Speedo not reading correctly - cable or gauge?

[hepkat63 11]

Hi All,

my speedo is out by about ten miles an hour - but varies depending on what speed you're doing - up to fifteen miles an hour.  It is very annoying !   Would you think this would be the gauge or the speedo cable causing the issue?   Is this common on lotus esprit turbo cars?

[Escape 743]

The cable is a fixed connection between the gearbox and gauge, so can't cause an incorrect reading (unless it's broken). The cable can cause the needle to bounce around if it sticks. Overreading is typically not an absolute value, but a percentage of the actual speed. Calibrating the gauge is possible (by adjusting the spring tension), but difficult to get right. A known problem is the magnet on the cable drive working loose and catching on the needle drive, giving sudden deflection and general overreading.

[molemot 521]

ALL speedometers over read by about 10%. Checking against GPS data show this clearly. I assume it is to prevent people suing the manufacturers if prosecuted for speeding, on the grounds that "They were doing 70 on the speedo"....when that would really be about 63mph and thus they are telling porkies. Haven't looked at a speedo for ages..always use the GPS, set the car to be on the limit on the cruise control, relax and steer...

There are still people out there who don't believe in GPS. My brother got nicked for speeding, full of the joys of spring, taking his daughter to church for her wedding in the Triumph Stag. He did one of these "speed awareness" things....and one chap there refused to believe that speedometers were inaccurate; "Why would the manufacturers spend all that money to produce an inaccurate instrument?" Even explaining the cost and accuracy of the GPS satellite system wouldn't convince him....of course, the Americans are going to spend billions on nav kit that doesn't work, aren't they??

[gvy 364]

Your turbo esprit originally had 235/60 VR15 at the rear .

What is the tyre you are using currently ? A tyre with a smaller overall diameter will give a higher reading on the speedo . Infact , you can expect the reading, with the correct tyres already to be out for about 8% (higher then reallity). Using a tyre with smaller overall diameter will worsen it.

You can calculate the extra difference quickly wit this tool : https://tiresize.com/speedometer-calibration/

greets

Geert

Edited August 30, 2016 by gvy

[hepkat63 11]

thanks guys for the replies.  Yes, using stock 235/60 tyres.  I guess it is just one of those put up with it things !  I did think about using my iPad mini and velcro it to the dash as a satellite speedo

[gvy 364]

I use a TOMTOM go installed in the Esprit.

I have considered velcroing it to my speedo. That way I would have navigation ,handsfree phonefunction (tomtom connect with my cellphone when entering the car),and a constant visible acurate speedreading right in front of me while the other dials are still readable. I dont need to touch it, while driving because I use  the voice control on the TOMTOM ( works perfect).

In the end I placed it next to the mirror on the window , because in my case the speedo is pretty accurate (about 5km/u wrong)  and I find it a pity to cover the classic dials with a TOMTOM .

In your case , with an incorrect/useless speedo, you can

Geert

Edited August 31, 2016 by gvy

[peteyg 300]

My speedo reads bang on up to about 100mph then over reads a bit. Bizarrely the most accurate speedo of all the cars in out household...

[Escape 743]

On 31/08/2016 at 16:59, gvy said:

I use a TOMTOM go installed in the Esprit.

I do similar, the original mechanical speedo was very bumpy and I never got round to sort the wiring of the replacement electronic one (there's a good chance it's actually faulty) but I have a Coyote attached to the dash. Besides speed it also indicates the maximum allowed speed (though not always very accurately) and warns of known or likely speed traps.

[benja-p 13]

On 30/08/2016 at 18:45, gvy said:

Your turbo esprit originally had 235/60 VR15 at the rear .

What is the tyre you are using currently ? A tyre with a smaller overall diameter will give a higher reading on the speedo . Infact , you can expect the reading, with the correct tyres already to be out for about 8% (higher then reallity). Using a tyre with smaller overall diameter will worsen it.

You can calculate the extra difference quickly wit this tool : https://tiresize.com/speedometer-calibration/

greets

Geert

I've checked this out on the link Geert and I think it's the other way isn't it? I have an SE running 245/50's on the rear but its time to change and due to lack of options I'm considering dropping to 245/45's. Inputting this into the link shows me that rather than doing 70mph the speedo would show 67mph? Now I'm even more confused! haha!