Tyre diameter

Apologies if this has been discussed before but why does an increase in width of a tyre increase diameter of the wheel?
I am thinking of changing rear 255/35R18 and front 225/40R18 to 225/45R17
for better ride characteristics on an 02 E46 sport convertible. Using a tyre size calculator http://www.kouki.co.uk/Utilities/tyresizer.asp the diameter of the 255/35R18 is 63.57cm. The 225/45R17 is 63.43cm so this would be a good replacement where the speedometer is concerned. However if a 255/45R17 is used the diameter shows 66.13cm. I understand the 255 is the width of the tyre so why does the diameter and rolling radius change?
Another question is just how much would the ride improve? Has anyone got a particular recommendation? Would it be possible to use 225/50R17 (65.68cm diameter) within the arch of the E46? The calculator indicates that using this tyre would incur a difference in the actual and indicated speed of 3.3% . This is 2mph at 60mph.
Advice appreciated
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It doesn't necessarily. The overall wheel, inc tyre diameter, is governed by the rim diameter, and the tyres width and profile, where the profile is expressed as a percentage of the tyres width. So a wide tyre with a low profile, could actually have a smaller overall diameter than the narrower tyre with a high profile, on the same 'diameter' rims. Not necessarily the same width rims though. Mike.
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As I indicated and forgot to add in my earlier post. The difference is that with a 225/45 tyre, the hight of the tyre is 45% of 225 1.25mm Using the same calculation makes the hight of the 255/45 tyre, 114.75mm, so the overall diameter is greater.
> Another question is just how much would the ride improve? Has anyone got a

Dunno. Mike.
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IIRC, The 225/45R17 and 205/50R17 are OEM recommended sizes for the E46 convert., but the 225/50R17 isn't. Two other benefits of changing from your staggered tire sizes to 225/45R17 all around are less expensive tires and reduced understeer.
Tom K.
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Tom is correct. In the US, the 205/50R17 is the standard OEM size for 330i models without sport package, and the 225/45R17 is the standard OEM size for 325i models with the sport package. I use both; the ride is very similar, but the 225/45R17 tires tramline much more. However, they give much sharper steering response.
IIRC the 225/45 tires are on a 8x17 rim, while the others are on a 7.5x17 rim.
FloydR
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..or increased oversteer but anyway...
On the M3 the tyre sizes are larger than other E46s at 225/45 18 on the front and 255/40 18 on the back so yes I reckon you could easily go up to those diameters if you wanted. I reckon there would be an increase in ride with 17s over 18s depending on your tyre choice.
I'm not sure if you can fit 16s unless they are an option, sometimes the brakes are too big to fit inside a 16" rim.
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Mike, Doesn't the 45 refer to the ratio of "width of tire" to "bead to tread" height? This is based on a cross section of the tire. Thus, higher numbers result in wider tires. Squat.

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If I've understood your question correctly, the 45 figure, the aspect ratio or profile to use it's more common name, has no affect upon a tyres width, as it's a percentage of however wide the tyre actually is.
A selection of 17" tyres could have profiles of anything say, between 30 and 80. All having the same width. The only difference between them would be that the tyre with the 80 profile would have a greater distance between the rim and the tread. It would be a taller tyre relative to the width, and have a larger overall diameter than any of the others with a lower profile. Mike.
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This is correct. A 225/<anything> is 225mm wide, and the profile (aspect ratio) is 55, 50, or 45 percent of that, depending on the rim diameter -- in the case of fitment on a BMW.

This too, is correct. The 225/80 is not an available size, or at least not a size that would be found on a BMW, but assuming it was a valid size, the tread width would be 225mm and the sidewall would be 80%, or 180 mm. A 225/80 would require a rim diameter of 10 or 11 inches to give an overall diameter that is in the ball park of the size of a 225/45x17
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Jeff Strickland wrote:

Nope. The 225 is the width of the tire at its widest point. Not the tread. Look at the tire specs on tire rack and you'll see what I mean.
--
-Fred W

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For the purposes of this discussion, I do not consider the distinction to be critical. The cross sectional width and the tread width are functional equivelents. Yes, they are different terms and are in fact different measurements, but a wider or narrower cross sectional width will always result in a wider or narrower tread IF the tire fits properly on the rim. A very wide or very narrow rim, relative to the width of the tire will have an affect, but I was ignoring that as an unhelpful degree of complexity.
As a practical matter, the 225s on my car fit properly on the 17" rims, and the cross sectional width and the tread width are the same, within about 10mm. I'll take is as equivelent, mostly because I lack the measurement devices to exactly guage the difference.
And, the widest point is at the top of the tire, not the bottom, where the weight of the car and the air pressure inside the tire will affect the dimension of the tire.
All of this is a level of detail that exceeds the need of the explanation.
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Sorry, Jeff, but I have to disagree. The OEM tires on our Toyota Highlander were 225/70R16, the OEM (for 325i sport package) I use on my 330xi are 225/45R17. The tread width on the Toyota tires is (was, those Toyo tires were trash) substantially - 2-3 cm - smaller than the tires on the BMW.
I just went out and *measured* the tread width: BMW - Goodyear F1 GS-D3 225/45R17 on 17x8 rims: 8" Toyota - Michelin Energy LX4 225/65R17 on 17x7 rims: 7" (Similar results for the 225/65R15 on our '91 525i.
I submit that 25.4 mm is not insignificant, and that the tread width is entirely different (due to many factors) than the tire width specced in the tire size.
FloydR
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Get last weeks copy of auto express - full explanation on tyre markings.......
On 4/10/07 16:38, in article snipped-for-privacy@corp.supernews.com, "Floyd

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"Squat" wrote

No. Higher 'profile' numbers result in taller tires, not wider. The profile does not tell you anything about the tire's width. But the width and the profile combined tell you the sidewall height.
Pete
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205, 225, 235, 255, etc. all refer to the cross sectional WIDTH of the tire. This width is defined by the number, so the TREAD WIDTH is a direct corelation tyo this number. A tire with a 205 cross section has a tread width that is narrower than a 225, without any6 regard to the other numbers that will be present. Larger numbers will result in a wider tire, if the aspect ratio remains the same, then the sidewall will also be taller.
35, 40, 45, 55, 70, etc., all refer to the Aspect Ratio. The Aspect Ratio is the height of the sidewall as a percentage of the cross sectiional width. A 205/50 has a sidewall height of 102 mm, a 225/50 will have a sidewall height of 113 mm. A 265/75 (a common light truck or SUV size) is 265 mm wide with a sidewall that is 75% (just under 199 mm) of that. A 255/35 will have a tread width of 255 mm and a sidewall height of 89 mm.
As a practical matter, the numbers generally work out (in the size ranges that we want to know about for a BMW), that if the RIM diameter increases 1 inch, the aspect ration will be reduced 5%, if the cross sectional width remains unchanged. A 225/55x15, a 225/50x16 and a 225/45x17 are all equivelent. I suppose this would be true of all tire sizes, but I've not actually done the math to know for sure.
In this example, the 15 is the smallest tire and the 17 is the largest, but the difference is only 7 revolutions per mile, so the effect on gearing, speedometer accuracy, power band, etc., will be insignificant. The 17 makes the most accurate speedo output -- I know this from first hand experience. My car when the cruise is set to 85 on the speedo, passes the mile markers with astounding repeatability in a time that says the car is doing 83.7 mph. The Trip Computer also reports a number that is the same, as near as I can carry a decimal place.
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Jeff Strickland wrote:

Nope.
The cross sectional width is at the widest point of the tire, which is always well up the sidewall from the tread. Tread can be narrower or wider even on tires with an identical cross sectional width and profile.
Generally the tread width increases with cross section width on tires of the same make and model. It's when you are comparing different makes and models of tires that it gets tricky.
--
-Fred W

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Ah yes, now I remember. I had forgotten how that worked. Thanks Memory is one of the first things to go with age.. I forgot the others... Cheers, Squat.
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Tyre size = width [/ profile] [x]R rim_diameter
Diameter = rim_diameter + 2 * (width * profile / 100)
so 225/55 VR 390 = 637mm
165 R 13 = 594mm (convert inches to mm, default profile is 80%)
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Okay, 225 / 45 x 17 ...
225 is the cross sectional width of the tire. It is essentially the width of the tread. Not exactly, but close enough.
45 is the percentage of the cross sectional width that is the width of the sidewall. This number is referred to as the Aspect Ratio. As the diameter of the rim changes, the aspect ratio also has to change to keep the overall diameter the same. A 225/55x15 has the equivelent overall diameter as a 225/45x17, you noticed that the 17 and the 18 sizes in your investigation gave nearly identical overall diameters, but the aspect ration was smaller on the 18s because the rim is larger.
17 is the diameter in inches of the rim. The rim also has a WIDTH specification too.
In your example, the 255/35x18 and the 255/40x17 are identical replacements. The difference is less than the rotational speed differences from left to right when going around a corner, not to mention the differences from front to rear.Your investigation also found that the 225/45x17 is also a suitable replacement. The difference here is only about 2 tire revolutions per mile. This is an insignificant change.
Personally, I think you are getting ready to buy a new set of rims and tires, and will not even notice any difference. your money would be better spent on a set of 16" rims and tires to use as snow tires, but snow tires on a 330i Convertible is probably not going to happen. My convertible would stay in the garage on snow days, and my truck would be on the street.

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"Jeff Strickland" wrote

Agreed. There isn't that much difference betw. 17s and 18s when it comes to ride comfort. A lot depends on a specific tire model, too. In the same size, some are harder/stiffer than others. If ride quality is a concern, going down to 16" rims with 205/55 tires would make a noticeable improvement and 205/55/16 tires are much cheaper too. It's definitely a downgrade in terms of the 'look' factor though, so it just depends what the OP values more.
Pete
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