Attached is a table of the effective speed on our Thunderslots, as a combination of gear-ratios and tire diameter. This makes it obvious that it pays to measure your tires after a post-race clean-up true. The difference between a tire trued to 19mm and 19.5 is 0.5 MPH true scale and 16mph at 1/32 scale speed. 19.5mm vs 20mm = 0.4mph (13mph); 19-20mm = 0.9mph (29mph) difference. The 20mm size is somewhat of an outlier though, because that’s effectively an untrued virgin tire on a 14.85-rib wheel; so the lower sizes are more significant.
Another factor not covered here is that for the smaller diameter tires your magnetic-downforce should be higher; which is also affected by how loose your motor pod is (hence how close to the track it sinks).
"... get on your bad motor scooter and ride!"
Sammy with Montrose