My speedometer is +5kms out

[rakehell]

I drove a lot this weekend and noticed that my speedo is consistently reading 5km over my actual speed - according to my Garmin GPS and two "check your speed" gates.

Has anyone else experienced this? Is it something that built in or could there be another cause?

[Dzl_Dubba]

Its the norm with many cars, some are even greater than 5km out. A change in wheel/tyre size may bring it back in line but its not too bothersome for me. At least your speed isn't greater than what is being shown!

[Guest001]

Lucky you one of mine is 6 and the other one is 7.

The law says they can be up to 10k +- 4% fast but not read less than actual speed To bring either down to match speed would require an illegal tyre size.

[rakehell]

Thanks. My last car was a Volvo and it was bang on. Didn't realise this could be a "feature".

[woofy]

Mazdas are pretty much exactly 5 out, our Skoda is more like 7, same with our Hyundai. OZ built cars tend to be bang on.

[The_Hawk]

It could be a Euro thing making sure it's always under.

To be 100% legal you can never read *under* the actual speed, but you can read over by a margin (Hillbilly's numbers sound about right). Now I'm a giant nerd and did the maths on this one and can tell you a few things.

* For a manufacturer to remain legal regardless of which wheel and tyre combo you might ship a car with you would have to ensure that the speedo was calibrated to the *largest* wheel and tyre combo available.
* If you speedo was 100% accurate with brand new tyres they will wear over time and get smaller making your speedo closest to accurate with new rubber (ie it only gets further from reading correctly the further the tyres wear).

Now I did these numbers for the Golf, but the principle holds true across the range.

The variance is only 2.3km/h. So if they set the speedo to be 100% correct the *worst case* for a new tyre would be reading 2.3% under.

Now lets factor in wear. Lets say you get 10mm of wear between a new tyre and a used one. This only gets you an extra 3.1km/h under from new to worn.

So if you had the smallest tyre and it was at it's end of life you could be out by 5.4km/h... yet somehow new tyres seem to be out by 6+ km/h and theoretically that will blow out to more like 9km/h by the time the tyres are dead.

I would be very interested to see someone with old tyres run the GPS check before and after having them replaced to see if that 3.1km/h shows up??

An interesting note. The car knows it's indicating a higher speed incorrectly. The odometer seems to read *slower* than the indicated speed. The average speed also seems to read the correct speed. For those with the appropriate function, try setting the cruise control to 106km/h on a flat straight freeway run.... then reset the average speed counter and let it recalculate. Theoretically it should come up at 106 since while on cruise that should be your average ... right... nope. We know the indicated speed is wrong and your real speed is more like 100km/h... which is exactly what the average will show.

I hope my rambling all makes sense.

[KWICKS]

96 actual through the speed gate when cruise is set at 100. New tyres.

[chriskan]

I had that in several of my cars, Honda, Subaru - used to read about ~10% over. We have a volvo xc90 and the speed on that is pretty spot on. I was out at Geelong last week and did the speed check on the way back. 105 on the digital speedo in the RS (with 19" Xtremes) showed as 101 on the speed check on the Princes Fwy.

[Guest001]

If you go here and put your different tyre sizes in it will tell you exactly the difference

Custom rims, wheel tire packages for your ride - RIMSnTIRES.com

Also if you have Satnav in your VW and Kamolds firmware with the secret menu you can get the actual car speed off the Canbus.

The figures I quoted above are from Australian Design Rules.

I worked out to get the Polo which is 7k over to read actual I would need tyres 40 mm larger, which are illegal

[brad]

I drove a lot this weekend and noticed that my speedo is consistently reading 5km over my actual speed - according to my Garmin GPS and two "check your speed" gates.

Has anyone else experienced this? Is it something that built in or could there be another cause?

Is it reading 5km at both 40kph & 100kph? That's odd because usually it's a percentage out.

ADRs state that the speedo has to read over. It's allowed 10% of the actual speed +4kph - at an indicated 100kph you might only be doing 86kph actual. Most manufacturers are about 4-5kph out. Skoda / VW take it to a whole new level.

Interestingly, Jeep Cherokees & the Audi A5 2.0tsfi read 100=100 which makes them in contravention of ADRs.

If you have an Octavia with 205/55r16 tyres it will be 9-10% over. 110 on the speedo will be 99-101kph. 225/45r17 will be 6-8% over.

You might be able to tweak the speedo a fraction using VCDS.

I fitted 26mm bigger diameter tyres and managed 100=100 but legally (in NSW) you can only go 15mm larger than the biggest diameter tyre on the tyre placard in the fuel lid.

A few people on this forum will tell you that it's a great help in not getting speeding fines but I don't see how a law abiding citizen unknowingly doing 86kph in a 100kph zone with a tailback of 50 cars behind them getting more & more frustrated and dangerously undertaking on the LHS or even overtaking on the verge can be safe for anyone.

I've managed 36 years of driving with only 3 speeding fines. None of those fines would have been avoided no matter how much "gimme" the speedo had in them.

edit:
Synopsis of the ADRs (from RACQ website)

Before 1 July 2006
For vehicles manufactured before July 2006:
An accuracy of +/- 10 percent of the vehicle’s true speed is needed when a vehicle is travelling above 40km/h. This means if a vehicle is travelling at a true speed of 100km/h, the speedo is allowed to indicate a reading between 90km/h and 110km/h.
An odometer accuracy of +/- 4 percent is also a requirement.

After 1 July 2006
From 1 July 2006 all newly introduced models of a vehicle available on the market must comply with the following requirements:
The speedo must not indicate a speed less than the vehicle’s true speed or a speed greater than the vehicle’s true speed by an amount more than 10 percent plus 4 km/h.
The speedo must always read 'safe', meaning the vehicle's true speed must not be higher than the speed indicated by the speedo. So if a vehicle travelling at a true speed of 100km/h, the speedo must read between 100km/h and 114km/h. Another way of looking at this is if the speedo indicates a speed of 100km/h, the vehicle's true speed must be between 87.3 km/h and 100km/h.
There is now no requirement to have an odometer.

From 1 July 2007
All newly manufactured vehicles (excluding mopeds) must comply with these rules.