To offset all of this, and to help prevent you from getting a speeding ticket, most speedometers are designed to read slightly high.
First and foremost, speedometers in most vehicles are designed to overestimate the speed of travel. International law has long required modern cars to overstate true speed.
The Australian Design Rules require that a vehicle's speedo must not indicate a speed less than the vehicle's true speed or a speed greater than the vehicle's true speed by an amount more than 10 per cent plus 4 km/h. This means that the vehicle's true speed must not be higher than the speed indicated by the speedo.
Most speedometers max out around 140 or 160 mph, even though the cars aren't designed to go that fast. The practice serves automakers' needs to mass-produce standard gauges for different cars. It also adds psychological benefits to drivers, who may want to think of themselves as amateur racecar drivers.
Downsizing, or installing a shorter tire, will cause the speedometer to read a faster speed than you are actually driving. This is because the smaller tire will have a smaller circumference, causing the tire to travel less distance per rotation than the original equipment tire.
The most common causes of a speedometer that stopped working include are a faulty speed sensor, a broken gear on the speedometer, damaged wiring, or a faulty engine control unit.
A speedo must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph. So if your true speed is 40mph, your speedo could legally be reading up to 50.25mph but never less than 40mph.
The regulation states that speedometers must never underreport a vehicle's speed, while it must never overreport by more than 110% of the actual speed + 6.25mph. So if you're going 40mph, your speedometer may read up to 50.25mph - but it can never read less than 40mph.
Driving about you may have noticed that your GPS may calculate a different speed than you car's meter. Speedometers are allowed to overstate speed by up to 10 per cent, but never to understate it. Speedos therefore often indicate a higher speed than GPS.
Problems With Tire and Wheel Size Changes
A larger tire has a higher circumference and fewer rotations as you roll along the highway. Because the tires rotate slower, the speedometer reads this as a lower speed. The faster your drive, the more your speedometer reading will be off. Your odometer will read lower, too.
"At 60 mph this means your speedometer should read no more than 1.5 mph higher or lower than your actual speed," Edmunds explains. Manufacturing tolerances and difference between tire brands and air pressure can cause tire diameters to vary, which may lead to an erroneous speedometer.
Are Speedos GPS legal in Australia? Australian Design Rule (ADR) 18, which came into force in 2006, states that the speedometer shall not display a speed lower than the actual speed of the vehicle, or a speed 10 per cent at 4 km/h higher than the actual speed of the vehicle. This is where the GPS speedometer comes in.
A vehicle's speed can be made by measuring the time it takes to travel one mile. The best way to conduct the test is to set cruise at one speed and measure the time it takes to travel between two green mile markers on interstate or other US/state highways.
There are laws around calibrating speedos
Legislation says that a speedo can't over-read by any more than 10% plus 6.25mph. If you're doing a real 30mph, legally, your speedo can't say you're doing more than 39.25mph. If you're doing 50mph, it can't show you to be doing any more than 61.25mph.
Because that measurement changes continuously, as the tire expands and contracts with use and environment, your speedometer is never accurate. Circumference is a simple function of a tire's radius, or the distance between the center of the wheel and ground.
It was traditionally a purely mechanical device but, in most modern vehicles, it is now electronic. Many modern vehicles often have multiple trip meters. Most mechanical trip meters will show a maximum value of 999.9.
With a clear view of the sky, GPS speed has shown to be more accurate than most vehicle speedometers.
Your car speedometer may have an error of plus or minus 4 percent in the United States. This indicates that you could be going faster than what the speedometer reading reveals to you for much lower speeds. But for higher speeds, you could be going 3 miles per hour slower at the minimum.
The issue with your speedometer not working properly is the transmission output shaft speed sensor has a faulty pickup or broken wire inside it. The speed sensor has a magnet that reads between teeth on a gear within the tail shaft of the transmission.
A dash cam with GPS works from a satellite, so as long as there isn't too much tree or building cover to interfere with the satellite reception, dash cams can be incredibly accurate. That being said, both methods of measuring speed are quite accurate and will only have one or two miles per hour difference in result.
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.
After taking various issues into consideration, it was determined that there is no need for passenger cars in Japan to travel faster than 180 km/h, and therefore most Toyota passenger cars are equipped with speedometers that only go up to 180 km/h. The speedometers of cars used for circuit racing go up to 200 km/h.
In some cars, drivers must account for as much as a 10% margin of error in terms of speedometer accuracy; tire wear and aftermarket modifications, such as different tire sizes and gearing, can cause erroneous speedometer readings. Most new speedometers are believed to function with a 5% margin of error.
Toyota spokesman Paul Hogard said the automaker wants speedometers to be easy to read, so there's value in placing the typical operating speed of American cars, 45 mph to 70 mph, he said, at the top of the speedometer, which is the easiest place on the speedometer for the driver to read.
A GPS speedometer is most accurate when you are traveling in a straight path with no overhead interference. Entering tunnels or traveling on windy or hilly roads may impact GPS speedometer accuracy when traveling via motor vehicle.