250 in a stang??
Dont know what you have loaded on your plane, but I can easily cruise at over 300 kts throttled back to 50 (manifold pressure) in the stang. If your loaded with fuel, rockets, bombs, or drop tanks it is alot less. Also, without trimming it out the stang has a natural tendancy to climb at 2 or so. Make sure its level and your not climbing.
Granger