On 9/17/2010 9:11 AM, Don Q wrote:
> On 2010-09-15 09:29:30 -0400, Arye > said:
>
>> It's true that that 185 and 195 indicate the tread width. However, the
>> 60 indicates the ratio of diameter to width, so it goes up
>> proportionally.
>
> Strictly speaking you are correct of course.
> However, 60% of 185 versus 60% of 195?
>
> Without going into heavy (for me) math, I will speculate that the
> difference in diameter will come to about 2%. If I'm going 51 and my
> speedometer is showing 50, I can certainly live with that margin of error.
>
> cheers
>

the speedometer would indeed read 2% too slow if going from a 185/60/14
to a 195/60/14. our beloved miata.net website has a handy little
calculator for this: http://www.miata.net/garage/tirecalc.html