Speedo reading accuracy

[readerr0r]

What is this Canbus secret reading in the menu? I'm very curious.

Yes the RNS510 can tell you both the CANBus speed and the GPS speed. The RCD510 can get the CANBus speed too, but you need VCDS to enable the dev menu.

To find your GPS Speed, go to the Secret Menu (Hold down setup for 20 seconds), then Navigation -> Matched Position, scroll down and you'll see the speed the GPS thinks you're doing. On my Polo this shows a 110 KM/hr when my MFD shows 117 (I had 10 Sats). I'd recommend having at least 4 sats/3D fix if you're relying on this to not get a ticket.

To find your true CANBus speed, go to the Secret Menu, then Navigation -> Dead reckoning, scroll down to CAN Speed.
This is what your car thinks it's going at. On my Polo this showed as 111KM/hr when my MFD shows 117. Non-standard tyres or wheels could mess this up though.

Or fit a PolarFIS if compatible with your car. Auto-Polar FIS+ Advanced - Complete Guide

PolarFIS cannot be fitted to a Polo atm. We don't have a separate CAN Gateway.

[brad]

Maybe so but I used a tyre comparator and it gave me 40mm larger from whats on it but I only have a basic Comfortline

This one Wheel-Tire size comparer - RIMSnTIRES.com

Indeed.

The only way i can get 18" rims with 35 series tyres to be anywhere near a corrective height is to go 305 or 315 width. In fact, it appears a 225/45x18 could well be the ideal width/aspect combo on an 18" rim on a Polo.

I had the same issue (speedo reeding 8% wrong) on the Octavia with 205/55x16 as OEM. 225/45x17 (with 17" rims of course) helped a bit but 225/55x16 have got it as accurate as I want (it still reads ~3% under).

None of the other cars I've owned in the last 10 years have been half as innacurate as the Skoda - includes Mk3.5 Cabrio, Avensis, Outback, Outlander, Calais, Falcons, Bluebirds, Mondeo, Liberty and a few others. The VWs are within the ADRs but only just.

The other thing to note with the innacurate speedos is that the odometer is also innacurate - but not as bad as the speedo. On the Octavia, if you checked the odo against a check length on the hwy 5km was actually 5.3km on the odo. This means that when you get the 15,000km service you have only done ~14100km.

[Umai Naa!!]

There was a bulletin floating around the dealer network about it.

Volkswagen claims that the odometer is not affected, but I wasn't about to sit down and do the math to prove them wrong. I wouldn't go as far as suggesting conspiracy theories, either.

[Hillbilly]

Indeed.

The only way i can get 18" rims with 35 series tyres to be anywhere near a corrective height is to go 305 or 315 width. In fact, it appears a 225/45x18 could well be the ideal width/aspect combo on an 18" rim on a Polo.

I had the same issue (speedo reeding 8% wrong) on the Octavia with 205/55x16 as OEM. 225/45x17 (with 17" rims of course) helped a bit but 225/55x16 have got it as accurate as I want (it still reads ~3% under).

None of the other cars I've owned in the last 10 years have been half as innacurate as the Skoda - includes Mk3.5 Cabrio, Avensis, Outback, Outlander, Calais, Falcons, Bluebirds, Mondeo, Liberty and a few others. The VWs are within the ADRs but only just.

The other thing to note with the innacurate speedos is that the odometer is also innacurate - but not as bad as the speedo. On the Octavia, if you checked the odo against a check length on the hwy 5km was actually 5.3km on the odo. This means that when you get the 15,000km service you have only done ~14100km.

The problem one has is not only finding the right size but that it is legal.

In QLD you can only go up in diameter by 15mm over the largest tyre fitted to the model of car or under by 26mm.

You must also put on a tyre with at least equal or better load index as well and some odd size tyres are a bit sus in this.

You must after changing still have your speedo with ADR specifications.

Changing to larger diameter rims does NOT allow you to exceed these measurements

[wai]

Basically the rule is that speedos can be up to 10% fast plus or minus 4kph

Just a slight correction. The formula shown in the ADR is essentially that the speedometer can overread by up to 10% of the true speed PLUS 4 km/h (not +/-)

0 £ (V1 - V2) £ 0.1 V2 + 4 km/h (the £ is actually the > symbol; some weird formatting in the ADR)

Of course, there is this is with the standard tyre, at a certain pressure, a certain weight, and certain temperature.

The best thing to do is to use a GPS unit to check. It is much more accurate (on level ground) than a speedometer. Now work out what the variation is. My Caddy is roughly 5% over the correct speed throughout the range. I have both an ANS610 and an AlerteGPS. Both the GPS units indicate the same speed.

[ATYPIC]

Indeed.

The only way i can get 18" rims with 35 series tyres to be anywhere near a corrective height is to go 305 or 315 width. In fact, it appears a 225/45x18 could well be the ideal width/aspect combo on an 18" rim on a Polo.

Are you insinuating that my own findings and experiences to date with my current wheel/tyre setup are a load of baloney? Did I not just previously note that my speedo is bang on? Is that really so difficult to comprehend?

You just stated in the same sentence... in fact, it appears 225/45r18 could well be the ideal combo for the polo...well that is just bollocks mate and I just think you're throwing it out there in hope that your assumptions are correct. That size tyre is ridiculously large and will not even fit under the polo.

[Hillbilly]

Just a slight correction. The formula shown in the ADR is essentially that the speedometer can overread by up to 10% of the true speed PLUS 4 km/h (not +/-)

0 £ (V1 - V2) £ 0.1 V2 + 4 km/h (the £ is actually the > symbol; some weird formatting in the ADR)

Of course, there is this is with the standard tyre, at a certain pressure, a certain weight, and certain temperature.

The best thing to do is to use a GPS unit to check. It is much more accurate (on level ground) than a speedometer. Now work out what the variation is. My Caddy is roughly 5% over the correct speed throughout the range. I have both an ANS610 and an AlerteGPS. Both the GPS units indicate the same speed.

Yes you are correct which can only make them less accurate It used to be +/- 4% but changed to what you say in 2006 I think.

A GPS reading probably should also be done on a flat straight road as the satellites dont take into account rise and fall and in some cases corners as they work on time between two points ( I think) and are always just a fraction late as well.

[brad]

There was a bulletin floating around the dealer network about it.

Volkswagen claims that the odometer is not affected, but I wasn't about to sit down and do the math to prove them wrong. I wouldn't go as far as suggesting conspiracy theories, either.

I doubt it's a conspiracy. If i had the optional 17" rim package on my car then the odometer discrepancy would be halved. It's just luck of the draw that of all the tyre options the 205/55x16 has the smallest overall diameter.

[brad]

Are you insinuating that my own findings and experiences to date with my current wheel/tyre setup are a load of baloney? Did I not just previously note that my speedo is bang on? Is that really so difficult to comprehend?

You just stated in the same sentence... in fact, it appears 225/45r18 could well be the ideal combo for the polo...well that is just bollocks mate and I just think you're throwing it out there in hope that your assumptions are correct. That size tyre is ridiculously large and will not even fit under the polo.

As you only provided 2/3 of the tyre spec I'd ask what tyre size are you running?

And sorry I was working on a 10% innacuracy whereas re-reading the thread it looks more like 8%?

Here's my calcs (tell me when i stuff up):
77tsi std tyre is 185/60x15 which has a nominal diameter of 603mm
GTI std tyre is 215/40x17 nominal diameter of 604mm
You would agree they are pretty close.

A straight +1 to 18" staying as close to 604mm will be a 215/35x18 which will be 608mm? Maybe 614mm if you have a large case tyre?

608mm will be 0.7% difference so whereas before you had to do 108kph to do an actual 100kph, now it's 107kph.

614mm will be 1.6% difference so an actual 100kph will 106kph on the speedo.

To get to around a 2kph misread (which is about where you want to be due to the thickness of the speedo needle etc) then I'd think you'd need to pull about 5% out of the error which will be ~30mm giving a 634mm OD which will be like a 245 or 255/35x18 or 215 or 225/40x18 depending on if it's a tall case or not.

What did you check your speedo against?

[nomadx38]

The 77TSI has 195/55r15 as standard just to correct you.

With the sports pack they are 215/40r17 same as the GTI.

Not to sure about the bottom of the range Trendline...