Speedometer

[Callipygous]

how does a cars speedometer work?

 

my brother suggested some kind of friction based device, where the faster one piece is turning the more the piece touching it gets raised.

 

i was very disappointed to find that howstuffworks didnt have an entry for this one : (

[Cap'n Refsmmat]

Usually, in a modern car, there's some sort of device (a magnet, most of the time) mounted on the axle/driveshaft that goes past a sensor. The sensor detects the magnet passing by so many times a second, and then uses the diameter of the wheels and such to calculate what that rotational speed translates into in mph.

[JesuBungle]

http://www.answers.com/topic/speedometer

 

This site explains a couple of types used in cars.

[weknowthewor]

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).

[Callipygous]

Usually, in a modern car, there's some sort of device (a magnet, most of the time) mounted on the axle/driveshaft that goes past a sensor. The sensor detects the magnet passing by so many times a second, and then uses the diameter of the wheels and such to calculate what that rotational speed translates into in mph.

 

so what happens when some guy decideds to trick out his car and puts 4 inch bigger tires on it?

[D H]

so what happens when some guy decideds to trick out his car and puts 4 inch bigger tires on it?

 

The speed on his speedometer is low.

 

Here's an experiment to validate this hypothesis of how speedometers work for those of you who live in the frigid North. Find a nice icy parking lot and hit the gas hard. Your speedometer might indicate you are going 120 MPH, but all you are really doing is spinning out of control at a nice leisurely 30 MPH.

[Callipygous]

so the size of your tires really does effect the accuracy of your speedometer?

 

i was really hoping that with all our moving pictures and flying machines and whatnot we had come up with something to get around that.

[Glider]

The tyre diameter thing is the easiest. If you don't go changing the tyre diameter, there's no reason to get all high tech.

 

If you wanted a truly objective (and accurate) reading, the way would be to take readings directly from the surface on which you're travelling, possibly using lasers under the car.

[tinnmann]

Yeah that would be the easiest but what about the differences in height and shinyness of the surface that the vehcle is driving on. Im sure that would have some effect on the accuracy of the speedometer unless a computer took an average of different readings from the laser sensor.

[Phi for All]

so the size of your tires really does effect the accuracy of your speedometer?

I know a policeman who sent each of his kids off to college with a new set of tires on their car. They were oversized so his kids never realized they were going 5-10 mph slower than what the speedometer read.

 

The kids never figured it out and they never got a speeding ticket either.

[Cap'n Refsmmat]

Wouldn't they have to be undersized?

[Glider]

Yes, they would.

[insane_alien]

supersized tires would make you go faster. wonder if its grounds for getting off a speeding ticket? 'my speedometer said i was going 30 i honestly*cough* didn't know i was going faster than the speed limit'

[Phi for All]

Wouldn't they have to be undersized?

I did have that backwards. It's also easier to go smaller since going bigger might cause a tire to bind in a wheel well if you changed by more than a couple of inches.

 

The trade-off here is that you may not get a speeding ticket but your odometer is going to show more mileage if your tires are undersized. Less value when you go to sell.

[tinnmann]

And with bigger tires youre going to have a harder time accelerating if you dont change a few gear settings in the transmission.

[EvoN1020v]

I have heard this way: Undersized tires goes faster than the accuracy of the speedometer.

 

i.e. Speed of the undersized tires: 140 mph.

Speedometer reading: 100 mph.

 

A friend have told me this story as her husband was caught by the police with a speeding ticket. He protested that he was going at 100 mph, while the police's laser gun read 140 mph. He had a friend who is a policeman and they did a private test. The first policeman was right, as the tires were going faster than what is said on the speedometer. The husband didn't want to buy the correct size of tires because bigger tires are more expensive than smaller tires. Who knows it might be more benefict that you buy the correct size tires than being caught with loads of speeding tickets?

[Callipygous]

I have heard this way: Undersized tires goes faster than the accuracy of the speedometer.

 

i.e. Speed of the undersized tires: 140 mph.

Speedometer reading: 100 mph.

 

A friend have told me this story as her husband was caught by the police with a speeding ticket. He protested that he was going at 100 mph, while the police's laser gun read 140 mph. He had a friend who is a policeman and they did a private test. The first policeman was right, as the tires were going faster than what is said on the speedometer. The husband didn't want to buy the correct size of tires because bigger tires are more expensive than smaller tires. Who knows it might be more benefict that you buy the correct size tires than being caught with loads of speeding tickets?

 

 

like we just discussed, thats backwards. if he bought smaller tires they would cover less distance with each rotation, so the car would think it was going faster than it actually was.