SPEEDO ACCURACY

[maydharm]

Hi All,

Just how accurate are the speed readings on sat nav's? At 70mph on the nuvi the clock on my Esprit reads 76-78mph. If the speedo is out is there a way to recalibrate it, or do i have to take it somewhere?

[neal]

Have you the later style dash with the VDO speedometer?

[Günter]

there is a reason 'for safety' that your speedo reads something more on European production cars. It is legal to calibrate it to read more, but it is not allowed for the manufacturer to offer an car on the market that reads to slow... . Something with ECE conformity !

My sat-nav shows around 10km/h less -as soon as I'm in an speed region above 80km/h Top speed on speedo is something to 290 with 6800 rpm (so it is in fact not more than the official certified 275km/h for an GT)

My guess you can on all VDO speedos -pre '98 and past- recalibrate the impulse readouts with the vdo-manual (documentet in the vdo-web page, and in this forum) and the tripcounter button [press it] /switch ignition off/on

[Tentenths]

Satnav speedo accuracy is pretty spot on - within <1% iirc.

As Gunter says, car manufacturers selling into Europe are required to ensure that speedos err on the conservative side i.e. must not under read. Hence the reason why all are calibrated to over read by, in my experience, anything up to 10%. Don't forget that tyre wear also impacts on speedo/odo accuracy.

[maydharm]

Thanks Guys and thank you Gunter i'll have a look on the forum for the info.

[freshcut]

Mine seems to read a pretty consistant 3MPH higher than my sat nav indicates. I've used it on all the cars we have and each seems to be 2-3mph higher on the speedo than the GPS indicates except one which reads about 3mph low, i.e. going 65 on GPS indicates 62 or so on the speedo. Ironically it's also the same vehicle I recently got a speeding ticket in to which I claimed to be doing 65, but the officer said he clocked me at 69, so I would say that the GPS reads pretty spot on.

[Günter]

the reason is pretty simple : sat nav gets informations by time code-packages and satellite identifier codes. Your speedo is an simple (manually) adjustable -impulse referenced- instrument. It shows speed in relation to a pre calculated wheel diameter/ turnaround-distance vs. impulses. If the wheels have differences to the pre set data, or you just spin the wheel to much it should show speed where no speed is (for example on quarter mile-run starts) You can simply foul your speedo, but not the sat nav

[Jonathan]

Ive just done this test.

I used Tomtom SatNav - a dedicated GPS HUD, the speed and the rpm vs gear ratio vs wheel diameter.

On a level run - all but the mechanical speedo are in tolerance.

The Cars vehicle speed sensor is spot on to the GPS and calculated speed using the wheel diameter/gears/rpm.

I experienced GPS running slightly under if anything but not by much - 1-2% if that.

The dial itself on the dash is another matter, I'm currently building a error correction bit of electronics to try and get it as accurate as possible.

[Günter]

good point Jonathan, but to be fair -does it matter what speedo says exactly ?!,

I mean it's just there for gatherings to compare the 'needle' with other guys. 275km/h or 290 or 300km/h the straight line speed of the V8 and even the S4s is fantastic anyway !

Edited March 15, 2009 by G

[Jonathan]

IMO yes.

I dont see the point of a gauge if it doesn't read right - plus at the speed I drive at the gauge is out by quite a margin.

[freshcut]

Actually, if you have the VDO gauge with the digital odometer, you can calibrate the speedo to read any speed you want it to. The link for calibration is as follows, but be careful as you should have proper testing equipment, except in the "manual" setting in which you manually input the pulse per mile figure to get it to read exact.

http://www.gomog.com/allmorgan/VDOCALIBRATION.html