The formula is (pi) 3.14 x tire size x pinion / spur = rollout

Gear ratio. The higher the ratio the slower you go.
Rollout. The higher the rollout the faster you go.

With a touring car you have to include the gear box ratio in rollout

I seldom used the gear box in the formula when I ran touring unless I was converting rollout of a fellow racer with a different gear box ratio.

Rollout is especially helpful when changing tire sizes, or running foam tires. Foams get smaller each run & adjusting your rollout with the pinion to keep the desired final ratio or to adjust for more or less speed, or motor temp.

I have a widget on my computer a friend gave me years ago that I type in the ratios & it figures it.

Hope this helps
Tom