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I may be doing my math wrong (I was good at it a long time ago in school, now it makes my head hurt!), but I get different numbers:
165/65/15 = 860 revolutions per mile
205/60/15 = 817 revs per mile
205/65/15 = 791 revs per mile
If I understand it right, the second number is the aspect of the tire expressed as a percentage of the tire width, so I multipied the first number by the second (as a percentage), converted from metric to english, multiplied by two ("top" and "bottom" of the tire) and added that to 15" to get the diameter of the wheel and tire, multiply by pi for the circumference, then diveded 63,360 by each circumference to get the revs per mile.
As I said, I might be making a big error somewhere.... but if I'm lucky, it seems like the bigger diameter I'll have with the new tires will just about compensate for the almost 10% error my speedo and odo seem to have! Or wait, will it make the error now 20%...? Nah, I think it's the former.
Tom
NYC
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