225 or 215 ?

[Rome_BRZ]

Currently have 215/45 17" and speedo is about 9kph out. (if doing 100, real speed is 109kph)
Cheers
AP

Are you sure you don't mean the other way around? i.e. if you're doing 100 on the speedo you're actually doing 91? Cos if not you have some big issues there with that speedo difference, it's illegal

[AlpineRaven]

Currently have 215/45 17" and speedo is about 9kph out. (if doing 100, real speed is 109kph)
Cheers
AP

Are you sure you don't mean the other way around? i.e. if you're doing 100 on the speedo you're actually doing 91? Cos if not you have some big issues there with that speedo difference, it's illegal

Nah I'm correct it's been like that since we bought it... Only way to resolve it is to install corrector in.
Cheers
AP

Sorry this made me thought twice... If I am doing 109 real speed is 100.
Cheers
AP

[HyRax]

How can you be sure that your trip computer is out? If the speedo was closer to accurate, then wouldn't the trip computer be more accurate?

ADR's dictate that a car cannot read slower than your actual speed. Accounting for general inaccuracies of measurement due to tyre pressures and the penchant for people to upgrade to larger wheels means most manufacturers deliberately configure the speedo to read higher than what you're actually doing, ergo when the speedo reads 100kph, in reality you're doing around about 96kph.

A larger tyre means that I will travel further distance on one rotation. My car is a 2.5i - it's factory setup has 215/45/17 tyres and the number-of-ticks-per-20-metres expected from the car's axle is calibrated as such for that wheel diameter. By going a larger overall diameter than factory, the number of ticks generated over 20 metres will be less. If I ripped out a GT speedo and put that in my car, it would read more accurately for these wheels as the factory wheels and tyres are larger than the 2.5i overall and therefore the speedo is calibrated differently. I could get it fixed of course by going to a meter calibration house and have them recalibrate the speedo rather than go the larger tyre route.

Ive thought about resetting ODO and rely on my Speed GPS application in my iPhone - it should be able to tell me if its accurate etc... Currently have 215/45 17" and speedo is about 9kph out. (if doing 100, real speed is 109kph)

Don't you mean the other way around? It's not legal to have a speedo read 100kph and actually be going faster than that. As your tyres wear, your actual speed compared to speedo speed should be even slower again, eg: 100kph on speedo equals, say 94kph actual on a seriously worn tyre, but on a new tyre with full tread, goes back to, say 96kph.

EDIT: You just corrected yourself with your last post.

[PinkK]

How can you be sure that your trip computer is out? If the speedo was closer to accurate, then wouldn't the trip computer be more accurate?

ADR's dictate that a car cannot read slower than your actual speed. Accounting for general inaccuracies of measurement due to tyre pressures and the penchant for people to upgrade to larger wheels means most manufacturers deliberately configure the speedo to read higher than what you're actually doing, ergo when the speedo reads 100kph, in reality you're doing around about 96kph.

That explains a lot, I am glad they fixed that as I thought it a little silly that ADR's could be +/- 10% yet some states didn't allow that leniency in their speed cameras.