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Discussion Starter #1
Hi all,

I was wondering how much can your tyres differ from the originals, and still work properly?

For example, you have 18" rims with 225/45 tyres. Can you fit 205/45 instead? Or 255/35? What is the acceptable margin? (if any)
 

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What wheel and tyre size do you have?

There is a +/- percentage difference that must be adhered to and it depends on the width of your wheel which will be stamped into the rear of the wheel on one of the spokes:thumbsup
 

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Discussion Starter #3
My tyres are 205/60R15 at the moment (that's why I'm fishing for some new alloys/tyres :p). I know you have to maintain the same circumference/diameter of the wheel, so the new tyres need to have correct aspect ratio. So my question was mainly about the width of the tyre - how wide or narrow a tyre can you choose for the same alloy? (assuming you will pick one with the right ratio)
 

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ok you can go for a 225/45/17 all round or 225/45/17 front and 245/40/17 rears on 17s or if you want 18s, then it should be 225/40/18 all round or 225/40/18 front and 255/35/18 rears that would keep the same circumference and also is what bmw recommend on 3 series
 

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My tyres are 205/60R15 at the moment (that's why I'm fishing for some new alloys/tyres :p). I know you have to maintain the same circumference/diameter of the wheel, so the new tyres need to have correct aspect ratio. So my question was mainly about the width of the tyre - how wide or narrow a tyre can you choose for the same alloy? (assuming you will pick one with the right ratio)
i think the 205 is as wide as you can go on standard rims and 195/65/15 is about as narrow as you can go
 

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Discussion Starter #7
I was thinking 17" or 18"... With preference for 18"s. They look just right to me.

The thing is, I'm quite picky when it comes to choosing the style :p And there's other things that I want to upgrade in my BM, so I want to keep the wheels upgrade reasonable finance-wise too.
In practice, that means eBay :p Or some good used wheels dealer maybe - but I don't really know where to look for them around Preston (I would appreciate any hints on that).
One thing I do know is to stay well clear of Halfords/KwikFit etc :p

The problem I find with eBay is that you have to know perfectly what you want&need. Otherwise you could end up with something of wrong size or with some shite not worth half its price... Hence my questions.

Anyway, I did some googling, and I dug up this:
Tire Specs Explained

For the lazy among us here is the relevant part:
Rim Width Range

Because tires have flexible sidewalls, a single tire size will fit on a variety of rim widths. A tire's rim width range identifies the narrowest to the widest rim widths that the tire is designed to fit. The width of the rim will influence the width of the tire. A tire mounted on a narrow rim would be "narrower" than if the same size tire was mounted on a wide rim. NOTE: Because the overall diameter of a steel belted radial is determined by the steel belts, there is little, if any, change to the overall diameter of the tire due to differences in rim width.

The industry rule of thumb is that for every 1/2" change in rim width, the tire's section width will correspondingly change by approximately 2/10".

For example: a tire in the P205/60R15 size is measured on a 6.0" wide wheel and this size tire has an approved rim width range from 5.5" to 7.5" wide. The tire has a section width of 8.23" (209mm) when mounted on a 6.0" wide wheel. If that tire were mounted on all of the rims within its range, the tire's approximate section width would change as follows:Difference from
Code:
Difference from  Rim     Approximate Tire
Measuring Rim	Width   section width

0.5" narrower	5.5"	8.03"
Measuring Rim	6.0"	8.23"
0.5" wider	6.5"	8.43"
1.0" wider	7.0"	8.63"
1.5" wider	7.5"	8.83"

Because of the different wheel widths used in the above example, there is a 8/10" projected difference in tire section width when comparing a tire mounted on the narrowest rim to the widest rim within its range. This may affect fenderwell and frame clearances when selecting optional aftermarket wheel and tire packages

Additionally, some vehicle manufacturers and tire companies have permitted rim widths that are not within the tire's original approved rim width range. For example: BMW has combined 235/40R17 sized tires on 17x7.5" rims (which are 0.5" less than the narrowest 8.0" wide rim listed for the size) on certain M3 models; and Chevrolet has combined P255/50R16 sized tires on optional 16x9.5" rims (which are 0.5" wider than the 9.0" wide rim now listed for the size) on certain Corvette models. While these applications have received the approval of the vehicle and tire manufacturers, staying within the approved rim width range helps assure that the tire's internal stresses are within its design parameters.

Measuring Rim Width

The measuring rim width is the industry standardized rim width upon which the tire must be mounted in order to confirm it meets its dimensional targets. Because the width of the rim will influence the width of the tire, a standard rim width for every tire size is assigned and must be used. This standardized measuring rim width allows all of the tires produced around the world to meet the same dimensional standards and therefore, be equivalent with regards to their physical size. The measuring rim width is sometimes referred to as the tire's "design rim width".

The assigned measuring rim width changes with the tire size's section width and with the tire size's aspect ratio. As tire section width increases, the measuring rim width increases proportionately in 1/2" increments. Therefore, relatively narrow wheel widths are assigned for smaller tires while wider wheel widths are assigned for larger tires.
Code:
Tire Size      Measuring
               Rim Width
P175/75R14	5.0"
P195/75R15	5.5"
P215/75R15	6.0"
P235/75R15	6.5"


Additionally, relatively "narrow" measuring wheel widths are assigned for taller profile tires (75-series sizes) which graduate in 1/2" increments to the wider wheel widths assigned for lower profile tires (40-series sizes)
Code:
Tire Size       Measuring
                Rim Width
P225/75R15	6.0"
P225/70R15	6.5"
P225/65R16	6.5"
P225/60R15	6.5"
P225/55R15	7.0"
P225/50R15	7.0"
P225/45R17	7.5"
P225/40R18	8.0"

According to this, you have quite a degree of freedom when matching tyre width to rim width?

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