Originally Posted by HarryLeach

Yes, but...

Internal ratio is 1, but since foam tires can vary in diameter, rollout is what you need to gear toward, as it will take the tire diameter into account.

Rollout = ((tire diameter * pi)*pinion)/spur

So if you know the rollout you're looking for and trying to figure out which pinion to use, it's

Pinion = (Rollout * Spur) / (pi / tire diameter)

There's other rollout calculators online, or a spreadsheet is easy to make for your own use when you know what your targets are.