[neilep (OP)]

Dearest Speedometeologists and owners of cars !

See this speedometer ?

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Nice eh ?...notice how kilometer-and-non-miles-per-hourtastic it is ?...c'mon...lets push it to the limit !..I'm a Miles Per Hour man Sheep myself !!


How accurate are they ?..I heard once that they are always calibrated so that if it shows you're doing 70..then you're really doing a little bit less !!..Is this true ?....

This actually leads me on to another question !  (Two questions in one thread..I am spoiling you !!)


How accurate are these things ?

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How do they know how fast I am going ?

How does it do it ?









whilst you ponder this post,
pray, enjoy this poem I wrote when I was 14
about 'weighing scales '

Wheeze, puff, pant, phew
Mrs. Bellyflop oh no not you
Three tons of chin
Makes me wanna breath in
Mrs. Bellyflop oh no not you

Spring bust, scales screetch
Stomach of yours, food sucking leach
Triple double treble chin
Stand on my tummy , my god I give in
Oh no Mrs. Bellyflop not you

Floorboards creak, footsteps thump
Prepare myself for the Bellyflop lump
Rock formatted fleshy mass
Twenty Ton Tesse, backside killer gas
Mrs. Bellyflop oh no not you

Thighs that itch, breath that smell
Oh take me now Heaven or Hell
Clothes stretch, stockings tear
Weigh herself, TOTALLY BARE
OH NO MRS. BELLYFLOP NOT YOU

Eyes that bulge, nose that runs
Mrs. Bellyflop squashed my tum
My calibration could not resist her
And now I must weigh her big big sister !! []

[another_someone]

Not sure about the radar controlled speed indicators - a lot depends on how well they are set up.

Car speedometers are allowed to overindicate speed by up to 10%, but are not allowed to underindicate speed.  They are usually set up to slightly overindicate.  Bear in mind that tyre pressure can alter car speedometer readings, as can the condition of the road, and certainly if you change from the manufacturers recommended tyres (not in terms of brand, but in terms of size of tyres) you will expect it to increase the inaccuracy of the speedometer readings.

[Karen W.]

LOL Poor poor poor scales...Oh my.. LOL!


I can't answer the question ..cause I wonder myself how they calibrate it myself, UNLESS you drive over or past certain sensors approaching said sign and it calculate how fast by timing the time your tires pass the sensor and the time they reach the second sensor and then shoot up a sign about your speed which it calculated by using the points between the sign..and the time it took to get from A to B... eh?? Just a guess!

[neilep (OP)]

Not sure about the radar controlled speed indicators - a lot depends on how well they are set up.

Car speedometers are allowed to overindicate speed by up to 10%, but are not allowed to underindicate speed.  They are usually set up to slightly overindicate.  Bear in mind that tyre pressure can alter car speedometer readings, as can the condition of the road, and certainly if you change from the manufacturers recommended tyres (not in terms of brand, but in terms of size of tyres) you will expect it to increase the inaccuracy of the speedometer readings.

This is excellent. THANK YOU GEORGE !

I did wonder about asking about Tyre tread and just concluded that the difference would be marginal. Now that you also mention, the other variables...I can see many avenues for margin of error !

[Bored chemist]

"I wonder myself how they calibrate it"
My guess is with a rolling road.

[JimBob]

The size of tires on the car can also cause the speedometer to read incorrectly. In the US some redneck guys like to put large over-sized tires on their pickups and they are getting stopped all the time for speeding. Other people also put over-sized tires on their autos.

[Simulated]

We have a car that's around 7 mph off hah

[engrByDayPianstByNight]

In the speedometer's case, here's my guess of how it probably works. With the manufacturer's recommended tire size, the speedometer calculates the number of revolutions per minute (RPM), then multiplies it by the circumference of the tire (=pi*diameter), which gives you the distance the car has moved in one minute, which can be easily converted to a unit of speed (e.g., miles per hour or kilometers per hour).

    However, this tire diameter must be a kind of system parameter that the speedometer knows and assumes this value never changes. In this case, when you replace the tires with larger sizes (meaning larger diameter now), the speedometer doesn't know and still uses the old diameter value. Since for a given speed, a larger tire takes longer to complete one revolution,, the number of RPMs is less than that with the smaller tire. As a result, the speedometer does the computations in the last paragraph incorrectly and displays a speed that's slower than the actual speed. So the driver may be tempted to step on the gas until the speedometer needle hits the desired speed, which means the actual speed is faster than that indicated.

    If my math is right, and the above reasoning is correct (i.e., assuming the tire diameter parameter is a fixed value accepted by the speedometer), then the ratio of the larger tire diameter to the smaller tire diameter is equal to the ratio of the speed run with the larger tire to that run with the smaller one. So knowing the diameters of the two different tires might give you some idea of how fast you're actually driving, assuming you can drive and compute at the same time. :-)

    I wonder if any new car now has a speedometer that can automatically sense the size of a new set of tires and therefore uses the new diameter value in its speed computations... Or would the GPS navigation system give a better, more accurate estimate of the speed than the on-board speedometer?

[JimBob]

In older cars the number of revolutions was hard coded into the speedometer by a cable that turned by gearing connected to the front axial. It may still be. There is a law here that makes all used car owners and mechanics reorde millage to prevent millage fraud. It was a common practice for used car dealers to get an electric drill, connect it to the cable to the odometer and speedometer and turn back the millage significantly. Don't know about newer all electric wonder autos these day. I stopped doing my own auto mechanical work 15 years ago.

[Soul Surfer]

Speedos in cars always overindicate in my experience but some can be quite accurate and others out by the full 10%  We calibrate ours using GPS which should be well within 1mph and I drive to accurate speeds based on the GPS readings for the speed limit not my car speedo.

Nowadays in the UK with different spped limits all over the place and it can be very difficult to know what the speed limit on a particular section of road is.  I feel that it is important for drivers always to be able to instantly confirm the speed limit at any moment he/she is driving the car and suggest that there should be different clolured spots in the lines on the road to indicate speed limits.  The current speed limit signs can be up to half a mile apart.

[another_someone]

Nowadays in the UK with different spped limits all over the place and it can be very difficult to know what the speed limit on a particular section of road is.  I feel that it is important for drivers always to be able to instantly confirm the speed limit at any moment he/she is driving the car and suggest that there should be different clolured spots in the lines on the road to indicate speed limits.  The current speed limit signs can be up to half a mile apart.

While I agree with your concern, I rather doubt the solution would be practical.  I suspect it will start with the best of intentions, but will quickly be overtaken by events, as speed limits are changed, and even if new markings are added to the road, the old ones will not be removed; or else the marking becoming faint over time.  It will lead to endless contentious court cases where drivers challenge speeding tickets because the road markings were ambiguous.

[engrByDayPianstByNight]

This may be off the current topic a little, but I once read some interesting posts about dynamic speed limit signs. Instead of a fixed number (e.g., 35[mph] in the U.S.), the sign is a LCD whose number changes according to the traffic flow at the time. This may require cars equipped with some wireless sensors that communicate with the sign so as to allow it to compute the average car speed, and along with some other input information the sign gathers from on the road (e.g., communicating with other speed-limit signs along the road), it may be able to compute a reasonable speed limit that changes with the change in the amount of traffic congestion.

To the best of my knowledge, this idea is still largely an open problem in the research community, although there exist some prototypical wireless sensor networks deployed for traffic control purposes.

[another_someone]

This may be off the current topic a little, but I once read some interesting posts about dynamic speed limit signs. Instead of a fixed number (e.g., 35[mph] in the U.S.), the sign is a LCD whose number changes according to the traffic flow at the time. This may require cars equipped with some wireless sensors that communicate with the sign so as to allow it to compute the average car speed, and along with some other input information the sign gathers from on the road (e.g., communicating with other speed-limit signs along the road), it may be able to compute a reasonable speed limit that changes with the change in the amount of traffic congestion.

To the best of my knowledge, this idea is still largely an open problem in the research community, although there exist some prototypical wireless sensor networks deployed for traffic control purposes.

We have variable speed limits along one section of the M25, to the west of London.  The M25 is a motorway (freeway if you are American), but heavily congested through much of the rush hour, and during the congested time, the speed limit can drop from 70mph (the national speed limit on UK roads) to 60mph (or possibly even 50mph).

There are, on most of our motorways, matrix signed (signs composed of a matrix of light bulbs that can display information).  These signs can often show recommended changes in speed limit (e.g. when there is a hazard ahead), but on this stretch of the M25, there is a red ring around the matrix signs, and they show a mandatory change in speed limit.  Have yet to be convinced they do any good, but they are there.