Re: changing Tahr profiles affect speedo?

["Bruce Bell" <bbell@bbell.com>]

Your method is most likely close enough for comparing
relative differences. There is however such a thing as
static loaded radius which depends on the tires profile and
sidewall stiffness that affects the actual rolling
circumference of the tire. Some of the tire company web
pages have these specs. look for revolutions per mile.

hth

Bruce

> What's the general guideline in switching profiles on
tires?
> 
> Let's say I start with a non-standard 195/65 R14
> 
> Wheel is 25.4 mm/inch * 14 inches = 355.60 mm
> Sidewall         2 * 195mm * 0.65 = 253.50 mm
>      total rolling radius         = 609.10 mm 
>  
> And I want to try a 205/60 R14 in its place:
> Wheel is 25.4 mm/inch * 14 inches = 355.60 mm
> Sidewall         2 * 205mm * 0.60 = 246.00 mm
>      total rolling radius         = 601.60 mm
> 
> which is a decrease of 7.5 mm or 7.5/609.10 = 1%
>