Incorrect Speedometer - Golf mkVII?

[CT69]

From my very simplistic point if view, I would just like to know exactly how fast I am driving; I had hoped that in this great age of technology, that might be possible!
I would accept that there would be an error of 1-2%.
Next question: if my speedometer reading is out by more than 5%, can I trust that my ODOMETER is not similarly inaccurate?
If VW or any vwwc subscribers know the answer to that I would be most interested (obvious ramifications for wear and tear, resale etc).

If the ODO is in fact accurate, then I could at least work out the amount of over-read on the speed (triggering lap timer and ODO-trip simultaneously, driving at a constant speed on the highway under cruise control for a while, then doing some simple maths). Not much point to this experiment if the ODO is buggered too.

Maybe I should check to see if the lap timer counts in actual seconds......

[b c]

I don't want to speed, but I also don't want to travel slower that I otherwise need to. I took a car GPS with me on a couple of trips and checked the MFD digital speedo against the independent GPS speedo (previously compared to roadside displayed speeds). Showed the digital over-reading by about 4kmh. Probably time I did it again.

[team_v]

From my very simplistic point if view, I would just like to know exactly how fast I am driving; I had hoped that in this great age of technology, that might be possible!
I would accept that there would be an error of 1-2%.
Next question: if my speedometer reading is out by more than 5%, can I trust that my ODOMETER is not similarly inaccurate?
If VW or any vwwc subscribers know the answer to that I would be most interested (obvious ramifications for wear and tear, resale etc).

If the ODO is in fact accurate, then I could at least work out the amount of over-read on the speed (triggering lap timer and ODO-trip simultaneously, driving at a constant speed on the highway under cruise control for a while, then doing some simple maths). Not much point to this experiment if the ODO is buggered too.

Maybe I should check to see if the lap timer counts in actual seconds......

The odometer uses the "uncorrected" speed from the engine/ecu so it will be correct, this was covered in detail in the mk6 section.

Again, a simple tyre size change can remedy this issue.

[brad]

My Skoda Octavia MK2 (like a MKV Golf) was 10% out ex-factory with 205/55r16 tyres.
225/45r17 brought it a bit closer (7%)
225/55r16 closer still (4%)
225/50r17 is about 2%- unfortunately it also exceeds the maximum increase in diameter, so is illegal. It's probably a bit too accurate for most people as teh width of the speedo needle is more than 2km

You haven't actually said what model MK7 you have or what size tyres you have but it is possible to go up a size & maintain the same profile & you'll make up a lot of the discrepancy.

The alternative is to buy a decent GPS & use the speedo function in that.

On some of the VWs/Skodas it is possible to tweak the speedo reading a bit using VCDS adjusting the distance impulse number.
Speedometer fast error adjustment and correction - mk5, mk6 | VW TDI forum, Audi, Porsche, and Chevy Cruze diesel forum

Check your odometer as well because that is also dramatically wrong - but not the full 10%. I think I worked out that 15,000km on the odo was only 14,300km actual, so you get services done earlier than needed & when you go to sell your car the recorded mileage is much greater than the actual milage. I guess the ideal situation would be to make the odo close to 1=1 & that will sort out the speedo with a bit of safety margin still available.

[DV52]

Again, a simple tyre size change can remedy this issue.

team_v: help me out here! Which way does it work? A larger tire results in a larger speedo reading - or the other way around?

[Dutch77]

Dutch77: yes, that's true. But in the 21st century, I would have thought that we could rely on better metrics in our cars.

if I could press you for a further response (please), you hinted in a previous response that the FIS might be able to display true speed. How does it do this?

As team_v suggested the odometer, and actual speed, can be read from the ECU.

People on the Mk6 confirmed this to be accurate - 116/117km/h speedo -> 110km/h Polar FIS -> overhead speed reading gantry 110km/h.

Polar FIS for Mk7 is still under development, I would guess based on previous estimates that it should be ready in the second half of this year.

[brad]

team_v: help me out here! Which way does it work? A larger tire results in a larger speedo reading - or the other way around?

bigger diameter tyre will make the speedo read closer to actual...

have a fiddle with this & you'll eventually work it out

Tire Size Calculator - Tire & Wheel Plus Sizing

[team_v]

team_v: help me out here! Which way does it work? A larger tire results in a larger speedo reading - or the other way around?

Increasing tyre sidewall size while maintaining the same width or increasing the width while maintaining the same sidewall value will increase rolling diameter.
This has the effect of increasing the actual speed of the vehicle compared to the speedometer reading.

Given the speedometer under reads by law, if for a rough example you upsize from say a 225/35R18 to a 225/40R18 (3.2ish%) or a 235/35R18 (1.6ish%) it will bump the speedometer reading closer to accurate while still maintaining legal sizing for tyre fitment.
Please note i just used random figures for tyre sizing so make sure your upsizing is done in accordance with what is legally allowable to increase rolling diameter by.



Now admitedly this is a costly route for a new car but when the tyres are getting on and need to be replaced this should be a consideration.
It also gives you more sidewall which is good for driving comfort.


This is a good website for comparison.
Tire Size Calculator - Tire & Wheel Plus Sizing

[Dutch77]

Increasing tyre sidewall size while maintaining the same width or increasing the width while maintaining the same sidewall value will increase rolling diameter.
This has the effect of increasing the actual speed of the vehicle compared to the speedometer reading.

..which is why some 18 inch Golfs run 235/40s after buying new tyres v OEM 225/40 I am guessing - but also because the 235/40s were traditionally a lot cheaper locally as they were a common Falcodore fitment.

[DV52]

Now admitedly this is a costly route for a new car but when the tyres are getting on and need to be replaced this should be a consideration.
It also gives you more sidewall which is good for driving comfort.

This is a good website for comparison.
Tire Size Calculator - Tire & Wheel Plus Sizing

bigger diameter tyre will make the speedo read closer to actual...


have a fiddle with this & you'll eventually work it out

team_v/brad: great minds think alike! Thanks both for the info. If I can extrapolate the content of your respective responses, this means, I think that as the tires on my car wear, the inaccuracy gets bigger (but only marginally, I hope)