Earlier posts were from the Hyundai Getz thread, I figured I'd just post it here just incase the mods reckon I'm pushing my luck by posting tech questions over there.

Quote Originally Posted by OTEP
I think www.tyrerack.com has a tire calculator. Basically, a 185/55r15 tire is a tire that is 185mm wide. The height of the side walls is 55% of 185 and is known as the ASPECT RATIO. 15 refers to the rim size intended for the tire. 5.5 jx15 means a 5.5 inch wide tire that is 15 inches across.
TY ulit!!! Looks like my assumptions were almost correct, I had a feeling the 55 had something to do with the side walls but didn't know whether it was the actual value or something else. I think I'm confused though, if used a 5.5 x jx15 which is 5.5" (139.7mm) how is it that a 185mm tire be appropriate for it? Or the 185mm tire rating is the width at the bulges?

Anyway of telling what x wide tire is appropriate with a y wide rim/mags? Oh and what about the letters, say like in 5.5 jx15, 185/55r15, 4.0Tx14, T105/70D14, what do the letters 'j', 'r', 'T' and 'D' stand for.BTW: I couldn't remember if it was in this forum that I got this but this is where I calc'ed the stock and compared it with the options: http://www.miata.net/garage/tirecalc.html
Oh, and uh, while I'm at it. I've been actually eyeing a mag wheel design but I'm clueless with regards to the size ratings nomenclature take for example this:


I assume the 15x6.5 is the same as previously discussed, 15in x 6.5in mags. What's the +xx for? Like the +18 or +33. The 4h/5h I assume are the number of holes(?) and the x/y right behind it are the diam/depth of the holes?

My main concern for these queries are basically safety. I wouldn't want to compromise my ride's safety should I manage to get a mag/tire combo that's not included in my ride's factory recommended options. Plus my function over form principle. If it would affect my ride, say in comfort, etc., in a significant manner, I'd rather opt for something within my ride's manufacturer's recommended options which are:
Rim Size Wheel Size
4.5 jx13 155/80r13
5.0 jx13 155/80r13
5.0 jx14 165/65r14
5.5 jx14 175/65r14
5.5 jx15 185/55r15
4.0 Tx14 T105/70D14
Lastly, am I correct at saying that to compute for the overall wheel diameter, in mm; one can use this formula:
((a*(b/100))*2)+(c*25.4)
where:
a=tire width (mm)
b=aspect ratio (%)
c=rim size (in)

and computing for rev/min at 20kph is:
(20000000/(pi*d))/60
where:
d=overall wheel diameter (mm)

TIA 4 d nfos-------