Need to know bike speed from engine rpm

[H100S]

Any websites out there you could suggest?

Ideally I could enter tyre size and gearing to determine this. Two different bikes with non standard gearing.

Reason pit lane has a 20mph limit with penalty if exceeded. Naturally I'm don't want to incur any penalties.

Motorbike does not have a speedo. I figured if using 1st gear I could work out 20mph from engine rpm.

Race is this weekend.