GPS speed accuracy Vs speedometer?

[if1977]

[mikealder]

And where is the diagnostic machine getting this data from, the gearbox sensor which is related to wheel speed - I would still trust the GPS speed to be more accurate, it certainly won't be 11MPH out.

Unless your diagnostic speed was from a calibrated speed sensor - Mike

[tgold]

[if1977]

[Badgerman26]

Ruddy hell, just read this thread after starting it with what I thought was going to be a simple question!

I did try and use the search button but TBH it seems to come up with so many posts that are not relevant to the search terms (compared to other forums I use). Not sure if I am searching incorrectly?

Anyhow sorry to open a can of worms!
_________________
TT Go 520
Fabia Vrs (Mine)
VW Golf (the wifes)

[Andy_P]

One line answer.... Trust the Satnav not the Speedo.

[Snudge]

[dhn]

[biggles150]

[Philgo]

Let's face it, if the Satnav can't work out your speed accurately it's certainly going to struggle with it's proper job. I'd say that the satnav is probably accurate to a fraction of a mph.

[jspencer]

[Gnomeface]

As an engineer, I can't let this go on without sticking in a few comments.

First, speedos on many (most?) modern cars are NOT driven from the gearbox output shaft (how ancient is that?) - the road speed is calculated by the car's computer from the data received from the wheel sensors that control ABS, DSC, etc, etc. The speedometer is then usually given an analogue signal (sometimes digital) which is generally higher to guarantee that it will not be illegal regardless of circumstances.

Second, different sizes of wheel options on a particular car model usually have different profile tyres, designed to give the same rolling radius so that there is no need for the manufacturer to adjust suspension or speedometer settings. The bigger the wheel, the lower the tyre profile that is normally fitted.

Third, in normal driving there's no way a tyre will be slipping or spinning on the road surface enough to affect the speedo, except during harsh acceleration or braking (when the speedo is inaccurate anyway).

Fourth, a flat tyre does have a smaller rolling radius and the circumference is effectively reduced because the tread is compressed and distorted within the (large) contact patch with the road. It is this continuous compression and release within the tread that generates large amounts of heat, destroying the tyre pretty quickly. The speedo will over-read by a small amount, not that you'd notice.

Fifth, tread depth is around 2% of a tyre's rolling radius and from new to bald can only affect the speedo by this amount.

Sixth, going down a steep (1 in 5, say) hill is likely to make your satnav show a speed that is too low (6% in this case) - possibly enough to get you a speeding fine if you rely on satnav rather than the speedo.

Finally, I've tested my own car's speedometer in several different ways, for example:
Using cruise control on flat straight motorway at a speedometer reading of 76mph:
Stopwatch timing between the motorway distance marker posts showed true actual speed as 70mph.
The TomTom showed the speed as 70mph - so the TomTom is accurate.
The car's computer trip readout stated 70-71 mph average speed - so the car's sensors are also pretty accurate.
The speedometer is obviously set to deliberately over-state the speed and I take that into account when using it. Pity that the "I'm doing the legal limit so I won't move over" outer lane hogs can't understand that their true speed is only 65.

So, that should have bored everyone to death.

[tgold]

[Andy_P]

[jspencer]

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