QuestionI just picked up a 2000 bmw 323ci. the previous owner gave me an extra set of wheels/tires for summer. I noticed the summer tires sizes are 225/40-18 front and 245/35-18 rear. can you explain why the rear are only 35, and what would happen if i went to a 40 on the rear? I notice many sites have a greater offering of 245/40-18. thanks!
AnswerJohn,
First, we need to start with the basics.
On every vehicle sold in the US, there is a sticker – commonly called the tire placard - that lists the original tire size and the proper pressure for that size. The placard is usually located on a doorpost or in the glove box – but sometimes it is located in the trunk or on the fuel filler door.
Notice that the original tire size is not 18". It is quite likely that these 18" are from a later model vehicle.
Second, here's a Tire Calculator:
http://www.1010tires.com/TireSizeCalculator.asp
Now put the tire size listed on your vehicle tire placard as the stock tire size and the others for comparison - and do me a favor and add 255/35R18 to the mix. So that's a total of 4 comparison tire sizes.
What should come out is that 225/40R18 and the 255/35R18 (the one I suggested you add) come the closest to the original tire's diameter.
I have a book that summarizes what is on the vehicle tire placards and in 2003 BMW offered 18" tires and rims in the 330Ci in those 2 sizes. One quirk is that the rear rim was wider (8 1/2").
But I don't know if the rims you have a stock or aftermarket. So you need to look to see what the widths are. They ought to be stamped on the rim somewhere. It will probably look like this: 18 X 8 1/2 JJ - or some variation of that.
If you have the 8" all around, you have to be a little careful about tire size selection, because a 255/35R18 requires a minimum of an 8 1/2" rim - and that's where I think you ought to go. After all, that's what BMW did!
But if you have 8" rims all around, the current set up is closest in diameter - and it's what Tire Rack recommends when it does 18" fitments for your vehicle.
But I noticed that while the 245/40R18 has over 100 tires available, both the 255/35R18 and 245/35R18 have 45 tires available . That's still plenty! Surely ought to 45 tires you should be able to find something that fits your needs.
