tyre sizes

teednab-el

ok guys can someone explain this one to me. Three cars were in the car park the other day. The first one was my car (Audi A4 B5 Model) Second car was a Scoda Octavia (2005) and the third was a VW Passat (B6 Model). I noticed that there was a difference in tyre dimensions. My car has R16 / 205 / 55, The Skoda had R15 /195 /65 and the Passat had R16 / 215 / 55. Could someone explain what each dimension stands for and if the tyre size of the Passat could go on the Audi? Would like to know for my own knowledge.

Thanks.

CJhaughey

First Number is diameter in Inches.
Second is width in Millimetres.
third number is profile expressed in percentage of tyre width.
so Yours has 205mm wide tyres with a sidewall height of 55% of 205mm.
a 215/55 will be significantly different in height as 55% of 215 is different and will affect the speedo and possibly foul bodywork depending on clearances.
Look here and see what I mean:
Link

rabbitinlights

Worth a read:

http://www.eiretyres.com/Identifying.html

Esel

teednab-el wrote: »

My car has R16 / 205 / 55, ...Could someone explain what each dimension stands for ...Would like to know for my own knowledge.

Firstly, for clarity and your own understanding, you're stating those specs a bit arseways. Your tyres probably read 205/55 R16.

As CJH said above, there is a fair bit of different info. there.

The most basic item is the last number, 16, which is the diameter (in inches) of the wheel the tyre will fit. So, you have 16" wheels. (The R means Radial, and describes the construction method of the tyre).

205 is the width of the tread in millimeters (mm)

55 (called the aspect ratio or profile) is the percentage of the thread width which equals the height of the sidewall. So, your sidewall height is 205 x 0.55 = 112.75mm.

You ask if the 215/55 R16 will fit your wheel. The basic size (16") matches - if it didn't, no go! The tread width (215mm) is only 10mm wider, no problem there. The sidewall height is 118.25mm, an increase of 5.5mm. Double this to 11mm to get the overall increase in tyre height. This 11mm is so small (less than half an inch) that it will cause no mechanical problem to steering/suspension, etc, but it will mean that the circumference of the wheel will be slightly larger, thus meaning that you would be travelling slightly faster for the same indicated speed.

How much faster? Not a lot, but let's work it out by calculating the difference.

Your present tyre diameter (think total height) is as follows: (we have to convert the 16" to mm first... 1" = 25.4mm )

16" x 25.4 = 406.4mm = diameter of wheel rim

Now, add twice the sidewall height ( 112.75 x 2 = 225.5mm)

So, 406.4 + 225.5 = 631.9mm total (your existing tyre)

The other tyre will have a diameter which is bigger by 11mm (see above)

Now (nearly there!) we calculate what percentage that 11mm is of 631.9mm,
which is 11 x 100 / 631.9 = 1.74%

So, for every wheel revolution with the taller tyre, you will have travelled 1.74% further than before, but your speedometer reading will be the same (because speed is calculated based on engine/gearbox ratio, not wheel revolutions)!

Long story short - the tyres will fit fine, and the difference should not be noticable in practice. It is important to do the sums, though, as it is possible to end up with major variations to the difference calculated above, depending on the figures you are working with...

You should be aware that, all other things being equal, the lower the aspect ratio, the harsher the ride! Hit a pothole with 205/40 tyres, and you could easily need a new wheel, as well as a new tyre....

Hope that helped!

Have a look on wikipedia.org for more detail

http://en.wikipedia.org/wiki/Tire

http://en.wikipedia.org/wiki/Tire_code

Have a look on carbibles.com for loads more detail! (changing wheels, offsets, tyres). It's a brilliant site overall.

http://www.carbibles.com/tyre_bible.html

http://www.carbibles.com/tyre_bible_pg4.html

teednab-el

esel wrote: »

Firstly, for clarity and your own understanding, you're stating those specs a bit arseways. Your tyres probably read 205/55 R16.

You are dead right. I couldnt remember the exact format but after looking at my tyres today they read 205/55/R16. Good to know and thanks for the information.