Speedometer issue

[johnwill]

Thinking to the jobs I need to get done this winter, one of them is to get my speedo sorted.

It's currently reading way too low, (1152 and worse on 1184) so my gauge, plus a friends spare 1152 and 1184 are all reading low.

I have 165 x 15 tyres, 4.1 diff, standard box with o/d.

I intended to get it to JDO or Speedy to re-calibrate but...

Are all speedo's out or something amiss somewhere else?

Any ideas welcome please.

(I've already put tippex on to mark the correct speeds...it looks awful)

Thanks

John

[RogerH]

Hi John,

the 4.1 diff is not standard.

I would have thought this would make the speedo over read  (more input for given output)

JDO do a very fast turn round 

 

Roger

[Hamish]

Jdo did mine this year great service and quick. 
their instructions for counting wheel turns can be done with car jacked up. 
I had had a 4.1 fitted and I think the Speedo number for this ratio is 1309. 
but I have 185/70x15 tyres.

[johnwill]

Thanks guys

[tthomson]

I seem to remember that there is a legal requirement for Speedometers to show faster than actual speed, but I don't remember by how much.

SatNavs normally show actual speed.

TT

[RobH]

Quote:

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 the true speed plus 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.

[this was a reply to a question in the Commons in 2001. My italics inserted to make the description read correctly]

So there is no mandate to over-read but the the required tolerance on the display in m.p.h.  is -0 +(10%+2.48).    That of course applies to modern vehicles built under the ECE regulations.

Checked against GPS both of my most recent moderns over-read by 3mph all the way up the scale. 

Edited September 29, 2020 by RobH

[tthomson]

Hi Rob,

Thanks for the info. To precis:

On average, for vehicles governed by ECE Regulation 39, their speedometers will over-read, but an individual speedometer should not do so beyond 10%+2.48 mph, and no speedometer should under-read. So if someone has had a speedometer refurbished within the time that this directive is in place, that speedometer would need to adhere to this directive.

I'm sure that most, if not all, original TR speedometers pre-date ECE Regulation 39, but I feel certain there was a similar UK regulation prior to that, but I do not know how one might find out what it specified.

TT