Speed in Gears
A vehicle's top speed is limited by drag and/or gearing. In road racing, a lot of effort is put into making the two limits coincide. Dirt bikes usually don't need a high top speed so other factors are considered when making gearing choices.
The nice thing about a single-speed gearbox is that you're never in the wrong gear. Conversely, you're never in exactly the right gear either.
The spreadsheet below summarizes the overall gear ratios and speeds in each gear for all the bikes examined in detail.
Overall gear ratios are computed from the product of the primary ratio, secondary ratio, and each gearbox gear's ratio (if present).
Speeds are computed in 1000 rpm steps from 1000 to 10000 rpm. Rows in red indicate the rpm value roughly equivalent to the motor's capability. The actual rpm limit may be different. I have doubts that the GasGas TXT300 revs beyond 10000 rpm - if even that high.
Gearing for the Montesa Cota 242 was estimated from a speed chart presented in a magazine test.