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In Reply to: tire sizes for 540IA posted by Jim Vomvas on May 07, 2000 at 19:04:12:
: Thanks...jim V
Gross calculation: 235mm (tire section width, not tread, i.e., o.d. of sidewall to sidewall) x.04 to convert to inches x .45 (aspect ratio which is percent of tire width sidewall height is) = sidewall ht. x 2 (for 2 sidewalls) + 17"(rim) = overall diameter or "tire height" in inches. Do same for 245s. Don't have calculator handy or I'd just give it to you--but not a bad pen and paper exercise either. After a few tries it takes about 5 seconds! Pretty accurate hand method. Check archives (from a few to several weeks back) for my post or response to someone where I posted a great website tire calculator which is even more precise where you can list a number of tire sizes you're interested for comparison (has rpm(iles), tire o.d., sidewall ht., % difference, etc. Best I've seen. It is actually a "Miata" site. (Other sites also can calculate different effective gearing, etc.) Whoever said that paper calculations don't always tell the entire story since sizes vary from manufacturer to manufacturer is correct...but variation shouldn't be too radical, and above methods will give you a good 98% rule of thumb. Good luck!.
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