Just be careful with 'rollout'... you need to keep track of your units. None were given above in the formula or the recommendation of 'high 30's'. Rollout has to have units, or it makes no sense (unlike 'ratio', which is unitless). Saying a 'rollout of 30' is like saying '30 long', which doesn't mean anything unless you say '30 feet long' or '30 inches long' or '30 mm long'.
The recommendation of 'high 30's' is probably 'distance in mm that your car should travel per motor revolution'. The measurement in the original question was taken in inches, not mm, so a conversion needs to be made in the tire diameter (from inch to mm) or the recommendation has to change to inches traveled per motor revolution.
I actually don't like using 'distance per motor rev' for my rollout calculations. I find it confusing that 'rollout' (when using these units) gets smaller as ratio (gennerally given as spur/pinion) gets bigger.
Instead, I like to use "number of motor revs per distance" (and you can choose your distance). At least that way, higher numbers mean the same thing (less top speed, more run time) whether you are talking 'ratio' or 'rollout'. I use 'number of motor revs per 10 inch traveled.' That gives me 1/12th scale 'ratios' of around 5:1 and rollout numbers around 7 or so, and both get bigger for more run time and smaller for less run time.
Just want to make sure that any new guys reading about rollout don't get confused when their numbers don't make sense.