Recently bought a new (ish- 2 months old ) F30 320i xDrive Sport Saloon from BMW dealer. It came with 17" Alloys - tyres 225/50/R17's. I have a copy of the factory delivery print out sheet with the specifications on the car. It states that the car was supplied with 18" alloy wheels as "additional equipment".  My question is this. If the car was direct from the factory with 18" alloys would it have been specially set up for 18" alloys and would the change to 17" alloys affect the calibration of things like the speedo, other electronic settings etc.?  I presume the 18" wheels would have been fitted with 45 profile tyres anyway - so there might not be that much difference in performance?        Thanks for your thoughts.

Hi Rob

It shouldn't make too much difference to the speedo reading as there is always a % fudge factor either way (you can always check it on an external sat nav) also if you are altering the profile of the tyres, as you say, there wouldn't be much if any of a difference in readings.

Cheers,   Trevor

Hi Trevor,

Thanks for the comments. Just installed my Speed trap detector which will also give me a speed reading on my journeys over the next week as a check.

Rob

Would be interesting to see how readings differ between the speedo and actual GPS readings with the current wheels/tyres combination

Hi Trevor. I've been away this week (but not in the BMW)  Just got back so I'll give you some figures in the next week or so.  Rob.

Ckecked speedo readings against GPS readings on a journey down the M6.  Surprisingly the speedo reads a consistent 2 mph over the GPS readins at 50 mph, 60mph and 70mph!  I thought it would be a bigger gap as the speed increased?  Perhaps I didn't get it quite accurate unless some electronic wizardry is at work? Will do more readings in the future for interest sake.    

Its nice to know the speedo is reading fast so that when you go through a camera at least it will be the right side of the limit :-)