[kchan9]

Hey guys, hoping someone can help me out here.

My buddy has a set of Rohana RC10 running 245/40/20 in the front and 275/35/20 in the rear. I brought them into a local tire store to install but the mechanic suggests I need different tire sizes (245/35/20 for front and 275/30/20 for rear). The overall diameter is bigger on my friends set of tires compared to oem. He says because they're bigger, the speedometer wouldn't be accurate and that it will ultimately affect my ABS, traction control, and all wheel drive system.

I know running a bigger or different size will throw the speedometer off but will it actually harm the x drive system? Should I ignore the mechanic and throw them on?

Any input is appreciated. Thanks guys!

[Infamouscrew02]

I'm not sure about the BMW X drive system but on my Audi prior, with the Quattro system, there was a small percentage that the tires could be off between the front and rear in terms of diameter. What will happen is if the rear wheels are turning faster than the front or vise versa, it will throw off the center differential. That will result in traction control and abs issues.

IMO best practices for a AWD system is to go ahead and run a square setup.

[.BMW.]

There is actually less difference in diameter between 245/40 275/35 than between 245/35 275/30, both in millimeters and therefore also in percent.

F: 245/40-20, 704 mm
R: 275/35-20, 700 mm
front tire 0.6% taller

F: 245/35-20, 680 mm
R: 275/30-20, 674 mm
front tire 0.9% taller

Regarding the speedometer error, I would say that it will be very accurate with the larger tire sizes. The speedometers in BMW's tend to show a few percent higher than the true speed.

By just looking at the numbers I don't see any issues with the tire sizes you mention with xDrive!

You will be closer to rubbing with taller tires, but that's another problem. And since you already have the tires I guess you can just test fit and see if it works.