Another question would also be what’s the difference between wheel speed and vehicle speed?
In: 620
The wheel on the car spins at some speed, say 1000 rpm. The diameter of the wheels is some distance, say 30 inches.
The car measures the rotational speed, is preprogrammed with the diameter, and calculates the final speed.
In this case, the circumference is 30pi inches, and it makes 1000 revolutions per minute, so 30,000pi inches per minute. That’s about 131 inches per second, or 89 mph.
There is either a mechanical cable or an electric counting wheel on the transmission output, calibrated to the wheel size the factory installed. As the wheels spin the sensor counts the amount of times it passes and does the math for you. A mechanical cable uses a gear drive calibrated in a similar way and just spins which holds the needle at the corresponding speed on the gauge.
Most cars use a cable that runs from the transmission to the speedometer. New/fancier cars just use GPS for speed.
There’s a electronic sensor or gear connected to a cable at the output side of the gearbox. The cable type uses the rotations to push the needle mechanically, the electronic type just uses a computer. Either way, the number of rotations converts to speed using the wheel diameter.
This gets interesting if you change out your tyres or wheels for a different size – unless you remember to re-calibrate the speedo it’s going to read the wrong speed. Even letting your tyre pressure get low can change the reading slightly, similarly, going around corners means the wheels on each side aren’t quite moving at the same speed and the point where speed is measures isn’t directly at either wheel so the measured speed will never quite be exactly the same as your actual ground speed
Magnets. It is always magnets.
[https://en.wikipedia.org/wiki/Hall_effect_sensor](https://en.wikipedia.org/wiki/Hall_effect_sensor)
Just to add to what others have said, a lot of bicycles use these. You enter the wheel diameter or circumference (better) into the unit and it spits out your speed. I sometimes use both but the wheel sensor can update your speed many times a second whereas GPS is usually longer than a second between updates. And like other have said it will not work consistently in thick canopy, tunnels and some urban canyons.
Engine RPM * Transmission Ratio * Diff Gear Ratio = Wheel RPM
Wheel RPM * Pi * Tire Height (with unit conversions) = Vehicle Speed
There are 4 main types of speedometers:
* Cable driven, from a worm gear in the transmission tailpiece (must be sized appropriately for final gear ratio). The speedometer itself is a magnetic clutch, and the faster you spin the input shaft, the higher the needle reads.
* Electronic speed sensor (basically an encoder that counts revolutions usually at the transmission). Some operate by resistance drop, others provide encoder feedback (analog signal) back to an ECU or gauge.
* Simple ECU calculated by engine rpm by ignition system count and known transmission speed/position
* GPS
speed = distance / time
there’s a sensor placed on one of the transmission wheels. This wheel have a small flag on one edge. Every time it spins the sensor counts one. If the wheel spins faster the sensor counts faster; if the wheel spins slower the sensors counts slower. The car computer has a clock in it so that’s how it knows the time between counts. The count is then converted in distance via a mathematical formula related with the tire size. 1 complete spin of the tire = x amount of inches. That’s why you can’t simply change the tire size on the car, you need to adjust this formula as well, putting in its diameter. Then the speed is calculated based on that distance over the time.
There can be some considerable differences in wheel speed from vehicle speed. Differences in diameter as well as turning can give you each wheel operating at a different speed, I’ve seen ~3mph difference from the inner most and outer most wheels during a turn at full lock. To break it down even further, lets I say I replaced 2 tires on my car while the other 2 are old and worn. This would make 2 tires have a larger diameter and thus a larger circumference. Because some are smaller then the others they have to perform more rotation to cover the same distance, more rotations for the same distance = fater rotational speed. Same idea while turning. The farther outside wheels will create a larger circle than the inner ones, but because the whole car is moving at the same speed, the wheels on the bigger circle cover more ground in the same period of time and thus are moving faster.
Fun fact most speedometers are not 100% accurate, manufacturers allow for a margin of error.
Speedometers are actually measuring the RPM’s of the wheels.
Then they do a multiplication with a constant, based on the circumference of the OE tires (their size). That “calculation” can be electronic or just mechanical, it’s a proportional relation.
However, that constant is dependent on the tire size itself. If one changes the OE size, the calculation will be off.
Here is an online calculator that can show the expected difference in speeds:
[https://tiresize.com/calculator/](https://tiresize.com/calculator/)
A phone’s GPS will show the exact speed at every time because is based on the speed calculated versus some very precise satellites.
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