, so that police could see how fast they were going! A speedometer is an instrument which provides the driver with instantaneous readings of speed. Traditional speedometers used gears and wires to determine speed, while most modern vehicles use speed sensors for the same. Common problems include, a faulty sensor, bad wiring, or dial malfunctions. Troubleshooting speedometer problems mostly call for a replacement of the speed sensor or cable, depending upon the vehicle. Both these repair jobs are simple to perform and can be done at home. Would you like to write for us? Well, we're looking for good writers who want to spread the word. Get in touch with us and we'll talk This could be due to two reasons. In older cars a break in the cable that connects the transmission to the speedometer is the most common cause. Cars produced after are usually equipped with speed sensors, which may crash and cease to transmit speed readings to the speedometer. A more serious problem could be a faulty speedometer head, which needs expert diagnosis. If cruise control is working, and the check engine light is on, it may indicate a problem with the speedometer itself, and may require a change of the instrument panel. In cases where the speedometer does not settle on any particular reading but keeps moving between speeds, it is almost always due to bad wiring, in case of a cable system, or a faulty speed sensor. In most cases, only the wiring needs to be changed, or the sensors re-calibrated, to repair the problem. Speedometers are calibrated according to the radius and diameters of the factory-fitted tires of your car. These determinants can change, if you get custom tires which are larger, or if they are of different dimensions. The rate at which your tires cover ground changes and if the speedometer is not calibrated accordingly, it can show a faulty reading.