Speedo accuracy - GJD
As I understand it (correct me if I'm wrong) a speedometer is allowed to overread by up to 10% and not underread at all. But when is this checked? I had assumed it would be done every MOT, but it's not on the list at www.motuk.co.uk/

Is the speedo checked at manufacture and then assumed to be OK? Would it only be checked again as part of an accident investigation? What if it doesn't work at all (ie reads zero all the time) - would the car pass an MOT?

Speedo accuracy - billy25
i always thought that the accuracy of speedo's was +/- 5%,though i stand to be corrected. i have never known testers to check them at MOT time, unless they do a "visual" while the cars on the rolling road for the brake tests, i do know however that it is illegal to drive a car with a defective (not working )speedo, as learning this knowledge cost me £20 back in 1993!!.
Speedo accuracy - GJD
I thought overreading was fine but underreading wasn't, because you'd be going faster than you thought. But I can't quote a law that states that so I stand to be corrected too ;-)

Not working at all is one thing, but if there are legal limits on the accuracy, I would have thought it would need testing.
Speedo accuracy - billy25
i agree with you,but it's not a matter that iv'e really thought about in depth before, speed on the speedo- is speed i'm doing!.i hope someone could clarify this issue for us it could be useful, however (for my own piece of mind)next time i see nice mr plod by the roadside with his radar gun, i might ask him if he will clock me whilst i drive at him at 40mph, then i will have a rough idea how my speedo reading is!! :-)
Speedo accuracy - Dwight Van Driver
Very strange as I was of the opinion that 10% error was as legal hence ACPO Guidelines on speeding ten percent plus 2

However on checking Reg 35 MV(Con and Use)Regs 1986 all that is stated is that speedometers have to be fitted to certain vehicle. No mention of accuracy.
Reg 36 outlines that they have to be maintained but again no criteria on accuracy .

To get Type Approval on Speedo the following test done
4.3. The accuracy of the speedometer equipment shall be tested in accordance with the following procedure: 4.3.1. the vehicle is equipped with one of the types of tyre normally fitted ; the test shall be repeated for each of the types of speedometer specified by the manufacturer; 4.3.2. the load on the axle driving the speedometer equipment must correspond to the weight complying with 2.6 of Annex I to Directive No 70/156/EEC; 4.3.3. the reference temperature at the speedometer shall be 23 ± 5º C; 4.3.4. during each test the pressure of the tyres shall be the normal running pressure as defined in 2.3; 4.3.5. the vehicle is tested at the following three speeds : 40, 80 and 120 km/h, or 80 % of the maximum speed specified by the manufacturer, if this is inferior to 150 km/h; 4.3.6. the test instrumentation used for measuring the true vehicle speed shall be accurate to ± 1.0 %; the surface of a test track when used be flat and dry, and shall provide sufficient adhesion. 4.4. The speed indicated must never be less than the true speed. At the speeds specified for the test in 4.3.5 above and between these speeds, there shall be the following relationship between the speed indicated on the dial of the speedometer
EEC Reg 75/443/EEC.
Again no error other than cannot show doing less that true speed.
Speedo accuracy - Big Vern
My 98R Vectra has never had its speedo checked since leaving the factory (if even then). When I had a tracking device in the car using GPS technology I made a point at traveling for long periods at a steady indicated 30MPH, 60MPH & 70MPH and was surprised to find that the tracking log showed up as only 1 ~ 2 MPH over what the speedo said I was doing at the time. So I was impressed.

I am in N.Ireland and as such we take our cars for a drive thru style goverment MOT, not a local garage where you leave your car and come back for it, as such I know what is tested and how, the speedo was not.
Speedo accuracy - SjB {P}
"Speedometer Accuracy UK" in Google returned a Hansard item from Monday, 12th March 2001, that states:

"The Road Vehicles (Construction and Use) Regulations 1986, as amended, allows the use of speedometers that meet the requirements of EC Community Directive 75/443(97/39) or ECE Regulation 39. Both the EC Directive and the ECE Regulation lay down accuracy requirements to be applied at the time of vehicle approval for speedometers. These requirements are that the indicated speed must not be more than 10 per cent of the true speed plus 4 km/h. In production, however, a slightly different tolerance of 5 per cent plus 10 km/h is applied. The requirements are also that the indicated speed must never be less than the true speed. A vehicle meeting these requirements would not be able to travel at a greater speed than that shown on the speedometer and a driver could not, therefore, inadvertently exceed speed restrictions. Her Majesty's Government have no plans to introduce instrument tests."

If the Road Vehicles (Construction and Use) Regulations 1986 act allows use of speedometers that meet this EU regulation, UK specific regulations can't be any tighter.

Interesting that it is illegal to be able to travel at a speed greater than the maximum that the speedometer can display. I'd never thought of this as a possible 'get out of jail free' before, but can see that the lawmakers did!

Speedo accuracy - billy25
cheers sjb,
thats cleared that up, and i stand corrected,original poster was correct with the 10% figures (however i knew there was a 5% in there somewhere! :-) )

Value my car