With foam tires and their always changing diameter, unfortunately fixed ratios don't make much sense. That's why people use roll-out on pan cars instead. But it's actually not that difficult:
Just calculate gear ratio: pinion/spur (=how often the tire will rotate per motor rotation*)
[* in TCs usually the inverse number is used: how often the motor needs to rotate to give the tire one full rotation (although a TC's internal gear ratio will modify that number again)].
Take that gear ratio times the circumference of your rear tires (which is simply: diameter*Pi) and you're done.
Example (random numbers): 100t Spur, 30t pinion, tire diameter 50mm:
gear ratio: 30/100 = 0.3
circumference: 50*Pi[3.1415] = 157.075
rollout: 0.3*157.075=47,1225mm or just 47mm
With foam tires (as opposed to rubber tires with their more or less constant diameter) this is the only proper way.