E46Fanatics - View Single Post - Tires replaced at discount tire, now dsc and yellow brake light are on.
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Old 10-03-2012, 02:24 AM   #23
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Originally Posted by SeanC View Post
Stock tire size for a 325 is 205/55/16, which results in an overall diameter of 24.87'', not too different than that of a 225/45/17, or 205/50/17, or 225/40/18, or 255/35/18 (all stock sizes).

The correct tire size combination for a staggered 18 inch set up is 225/40/18 fronts and 255/35/18 rears (for example, stock ZHP). The overall diameters are 25.08'', and 25.02'' inches respectively, the slight difference is negligible.

OP's problem is having incompatible tire sizes in the front and rear. He could have gone with 245/40/18's all around (even though not a stock size), or 225/40/18 all around (assuming wheel width allows). Putting one size in the front and the other in the rear is a NO GO.

Let me elaborate even further:

If you install 225/40/18 in the front and 245/40/18 in the rear, rear tires will be rotating 809 times per mile, while the fronts will be rotating 829.3 per mile, the difference is quite possibly large enough to be picked up by the DSC sensors. For the correct rear tire size, i.e. 255/35/18, revs per mile is 831.3, which is much closer to that of a 225/40/18 size tire, and the difference is within DSC's operating limits.
Nail on the head. The DSC, TCS and ABS all work by monitoring the wheel speeds relative to each other. When one wheel starts spinning faster than the others, it indicates that wheel is slipping and the DSC/TCS/ABS will intervene to try and stop it from slipping. Here, your tire sizes are such that your front wheels are ALWAYS spinning faster than the rears (since the rears have a larger diameter) and the DSC is basically telling you to go **** yourself since it has no idea what's going on or how to fix it. You can drive fine without it, but I would just keep the car happy and install the correct diameter tires.

Also, your ABS is likely inop since it works off the same principle as the other systems. You may get by just fine without it, but if you are not a skilled threshold braker I wouldn't chance it (especially in rainy climates).

Honestly I would expect a service manager to know all of this since it is very important with modern cars. I think it could be considered unsafe of him and you may want to speak to someone higher up about it and let them know about the whole situation.

Last edited by Wh33lhop; 10-03-2012 at 02:33 AM.
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