Speedometer Accuracy compared against GPS

[Flugel Hound]

I was reading through the MPG thread, it struck me that speedometer inaccuracies can account for mpg differences drivers are experiencing amongst all the other myriad of reasons.

http://www.audi-sport.net/vb/new-a3-8v-chassis/177067-a3-real-fuel-consumption-2.html

Im currently driving Passat Estate Blue Motion 1.6 TDI
Long term average is 49mpg
Short term to work and back is 55mpg
Long slow drive being careful 67mpg over 100mile

However the speedometer reads 76mph when in fact i'm travelling at 70mph displayed on a GPS.
55 = 50
44 = 40
33 = 30
22 = 20
The Speedometer reads increasingly high through the speed range.

So, how accurate are the A3 Speedometers?
Andoid phones can be used to display speed over the ground.
Nice free app here
https://play.google.com/store/apps/details?id=com.chartcross.gpstest&hl=en


Fluges

 

[Spacemarine]

speedometer inaccuracies can account for mpg differences drivers

I also thought so at first, but it seems that only the display of the speed is overstated, the speed that is used for internal calculations is the correct one. You can try that out very easily: Drive at constant 55 mph, then reset your average speed reading. After a few seconds it will show your "real" speed which is around 50 mph.

I tried this in my Seat Leon and there I found the speedometer to always show 5 % more than the GPS Speed. However the average speed was always exactly the same as the GPS speed.

 

[Flugel Hound]

Nice idea
In the past it did cross my mind how average speed interacted but didn't get round to proving either way.
Will try that out this afternoon.

Fluges

 

[NHN]

It's common for speedo's to be 5-10% out.

 

[jet_andy]

It's common for speedo's to be 5-10% out.

Partly true, but legally speedometers are not allowed to read low - you should not be able to be doing a speed higher than indicated. Commonly, and with my last three cars or so, speedos read about 5% high - so if you have an indicated 70mph you're probably doing about 67mph.

I would doubt the OP's figures somewhere - either the speedo is faulty or his GPS speed is inaccurate. Usually GPS will be very accurate, but you have to remember the quality of the fix is dependant on the number of satellites that are in clear view. And I wouldn't treat any App data as gospel.

 

[Flugel Hound]

jet_andy
What's to doubt about my figures?
The Android App always indicates the same as my dedicated GPS (Tom Tom)
My Dashboard speedo indicates higher than actual speed, in keeping with the law and your statement.

Fluges

 

[NHN]

I never mentioned high or low fella, I merely stated 5-10% out

 

[jet_andy]

That's why I said partly!

 

[NHN]

Sorry, still not getting how its partly true of my statement, but alas, maybe the heat is thickening my blood & making me slower than normal

 

[s33nyboy12]

Sorry, still not getting how its partly true of my statement, but alas, maybe the heat is thickening my blood & making me slower than normal

i think your right mate! Firstly in 5-10% part and this post

Sean

 

[h5djr]

The amended Road Vehicles (Construction and Use) Regulations 1986 permits the use of speedometers that meet either the requirements of EC Council Directive 75/443 (as amended by Directive 97/39) or UNECE Regulation 39.[SUP][/SUP]

This means, the speedometer must never show an indicated speed less than the actual speed. However it differs slightly from them in specifying that for all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower than this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph. For example, if the vehicle is actually travelling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph.

 

[jet_andy]

Accuracy is normally quoted as +/- ....that's all...not criticising....

 

[NHN]

Accuracy is normally quoted as +/- ....that's all...not criticising....

I never mentioned you were, just trying to see how partly came into it lol

 

[h5djr]

Accuracy is normally quoted as +/- ....that's all...not criticising....

As you will see from my post in the UK ( and the rest of Europe) a speedo can be minus nothing and plus around 10%. GPS is always the more accurate of the two. The digital speedo readout on an A3 is the same as the normal speedo, just translated into numbers and no more accurate.

 

[jet_andy]

I never mentioned you were, just trying to see how partly came into it lol

The part missing that says a speedo can read over but not under actual speed....."5-10% out" suggests either way...

 

[jet_andy]

As you will see from my post in the UK ( and the rest of Europe) a speedo can be minus nothing and plus around 10%. GPS is always the more accurate of the two. The digital speedo readout on an A3 is the same as the normal speedo, just translated into numbers and no more accurate.

Agree completely but consideration should always be give to quality of GPS fix, which can be compromised under heavy tree coverage or surrounded by high buildings.
Our company vans are GPS tracked and you do occasionally see speed discrepancies outside the normal limits, although for the vast majority of the time they're bang on.
One guy in particular has his speed limiter set at 70, and his GPS track on motorways always shows 67mph.

 

[Vertigo1]

It varies a lot and even the nature of the variation differs. One of my A3s had a speed which over-read by 10% at all speeds whereas another one was 2mph fast at all speeds. It's just manufacturing tolerances.

With the RNS-E in the 8P, you could use the engineering menus to display the GPS speed for comparison but I've yet to find out if there's such a screen on the MMI in the 8V.