530e60

Thanks all for your kind and very helpful replies.

With regards to the wheels supplier, I am pretty much on my own now. They have a variety of after market wheels (mostly Korea/Taiwan/China and some Italian) and don't seem to know exactly what offset is (perhaps like me). I could not work out an exchange or refund; so I am left to sort things out on my own.

To clarify, the rubbing noise comes more from the right wheel while turning left (especially sharp angle turn). What is strange is that after the left turning, once the wheels are straight again, there is still some noise. Compare this to abslutely no noise while driving straight prior to the left turn.

There is little or no noise, from either wheel, when turning right. Perhaps the plastic arch moulding's shape is not exactly a mirror image on both sides and hence the noise only on the right wheel during left turning?

In any case, it appears from everyone'e feedback that I need to go for 255/30 or 245/35. Given that I have just installed a set of 4 brand new Yokohamas 2 days ago and not very keen to spend much more, can I get away with replacing only the front tyres using the smaller size? No noise has been heard from the rear tyres so far.

Also, can someone post any details on understanding offset and the formula for wheel diameter?

Thanks

herb

530e60 on most of the wheels you can check the offset on back of it . you'll see some numbers and letters i.e et/20 ... as for the front tires you can just replace the front one to 245-35-20 and leave the back at 255-35-20's but then again you will have to check the tires height and make sure both of the front and rears have the same height check this out

rodybmw

Before I begin let me appologize for the long post. Hopefully you will find it useful.



The link that Herb posted explains what the offset is. Also, you mentioned a few posts back that you did not think the offset was changed because there were no spacers installed. Having a spacer or not does not necessarily mean that the offset is changed, since the wheel itself may have a different offset. Also, just because the offset of a wheel is not the same as original offset it does not necessarily mean that it's wrong. Sometimes when wheels are a different width from the original you may need a different offset to make it fit correctly. As mentioned above some wheels have the offset written on the back. For example, a +20mm offset would be written as "ET20" or "E20". If it is not written then you can get the model number (which should also be written on the back of the wheel) and call the manufacturer or importer of that brand of wheel and ask them what the offset is. By the way, if a wheel shop does not know what the offset is I would not trust them enough to do business with them anymore.

The formula for calculating the tire diameter is actually pretty simple. Lets take the original tire size of 245/40-18 as an example. The first number is the width of the tire in mm (245mm). The second number is the height of the sidewall on each side of the wheel in percent of the width. This is better known as the "profile". This means in this example each sidewall is 40% of 245mm, which is 98mm (there are 2 sidewalls in the overall diameter of the tire). The third number is the diameter of the wheel in inches (18").


In order to calculate the overall diameter of the tire we can use this formula:

Tire diameter = (sidewall height x 2) + wheel diameter

where sidewall height = Tire width x profile (in percent)

But remember that all the numbers are in mm except for the wheel diameter which is in inches. So to convert the wheel diameter to mm we should multiply it by 25.4. Or for those who want the overall tire diamter in inches they can convert the sidewall height to inches by dividing it by 25.4. Then they can double that number and add it to the wheel diameter in inches.

So here's the calculation for 245/40-18:

sidewall height = 245mm x 40% = 98mm
Wheel diameter in mm = 18" x 25.4 = 457.2mm
Tire Diameter= (98 x 2) + 457.2 = 653.2mm


The tire diameter for 255/35-20 is:

sidewall height = 255mm x 35% = 89.25mm
Wheel diameter in mm= 20" x 25.4 = 508mm
Tire Diameter = (89.25 x 2) + 508 = 686.5mm


The tire diameter for 255/30-20 is:

sidewall height = 255mm x 30% = 76.5mm
Wheel diameter in mm= 20" x 25.4 = 508mm
Tire Diameter = (76.5 x 2) + 508 = 661mm



Although 661mm of the 30 profile tire is still higher than the original 653.2mm, it is MUCH closer to it than the 686.5mm of the 35 profile tire.


I went through the above example to illustrate what all the numbers mean and how to calculate the diameter on your own, but in reality there is a much easier and better way to find out this information. If you click on the link below it will take you to a tire size calculator where you can enter the size of your original tires and then the ones you wish to upgrade to and it will tell you not only the diameter but a lot of other information in both mm and inches. It also tells you the speedometer error and percent change in tire diameter. As a matter of fact when I entered your 255/35-20 it gave a warning that they don't recomend changing the diameter by more than 3%, and this size changes it by 4.85%, which according to them may cause brake failure. Click on the link and try it out, it's pretty cool.

>>>>Click here for Tire Size Calculator<<<<

rodybmw

JDN

Thanks, rodybmw. That was informative and helpful.

530e60

Thanks all for the kind insight.

Unable to find a smaller tire size, I have come across the following offer which I want to check with guys before proceeding:

Option 1:

19*9 wheels with offset ET40 with 275/30 R19 tires
While these appear ok for the rear, arent they too big for the front?

Option 2:

19*8.5 wheels with offset ET13. This design is my second choice but now I am more concerned with it being a good fit rather than the looks. What would be good tire size for these?

Which option sounds better to herb, cidair, rodybmw and other friends?
Thanks again all for the info on the offset.

cidair

I have 8.5x19 ET10 with 245/35 in front and 9.5x19 ET22 with 275/30 at rear.

Go for 9x19 rear and 8.5x19 front if possible.