Maybe you doubt it.
An austrian guy flew a Balsa Bandit up to 491 km/h(305mph) with a 26lbs. engine.
This plane has a take off weight of around 14lbs a thrust of 26lbs. The wing span is 39.5 inches. GPS is very accurate. Go ahead get a GPS give it a professnial pilot so he can compare it with the flight insruments. He will get the same numbers at the GPS.