Incorrect Speedometer - Golf mkVII?

[par2]

OK, checked this out over a range of speeds this morning. The base data is 2015 model Mk7 GTI DSG, 4,500 km (tyres are new), correctly inflated to manufacturer recommended pressures. Wheel 18 x 7.5, tyres 225/40 R 18. GPS is a Tom Tom Xl. At a steady 80 km/h on car, GPS showed 77 km/h. 90 = 87, 100 = 97, 110 = 106. This indicates an under read by the car speedometer of about 3.5% to nearly 4%. Trying the reverse, Car cruise set to 83, GPS showed 80/81. Car set at 93, GPS 90/91. Car set to 103, GPS 100 exactly. Car set to 114, GPS 110 exactly. So, a general "plus 4km/h" rule is what I will use from now onwards.
For interest, I did the same with my 2010 Mk6 Golf, with 17 x 7 wheels and 225/45 R 17 tyres, manual 118 TSI. It has a cosistent under read of about 5%. That is, car at 80, GPS 76, 90 = 86, 100 = 96, 110 = 105. This was using the same Tom Tom GPS. I ended up with a general "plus 5" rule for it. This has kept me "off the Radar", literally, for four years now.....

[knightsy]

Just got my new Mk7 110 Tdi Highline on Friday. Decided to check the speedo. I used the following advices from this thread:

"As for checking the speedo itself - if you don't have a GPS handy this works well at getting the uncorrected speed out of the car..

- Set the cruise control to the speed you want to check, say 100km/h
- With the vehicle at speed reset the trip computer
- Select the average speed readout
- Initially it will show dashes but after a few seconds the average speed is shown which is the uncorrected speed and actually quite close to GPS speed..

For cruise control at 100km/h I get 94km/h"

and

"On the touchscreen, press car, go to settings (the cog wheels in the bottom right corner) scroll to multifunction display, scroll to second bottom option to reset "since start". I have never tried it while the car is in motion though."

I did a check on the weekend and with cruise at 105kph I pressed OK in "since start" and got 105kph avg. Thought I must have done something wrong. I did it again today, twice. The first time I got 105 and so pulled over into a lay-by. Naturally the avg slowly dropped and when the road was clear I set off again with the avg at about 49kph. Up to 105kph cruise and when I pressed OK on "since start" it immediately changed to 105kph avg.

Is it possible that my speedo is accurate? Has anyone else got this result? Or did I do something wrong?

[lambertia]

Have you got a smart phone? Download a speed app. My car indicates 104 at a gps indicated 100.

[knightsy]

So I downloaded a speedo app (Speedview) and went out with a mate today. The results are: speedo 115, gps 110; speedo 105, gps 100; speedo 85, gps 80. All while using cruise. This last was done without cruise in traffic and we got speedo 63-64, gps 60.

All of which leaves me thinking that the average speed function is sampling the speedo, not the odometer. Unless of course I was doing the original checks incorrectly.

At least I know now how to proceed on my trip to Melbourne in a couple of weeks.

[popcorn]

I know the mk5 climatronic had a diagnostic function built in and one setting would show uncorrected speed. Unsure if the mk7 air conditioning system has a diagnostic function in it though

[knightsy]

Returned from my Melbourne trip yesterday. Seriously impressed with the car! I did two 5km timings in northern Vic and they exactly confirmed the satnav readings - 115 on the speedo is 110 actually.

[wai]

Returned from my Melbourne trip yesterday. Seriously impressed with the car! I did two 5km timings in northern Vic and they exactly confirmed the satnav readings - 115 on the speedo is 110 actually.

If you went down the Hume, how did the speeds compare at the speed check gantry near Albury?

[knightsy]

If you went down the Hume, how did the speeds compare at the speed check gantry near Albury?

If I remember correctly the car in the left lane doing the same speed as me was 109, I was 111, and the car behind me, traveling at the same speed was 110 - just caught the speed as I went under the gantry. I have done that trip a few times and I think the gantry can say + or - 1 or 2 kph of the actual speed depending on how it feels at the time. But still a reasonable indicator - especially for those that think their speedos are accurate!

The car is new. I got 5.6 going down and 5.7 (rain in Vic and uphill ha ha) coming home. Pretty pleased with that. I am using BP Ultimate diesel.

[Mountainman]

The car is new. I got 5.6 going down and 5.7 (rain in Vic and uphill ha ha) coming home. Pretty pleased with that. I am using BP Ultimate diesel.

Is that MFD indicated or real calculated? I find my Golf indicates around 0.2L/100km less than the real amount. Also an interesting result for a diesel. My 90 TSI manual returned an MFD indicated 4.5 Toowoomba to Sydney and a 4.6 return travelling at 5km/h over the indicated limit - usually 100 but 110 on the M1. I can never get it as high as 5.6 or 5.7 even when travelling at indicated 115km/h on western QLD roads where it usually returns low 5's. Slight overall uphill gradients, rain and head winds all have a negative effect of up to 0.5L/100km.

[Baz 55]

I do a lot of highway driving and use a gps speed app for an accurate reading.
My MK 7 I'd doing 100 when speedo reads 106.
I borrowed a new Jetta yesterday and it was doing 100 when speedo reads 107.
These speeds have been cross checked with the speed checker on the Geelong Highway.