New golf gti wheel alignment

[juicelow]

Hi folks.....quick question from a stupid newbie

I'm taking delivery soon of a lovely new gti....it's coming from the factory with 17" wheels (I live in Denmark) and the first thing I'm doing when I get home is installing some 18" wheels with pilot sport 4 tires. (Didn't take the factory 18" for cost reasons)
Will I need to get the wheels realigned and will the change effect the speedometer reading?
Boring question I know but your help and wisdom is much appreciated.

[Daz Auto]

No and no.

The standard tyres are 225/45/R17

18inch alloys have 225/40/R18 tyres fitted.

So the diameter of the alloy has increased by 1 inch or 25mm. However, the depth of the tyre side wall will also decreased by 25mm. The 45 and 40 numbers are some sort of ratio and not an actual measurement. Therefore, the tyres should have the same circumference = does not effect the speedo.

Your tyres will need 'fitted and balanced', but wheel alignment will be fine until you hit a few potholes.

[SRGTD]

No and no.

The standard tyres are 225/45/R17

18inch alloys have 225/40/R18 tyres fitted.

The 45 and 40 numbers are some sort of ratio and not an actual measurement.

The 45 and 40 numbers represent the aspect ratio of the tyre; the ratio of height of the tyre compared to the width.

For a 225/45/r17 tyre, the height of the tyre is 225mm x 45% = 101.25 mm and a 225/40/r18 tyre it's 225 x 40% = 90mm.

[Bandit127]

Daz is right, the speedo accuracy is directly related to the circumference of the tire.

This website has a calclator.
https://tiresize.com/speedometer-calibration/

It says you will have a difference of 0.3 mph at 65 mph.

[juicelow]

Cheers guys.....you're awesome.