合金鈐啲尺寸其實點計

The example you provided, 215/65-15, is actually the size for tires, not 合金鈐 or wheel/rim. 215 is the width of the tire, or 215 mm; 65 indicate the side wall (how "tall" or "think" the tire is) in percentage -- in this case, the tire is 215 x 65% = 139.75 mm tall. 15 means the tires can only be mounted to rims that's 15 inch. Note that the width of the tires can only be mounted to rims with certain widths. The first generation Previa/Estima use 215/65-15 tires, and the car comes with 15" x 6.5" rims.

Offset is the distance from its hub mounting surface to the centerline of the wheel. Here's a good illustration that explain offset of the wheel:

http://www.tirerack.com/wheels/tech/techpage.jsp?techid=101

The letters "JJ" indicates what tire bead profile or rim contour designation the tires and rim "meet". Basically, most tires and wheel combo nowadays will have a "J" profile, which means when you mount the tires onto the wheel, and if you cut the whole thing into half, the area where the tire touch the wheel will look like the letter "J". "JJ" are usually mounted on 4x4 wheels.

One more thing. The R in your 215 65 R15 means the tire is radial tire, again, has nothing to do with the 合金鈐

2007-05-08 23:22:42 補充：
你講個215 65 R15 係tyre嘅size, 唔係合金鈴. 即係話條tyre係215 mm闊, 215mm x 65% 咁厚, 裝喺 15寸鈴上面.Offset嘅數字係代表咗條鈴係突出嚟D定係凹入去個fender入面多D, 用條鈴嘅橫切面中間做measurement. 如果條鈴係15" x 6.5", offset係"0"嘅話, 即係條鈴有一半, 即係3.25", 係喺個hub入面, 而另外一半喺個Hub 出面. 嗰D好闊, 望埋去隻碟好深, 一般用喺尾轆車嘅鈴就係negative offset.

2007-05-08 23:25:21 補充：
JJ係指條tyre 裝入去條鈴度時, 兩樣嘢嘅接觸面(即係tyre嘅橡膠同鈴嘅邊)橫切面嘅形狀似兩個相連"J"字.