How Does a Speedometer Work?

How Does a Speedometer Work? how accurate is it?
i hope this isnt a re-post.

Well, that depends. What car are you talking about? In some newer Fords, they now have the Hybrid Electronic Cluster (HEC). It works by reading the speed from the output shaft of the transmission via a speed sensor. As with any programming, it's as accurate as the person who programmed it.

In older Fords, there was a physical cable that went from the output shaft to the speedometer. It was as acurate as the person installing the little gear on the cable where the output shaft was at. Going up and down teeth on that little gear would move the speeometer needle up or down so many MPH (KM/H).

Hope that helps.

i thought they were based on how big the circumfrence is on youtr tires, then calculated how many times per minute the tire is rolling the complete circumfrence, and then it calculated that into mph

Both are basically right. My_porsche is talking about the theory and math used used, whereas the previous post is the actual mechanical application.

well put digital craft

im not owned d-quick, you get the same tire DIAMETER! and if you do get alot smaller wheels the speedo says you are going faster than you are, if you get bigger, ythen it says slower that it is

[QUOTE=my porsche]and if you do get alot smaller wheels the speedo says you are going faster than you are, if you get bigger, ythen it says slower that it is[/QUOTE]
i thought it was the other way around :confused:

A turn of a small wheel goes a shorter distance than a large wheel, so a small wheel could turn 10 times and only go 10 metres but measure 20mph wheras a large wheel can turn 10 times and go 20 metres but will still only measure 20mph (just an example). Personally I'd just fit a GPS Speedo if i was that bothered about pinpoint accuracy, theyre as accurate as you can get nowadays.

sorry, my bad, i meant the other way around sorry guys

yeah i realised that, i just had one of my alltoo often brain farts while typing

gps speedo eh? thats pretty cool

or you can use a calculator and a ruler. :D :D

[QUOTE=Rockefella]or you can use a calculator and a ruler. :D :D[/QUOTE]
lol, that would take a while

[QUOTE=Sweeney921]lol, that would take a while[/QUOTE]

not to mention dangerous lol

[QUOTE=my porsche]sorry, my bad, i meant the other way around sorry guys[/QUOTE]

Nah, I think what you initially said was correct.

The Falcon uses the ABS sensors to determine speed..

[QUOTE=my porsche]sorry, my bad, i meant the other way around sorry guys[/QUOTE]

You got it the right way around .. thats what i was trying to explain

DodgeNitroBIRM,
I am writing a paper for my college intro to Physics paper and i was wanting to thank you for discussing the mechanics of a Speedometer. My paper is to be a page long and about how speedometers and odometers work.

A small addition to this thread. It is not uncommon nowadays to have the Engine ECU calculate the speed at the wheels using multiple sensor signals, which are allready present anyway. IIRC the latest Delphi & Bosch Systems have the ability to do so.

I went to howstuffworks.com and found this (for mechanical, or "eddy" speedometers)

[quote]Let's say a car is traveling along the highway at a constant speed. That means its transmission and driveshaft are rotating at a speed that corresponds to the vehicle speed. It also means that the mandrel in the speedometer's drive cable -- because it's connected to the transmission via a set of gears -- is also rotating at the same speed. And, finally, the permanent magnet at the other end of the drive cable is rotating.

As the magnet spins, it sets up a rotating magnetic field, creating forces that act on the speedcup. These forces cause electrical current to flow in the cup in small rotating eddies, known as eddy currents. In some applications, eddy currents represent lost power and are therefore undesirable. But in the case of a speedometer, the eddy currents create a drag torque that does work on the speedcup. The cup and its attached needle turn in the same direction that the magnetic field is turning -- but only as far as the hairspring will allow it. The needle on the speedcup comes to a rest where the opposing force of the hairspring balances the force created by the revolving magnet.

What if the car increases or decreases its speed? If the car travels faster, the permanent magnet inside the speedcup will rotate faster, which creates a stronger magnetic field, larger eddy currents and a greater deflection of the speedometer needle. If the car slows down, the magnet inside the cup rotates more slowly, which reduces the strength of the magnetic field, resulting in smaller eddy currents and less deflection of the needle. When a car is stopped, the hairspring holds the needle at zero.[/quote]

and this for an electrical speedometer:

[quote]An electronic speedometer receives its data from a vehicle speed sensor (VSS), not a drive cable. The VSS is mounted to the transmission output shaft or to the crankshaft and consists of a toothed metal disk and a stationary detector that covers a magnetic coil. As the teeth move past the coil, they "interrupt" the magnetic field, creating a series of pulses that are sent to a computer. For each 40,000 pulses from the VSS, the trip and total odometers increase by one mile. Speed is also determined from the input pulse frequency. Circuit electronics in the car are designed to display the speed either on a digital screen or on a typical analog system with a needle and dial.[/quote]

Source: [url=http://auto.howstuffworks.com/speedometer2.htm]Howstuffworks "Eddy-Current Speedometer"[/url]
Beginning of article: [url=http://auto.howstuffworks.com/speedometer.htm]Howstuffworks "How Speedometers Work"[/url]

Exactly. That's basically the way all rotation sensors work. Which sensor to get the signal from is another thing. Nowadays the ABS-sensors are used a lot or data from the ECU's injection/ignition part.

So does that mean that the accuracy of any mechanical speedo is COMPLETELY reliant on the... erm... "springiness" of the hairspring? Does the temperature of the hairspring not therefore affect the accuracy of the speedo?

Or did I completely miss something?

most car and motorcycle are off a bit. On motorcycles its very common for them to read 5-10% higher than the actual speed

[quote=VtecMini;774016]So does that mean that the accuracy of any mechanical speedo is COMPLETELY reliant on the... erm... "springiness" of the hairspring? Does the temperature of the hairspring not therefore affect the accuracy of the speedo?

Or did I completely miss something?[/quote]
Nope. Your 100% right.
Just that the quantity of the variation is limited by the choice of the spring material and compliance.
However, it IS the reason why the manufacturers have always been asked by goverments to make the speedo read OVER so that when any individual variation occurs they dpeedo won't read significantly lower than the real speed.

[QUOTE=Matra et Alpine;774134]Nope. Your 100% right.[/QUOTE]Yeah? Booyah!

So how long since have mechanical speedos been redundant then? I'm guessing electronic ones have taken precedence now, right?

Does ANY new car still ahve mechanical ?
I don't know but would doubt it as electronic is cheaper

[QUOTE=Matra et Alpine;774174]Does ANY new car still ahve mechanical ?
I don't know but would doubt it as electronic is cheaper[/QUOTE]

Yeah I guess that is the situation but I bet that the mechanical one is more reliable. Especially if someone sends out an EMP :p (yes, i know that would fry your car :rolleyes:)

[quote=Fuerte100;774179]Yeah I guess that is the situation but I bet that the mechanical one is more reliable. Especially if someone sends out an EMP :p (yes, i know that would fry your car :rolleyes:)[/quote]
As someone who has had to fit and repair speedo cables and drive gears on many cars and Halda take-offs .... nope ... electronic MUCH more reliable ( well as long as it's not made by Lucas or the Italians :) )

So if the drive shaft rotating magnets, creating eddy's controls the speedo, why does the car register a slow speed, >5 mph? wouldnt the eddy spikes be to slow to accuratly measure? Are their simply so many "toothed needles" to not worry about slow rotation speeds?