[Rab]

Ninja 250 on the freeway, probably fine (never ridden one but its top speed would suggest this).

Honda Rebel or Nighthawk 250 on the freeway is marginal though. Although capable of 80 mph flat out, down hill and with a wind behind you, it’s very buzzy at high speed and you’ll soon have to change down a gear just to maintain an indicated 65 mph when you encounter a long uphill slope.

Same is probably true of most other non sport-bike 250 c.c four strokes (which is most 250s).

Trialsrider is correct, engine displacement is not a good indicator of speed, acceleration, power, etc. as there are other variables which come into play.