Car speedos

[Cycling Dan]

Is it true that car speedos have 2mph added onto them?
So true speed would be 30mph and then the spedo for the driver shows 32mph.
Is this true or just a myth?

 

[MontyVeda]

probably a myth... but I expect when a speedo is calibrated, it'll be done in such a way that i wont show you're going faster than you are... so there may well be some truth in it.

saying that, another possible myth is that one is allowed 10% over the speed limit... so if a speedcamera clock you doing 32 in 30 zone, you won't get fined because your speedo may be inaccurate.

I'm sure some of our members of the police service will put us straight before long

 

[asterix]

They always over-read. The faster you go, the greater the difference.

My speedo never agrees with the satnav.

I haven't tested it against a speed camera and shall continue to try not to.

 

[Globalti]

They always over-read by 5-10%. Manufacturers err that way to ensure they can't be blamed if you get nicked and because there are so many variables such as tyre diameter and pressure that affect the speedo's accuracy.

 

[asterix]

probably a myth... but I expect when a speedo is calibrated, it'll be done in such a way that i wont show you're going faster slower than you are... so there may well be some truth in it

I think you meant?

 

[Night Train]

The speedo has to be within 10% IIRC and so they are generally calibrated reading slightly fast so as not to read slow, which would not be legal.

Compared to my satnav my speedo reads 32mph at 30mph, and 65mph at 70mph. I drive at less then the limit as shown on the satnav.
The satnav is comparable to the roadside speed indicators.

 

[mickle]

I remember being told (by a Ford technician) that the average discrepancy was 8%. That was a long time ago, Cortina days, but comparing my car and the vans I drive I reckon it hasn't changed substantially.

My understanding is that the police, and speed sensing cameras, allow a margin of error of ten percent plus 2mph. So you really do need to be going a fair old lick (over the speed indicated on your car speedo) to earn an actual speeding ticket. About 85mph in a 70 zone for example.

 

[GrasB]

They're all over the shop, especially the ECU driven ones. At very low speeds (<10mph) often speedos under read quite a lot. Once above 15mph every speedo I've seen under-reads on factory wheels.

IIRC speedo error is measured at certain spot check speeds. Manufactures make sure the speedo reads within the -0% to +10% error margin. I worked out one speedo started under reading (5km/h = 3km/h) skewing to accurate at 20km/h. From there it continued to skew towards +10% over read at 80km/h & held there until you hit 225km/h at which point it it increased its over-read by 1% for every 10km/h faster you go.

Mechanical or magnetic pulse signal speedos tend to be liner within reason (most speedos have idiocracies at sub-15mph speeds). They're either accurate at very low speed & slowly over read as the speed increases or over read by a static %.

 

[MossCommuter]

Tyre wear between brand new and worn out will affect speedo reading by a couple of percent as will over/under-inflation of the tyres.

 

[pplpilot]

all you need to know here -

http://en.wikipedia.org/wiki/Speedometer#Error

 

[Fnaar]

Why would a car be wearing swimming trunks?

 

[Piemaster]

For IVA tests (for kit cars/much modified vehicles) its not allowed to read low, but is allowed to read high, think its 10% but not sure, so will be calibrated for that.
Providing it's actually looked at in relation to speed limit of the road a speeding fine should never happen.

 

[numbnuts]

Why would a car be wearing swimming trunks?

A WOMAN accidentally drove her car into a river while manoeuvring out of a car park last night.

 

[mattobrien]

Oddly the speedo on my Lotus Elise is pretty much bang on at 30mph and is very accurate until you get to considerable speeds

Strange that Lotus would engineer an accurate speedo, when most other things on the car do not have a similar tolerance - guess they must have bought in the part

 

[GrasB]

Oddly the speedo on my Lotus Elise is pretty much bang on at 30mph and is very accurate until you get to considerable speeds

Strange that Lotus would engineer an accurate speedo, when most other things on the car do not have a similar tolerance - guess they must have bought in the part

More accurately, they've not engineered the speedo they basically use an use an off the shelf 'aftermarket' item & simply inputted the correct tyre size parameters. Aftermarket speedos are normally as accurate as the calibration the owner sets up.