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      10-12-2019, 02:12 PM   #63
Artemis
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Drives: BMW M2C (BSM)
Join Date: Jun 2011
Location: Belgium

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Quote:
Originally Posted by hlm2c View Post
Normally the speedometer reports us as +3 kph over what we're actually travelling on 245/265 MPSS. When you swapped to 255/275 MPS4S, what change did that make in regards to the speedometer?
I can confirm what alscks0414 commented in the past in the dedicated thread here ("Increasing size to 255 & 275 any cons?"): minor deviation, speed needle is closer to real speed.

130 km/u indicated on my dashboard:
  • 127 km/h real speed with the 245/265 set-up (stock size);
  • 128 km/h real speed with the 255/275 set-up (adjusted size).
I've noticed that the ODB/ECU logged a P1766 code ('Engine Speed Plausibility'), but that code is only displayed with an ODB/ECU reader (no sign of the code on the onboard computer or iDrive).

Furthermore, no rubbing and a bit meatier filling up the wheel wells.
Quote:
Originally Posted by alscks0414 View Post
Hey guys,
All vehicles regardless of makes, do develop their electronic measures of cars based on their manufactured spec.
LSD, suspension, engine throttle, and other ECU controlled features will be basically working as "what manufacturer intended" when you are keeping all things as state as it came.
Tire sizes will be one of the component as well.
However, remember that a lot of different model vehicles do offer different wheel sizes but they will relatively change their tire sidewall profile so that rolling diameter of vehicle will not differ much. They set about 1% (or less) of difference of rolling diameter will not alter the driving characteristic of one vehicle and will not give much stress on ECU controls and other physical components. (Same thing goes for AWD, xDrive. If you wish to go with staggered wheels + tires setup front and rear, engineers recommend you to stay within 1% difference of rolling diameter front and back).
If you do go over this 1% boundary, you could potentially run into "different driving characteristic from what manufacturer initially intended".
All makes do spend tons of hours to develop their car and when it comes to sports cars that requires precisely measured values and input, they will spend more time for research to decide what they will do before any vehicle released for sale. But, of course, they are not perfect... resulting into recalls and extensive repairs, etc.
Going back to the topic, 245/35/19F + 265/35/19R and 255/35/19F + 275/35/19R does make their difference in rolling diameter about 1.07% front and 1.05% at the rear. And I would not consider this as "major difference" for ECU control measures and driving dynamics.

Cheers
M2 tire size changes impact (table by mToronto):
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