Talk:Speedometer

"Common misspelling"

"(commonly misspelled spedometer)"

?! —Preceding unsigned comment added by 203.37.47.29 (talk) 08:10, 4 April 2008 (UTC)

Any help?

Would it help if I add a picture of a speedometer showing 172km/h? Petros The Greek 18:23, 29 October 2005 (UTC)

That's all you can do? Nova SS 03:40, 25 March 2006 (UTC)

Factually incorrect statements

It is probably factually incorrect that tire inflation causes much of a difference in speedometer readings. I have removed that statement. Tires are a woven collection of rubber (that does not expand like a balloon) and a mesh of metal (which does not permit balloon-like inflation under normal circumstances). The rolling circumference of a tire is generally fixed and does not change with inflation.

Also, it is questionable that speedometers are intentionally calibrated to read too high. As of 1997, federal standards allowed a 5% speedometer error either way. Any contention that they are indeed calibrated high needs to be backed up with a citation. Otherwise, such a statement has the appearance of a communally-reinforced misconception.

I would suspect it IS done on purpose - it means that any natural variation in calibration, viewing angle, overly deep tyre tread (or use of chains)/overinflation and/or mis-installation of oversized tyres by the user/shady service agents has less of a chance of making the dial (appear to) under-read. As the opposite case could be about as likely, a sensible manufacturer may peg their normal speedo calibration some way up into the allowed range, but not too high. This would match with my (anecdotal, I know) experience of various vehicles vs all of GPS, "watch your speed!" radar-based roadside signs, and stopwatch milestone/km-post timings. 5% over-read is common (that's "only" 4mph at 80mph...) and 7-8% or more is not unheard of. I recently had new, slightly larger tyres put on my own car (2001 vintage, french made & sold in the UK) and it's now *almost* fully accurate... but still overreads by maybe 3%. As far as a digital display would be concerned, it may actually be under-reading; a self test mode I discovered (that supposedly counts up the electronic speedo in 20mph increments, the tacho in 1000s and the other guages in 1/4s) shows both it and the tach to overread a bit even when ostensibly showing exact speed multiples ... and it may not even be possible to perfectly calibrate it as the variation from absolute varies up and down at each speed/rpm point.

Besides, the general public sort of expect that now. I certainly do. I've gotten fairly used to driving at "33-36mph" and knowing that I should be immune from "30mph" speed cameras that would normally trigger at 33.1mph as an absolute minimum, and 35.1 in a more typical case. Finding out post-tyre-change that I now need to alter that to 32-34 on the dial to stay safe was a surprise...

Also, in a further anecdotal contribution, it's allegedly standard practice for the big german manufacturers - certainly the premium marques - to set their dials to +7%... which will typically keep them within +10% +10km/h (ie 6.2mph, the quoted euro standard) / -0% in most situations even with further drift. It may even be a TUV mandated thing to do that from the factory - depends whose story you believe. My old Polo was definitely optimistic about the speeds it could achieve. 95mph in level flight from a tired out engine that should have only been good for 90 even when freshly run-in, 105 downhill, and a 1-litre that stretched to 81mph at the top of 3rd gear...? Riiiight. 193.63.174.11 (talk) 12:46, 24 March 2011 (UTC)

Is there any other source than an out of print article in AAA from 1997 regarding federal speedometer accuracy regulations? I am unable to find any such regulation in actual US Code, and do not believe the source is accurate. If it is indeed true, another source (preferably the regulation itself) should be readily available for citation.

Fishbert (talk) 21:55, 13 July 2011 (UTC)

This statement may be substantially true, but needs a lot more work and citation before it is put back:

A GPS system may also be used as a speedometer. This is usually more accurate than car speedometers as GPS systems are not calibrated to display a higher reading. Another advantage of using a GPS as speedometer is that it is easy to install and only depends on a somewhat clear view of the sky and a power supply. However it takes up to a minute before it has "tracked" all the satellites and only updates the reading every second or two. Bad signal will lead to it being inaccurate.

Nova SS 23:12, 15 March 2006 (UTC)

GPS does suffer drift over time if you're stood still, but this is on the order of a a few tenths of a km/h at worst. The accumulated error can be 100 metres or more if it doesn't often try for a new initial fix. However that's over a half hour or possibly longer. The effect on an instantaneous speed reading where the distance between two consecutive points in the space of 1 second or less is minimal, particularly if your readout is in whole km/h or mph. It's distinctly more accurate than a speedometer dial unless your GPS signal is very poor - in which case a typical civilian receiver will simply claim "no signal" rather than risk sending the customer off course. Military or hardcore hillwalker ones may allow a more degraded service as the user should understand to only use it as a guide alongside standard orienteering-grade maps, compasses etc rather than an absolute, "twat-nav" style position/speed/direction fix (which typically snaps to the nearest road as probabistically identified by its routefinder, and known travel history). 193.63.174.11 (talk) 12:46, 24 March 2011 (UTC)

Sorry, but the above is a load of nonsense. Anyone who can be arsed head over to the library and grab one of the many books on satellite positioning then correct the text and add the relevant references. — Preceding unsigned comment added by 109.81.211.130 (talk) 01:57, 12 September 2015 (UTC)

Inventor of the speedometer

As far as I can tell Josip Belušić did not invent THE speedometer, that has been credited to Charles Babbage who died in 1871, if you look on Josip's page you will see it has "the first electric speedometer." which I believe is correct, I am trying to track down a patent No. or similar for Babbage to confirm though.

FzerozeroT 23:05, 9 June 2006 (UTC)

$Insertformulahere$== how does a speedometer work ==

"A speedometer is an eddy current device...ie, a permanent magnet rotating close to a conductive disk or cup causes electrical current to flow in small rotating eddies. These currents, being "shorted out" by the continuous conductive disk, cause a drag torque. The torque produced is a function of several things, but primarily the velocity of the magnet and the flux produced by the magnet. The torque winds up a spring on the needle until the spring torque is the same as the eddy current torque." -- http://www.pelicanparts.com/techarticles/Mult_gauge_repair/mult_gauge_repair.htm

I wish the article had a diagram of how (mechanical) speedometers work. The "pelicanparts" description (one magnet, one conductive cup) seems inconsistent with the "The speedometer itself is two rotating, barrel-shaped magnets." description currently in the article.

Are these 2 partial descriptions of the same thing thing, or are there really 2 different kinds of (mechanical) speedometers, working under different physical principles? --70.177.117.132 22:32, 3 September 2006 (UTC)

Now, here's a question: What mode of operation did the speed indication devices on steam locomotives use? They certainly had to operate under restricted speeds at certain dangerous points on the track, and Casey Jones was somehow known to achieve 90mph or more at peak (not just average between towns), long before the 20th century kicked in and electric devices were used. But something that worked close to or even exactly the same as the Eddy Current one may have worked without people being fully aware of the nature of interaction between the spinning magnets, and the outer, springloaded carrier... 193.63.174.11 (talk) 12:53, 24 March 2011 (UTC)

Most steam locomotives -- even in the 1930's -- didn't have a speedometer. Experienced drivers could judge their speed accurately enough by looking out of the window, listening to the exhaust puffs and feeling the vibrations of the locomotive. More accurate measurements were occasionally performed by the guard, who used his watch and the mileposts, which were placed along the track typically every 1/4 mile.

Anyway, it would be a good idea to have some more info here on speedometers in trains. Does anyone know something about that matter? PiusImpavidus (talk) 11:08, 22 June 2012 (UTC)

Units versus country

I have to hand a list of countries with their rule of road, international registration letters and their speedometer units - km/h or mph. Unfortunately, I don't know the provenance of the list, but, since I work in vehicle product certification, I can check a number of them. Additionally, where it is out of date, like the Republic of Ireland that recently metricated, I can update it. I had hoped to post the document to Wikicommons or my user page, but can't because I don't know who owns copyright. Hopefully a more experienced editor can suggest a way of proceeding - if it is worth doing. Obelix homologator 07:29, 16 August 2007 (UTC)

Actually, only the US, UK, and Canada have dual miles-per-hour and kilometers-per-hour speedometers. Here in the US and over in the UK, miles per hour is the primary calibration, with kilometers (kilometres) as the speed on the inner track. In Canada it is the reverse: kilometres in the outer, miles inner. I always claimed that Ireland would have the same type of speedometers like Canada... so if Canada has a small MPH for US trips, why can't Ireland have small MPH for British trips?

Harvey994 19:10, 30 May 2010 (UTC -5)

Recent hire cars I've driven in the US have only had mph markings on the speedo e.g. Corvette instruments. Not my photo but you get the drift. Mr Larrington (talk) 13:22, 11 January 2011 (UTC)

Standing Speedometer

What's a standing speedometer? Like the ones near roads that show how fast everyone is going? —Preceding unsigned comment added by 69.220.2.188 (talk) 22:59, 28 November 2007 (UTC)

Its a radar device, bassicaly it hits your car with two radar bursts (im not sure how far apart, probably 1/2 a second to a second) it measures how long it takes the first radar burst to return, measuring the distance from the emmitter to your car, and back. Then it hits the car again and does the same thing. Then it compares the two, and sees how far apart they were, then calculates the speed of the vehicle. Thats the simplified version anyway. —Preceding unsigned comment added by 170.140.118.80 (talk) 19:39, 22 April 2008 (UTC)

Error

Perhaps changing tire or wheel size should be discussed in this section as well, as it is a common occurrence for an error in your actual speed, and the kits you can get to compensate for this.

Also I am not sure if this is true or not but someone once explained to me a phenomenon only to be described by me as road tire expansion, and it had something to do with traveling at high speeds and either the tire getting warm and the air expanding inside the tire or something having to do with lift and the tire expanding due to less pressure from the vehicle on the tires. I believe that if it was true it might have an influence on creating error, but from the same source was told that this was already taken into account.

-Searauber 13:34, 2 July 2008 (UTC)

Long ago, when I drove a Volkswagan Karmann-Ghia, I observed that the speedometer changed noticeably according to the engine torque. This rear-engined car has very little weight on the front wheels, one of which drives the speedometer; so the force countering the torque applied to the rear wheels is a much greater fraction of the front-wheel weight. —Preceding unsigned comment added by 63.86.92.198 (talk) 14:37, 23 October 2008 (UTC)

Probably those tall, weak-walled old tyres significantly changed shape - and therefore rolling circumference - with the amount of weight being put on them through the suspension. More throttle... more acceleration... more weight at the rear rather than the front! 193.63.174.11 (talk) 13:00, 24 March 2011 (UTC)

This Section should be cleaned up. The format makes it difficult to read and understand. I think that starting with just basic explanations would be good. A reformat without all the formulas interspaced in the text would be appreciated. Perhaps a better design would have the formulas etc at the end maybe put into more of a list form. Currently it appears as if someone arbitraily combined an english and math textbook. — Preceding unsigned comment added by 72.11.91.140 (talk) 03:46, 2 December 2013 (UTC)