Re: Need Wheel/Tire Help (archive)

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In Reply to: Need Wheel/Tire Help posted by Trip on January 20, 2001 at 22:28:00:

What you need to learn is what tire sizes mean first.
When someone says 195/70R14, they mean the tire has a 195mm tread width, 70% of the width is the sidewall height, and it fits on a 14" rim. So the total diameter of your stock wheel/tire combo is:
195mm * 70% * 2 (because you have two sidewalls, the top and bottom) + 14in. * 25.4mm per inch.
The total diameter is: (195*.7*2)+(14*25.4)=628.6mm, which we will round to 629mm.

Obviously I could keep the original wheels and replace the tires with the same size, although I think that a 205/60R14
tire would fit and am wondering if this is a good option.

For your stock wheels, you have to consider how wide the wheels are. Most 6-series which had OE 14" wheels, were 14" in diameter and 6.5" in width. So I'm going to assume you have those. (There were 14" x 7" BBS-Mahle wheels fitted to the E12 M535i, and SOME euro 635Csi's, but those are rare, and if you have those, you'll have to inform me).

Ok, the maximum size you can usually fit on 6.5" rim is 215mm, and maybe get away with 225mm.

So I'm going to work with 215mm to guarantee fit.
215mm * 0.65 * 2 = 279.5mm, add to that the 14" diameter of the wheel, and you get 635.1mm diameter. This will fit, and you won't have too much error in the speedo. So the size you're looking for willbe 215/65R14.

225, IF you can fit them, will need to have a 60 series sidewall. (225*0.6*2 + 14*25.4 = 625.6mm), so 225/60R14.

205 tires need this size: 205/65R14 (205*0.65*2 +14*25.4 = 622.1mm), but this may be a little too much error.

Or do I bite the bullet and upgrade to a larger tire size? Obviously this will cost more, and is it really worth the additional cost? If so, what is better, a 15" or 16" upgrade, and what tire size would I use? I would want something that would work without any modifications.

I'm not looking to turn this car into an autocross champion, but if there was a simple way to achieve some marginal handling improvement it seems foolish to not take advantage of that option.

A 15" (or plus 1 in your case) wheel will improve handling a bit, without compromising too much ride harshnes. A 16" tire will probably be too harsh from what you're describing. So I'm going to concentrate on 15" options.

The best would be to try and find a late model 15" x 7" wheel from the E34 5-series. They came in 3 styles, the older BBS style with hexagonal cap, the later BBS style (on 540s) with flush cap, and the 525i-style 20 spoke bottlecaps. Now these wheels should fit with no rubbing problems on your car.

For the 15x7 size wheels, you will be looking for 225/55R15 tires (628.5mm diameter). 205/50R15 will work as well. And both fit fine on the rim.

My recommendation would be to get the 15" wheels. It'll improve handling, you will be able to find better performing tires, and you can use the 14" wheels with snow tires in the winter. Also, you won't kill your back or kidneys with super-harsh rides, if that's what you're looking for. Optimizing the tires will make your car handle better in ALL circumstance.

Go to www.tirerack.com for a good explanation on why you want two sets of tires for your car. You'll be very happy with the results. Also, www.discounttire.com has a few wheels for your car to choose from. Rob Anderson has a good write-up on what offset is, but with the size wheels you're looking for, it is doubtful you'll need to worry about that. (Tech articles, Officers Section, www.m535i.org).

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