Here's a formula for determining the outer diameter of a tire:
Using the aforementioned 185/65/15.....185 X 65% = 120.25...convert to inches by dividing by 25.4 (a constant)
= 4.734 which you then multiply by 2 (X 2 = 9.468) + the tire diameter (in this case 15) = 24.468....Any upgrade should equal this or be plus or minus .5 inch (1/2 inch).
So using this formula we see that the 195/60/15 = 24.212.
So the difference in diameter is .256 (approx. 1/4 inch). So your good to go as long as you have clearance.