Incorrect Speedometer - Golf mkVII?

[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)

[team_v]

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 I accuracy gets bigger (but only marginally, I hope)

Brad is one of the resident tyre experts on our forum.


As the tyres wear down, your rolling diameter decreases so the speedometer will read lower still.
So if you start off with a 6% error (speedo says 100 at 106km/h) then you might see it push out to 7% with tyre wear (speedo says 100 at 107km/h)

[tigger73]

One thing that you have to keep in mind is that car makers have factor in people going either 1 size up or 1 down on tyre size. So they'll generally set the speedo to be around mid-range of what is acceptable by ADRs so that if the owner does change their tyres they are still within the allowable limits.

Which puts you 6km/hr out at 100km/hr.

Probably the best thing you can do is "calibrate" your speedo with a GPS based device. So you know what your actual speed is relative to the indicated speed.

...and then hope that the speed cameras are all calibrated accurately and installed correctly also


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[brad]

...and then hope that the speed cameras are all calibrated accurately and installed correctly also

I look at the difference between actual & gauge reading as being similar to those folks that try & fool themselves to getting to meetings on time by setting their watch ahead of the real time (I have a friend who is smart enough to have a Doctorate but sets her watch 30 minutes forward to trick herself.)

Since I got my speedo to a 1=1 status I rarely exceed the speed limit. I actually treat the speed limit as a limit (maximum) rather than "this is the speed you should have the speedo on". It still means I'm driving faster than many other road users.

[team_v]

Since I got my speedo to a 1=1 status I rarely exceed the speed limit. I actually treat the speed limit as a limit (maximum) rather than "this is the speed you should have the speedo on". It still means I'm driving faster than many other road users.

I find the same thing.
Makes you realise all those people who think they are being safe by doing 90 in a 100 zone are just dawdling becasue they are really doing 85.

[Umai Naa!!]

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.

I recall a service bulletin from Volkswagen Group Australia that refutes this, as it is measured based on the car's "true" speed, not the "corrected" speed displayed on the speedo.

[hoi polloi]

I recall a service bulletin from Volkswagen Group Australia that refutes this, as it is measured based on the car's "true" speed, not the "corrected" speed displayed on the speedo.

Indeed... And it's quite easy to measure odometer accuracy using the odometer check signage on some motorways - they only go for 5km each but 10% of 5km is still 500m and the odometer is a lot more accurate than that..

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


- Anthony.

[DV52]

Indeed... And it's quite easy to measure odometer accuracy using the odometer check signage on some motorways - they only go for 5km each but 10% of 5km is still 500m and the odometer is a lot more accurate than that..

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


- Anthony.

Anthony et al: is it just me, or does anyone else find all this strange. What's being said is that the car actually knows the correct distance that it travels and the car also knows correct time. But for some odd reason, the car can't/won't tell the driver the correct speed. Doesn't this seem just a little bit absurd? Why would the good burghers in Germany take these correct metrics and manipulate the speedo reading to give the wrong number?

[Umai Naa!!]

Take a moment to think about how speed limits are structured in the car's country of origin.

If they didn't have to take over countries and their laws into consideration, they wouldn't print anything above 80 on the speedo.

[MGV]

...The fact that all those other cars kept passing me suggests that their speedos were more accurate...

Maybe they were all just aware that their speedos were out by a known amount. I set my cruise to 118 when I'm on the freeway, 107 when I'm on the highway.

[Guest001]

The ADR rules in Australia state that all car speedos may read faster than actual by up to 10% +/- 4.

It may NOT read less than actual speed

The odometer is not affected and should be accurate as to distance travelled

My Polo is 7 over the Passat is 6 and my Landcruiser was bang on at 100kph

Also I worked out to bring the Polo correct would take a size of about 40mm diameter larger.

This would be illegal as most states only allow up too 26mm larger.

It would also seriously overgear the car and make it a lot slower off the mark and put additional strain on the DSG