The goal I believe is to keep engine rpm and ground speed relatively close after tire change.
To calculate, need to know peak engine rpm, and both gears on original, along with the information provided in original post.
The formula would look like this,
Basic setup.
6000 rpm
15/60 gear ratio equals 4 to 1 ratio
33.75 tire rollout original
37.25 final tire size
Engine rpm/gear ratio = axle rpm.
6000/4 = 1500
Axle rpm times original tire rollout equals inches per minute distance covered.
1500 × 33.75 equals 50625 inches per minute
Distance covered divided by new tire size equals axle rpm.
50625 /37.25 = 1359.06 rpm
Engine rpm divided by axle rpm equals new gear ratio.
6000/1359 = 4.415
15 × 4.415 = 66
So 15/66 or equivalent would be starting point using the numbers I substituted.