True performance of Hawk 75?

[GuerraCivil]

I would be interested to know the "most reliable" information about Curtiss Hawk 75´s (P-36) performance - in this case Hawk 75A-planes, which were bought by the French and used against Germans.

There is quite a lot of difference concerning the top speed depending on different sources. According to American/English sources Hawk 75 could achieve the top speed of 500 km/h (311 mph) at the altitude of about 4000 m (approx. 12 000 - 13 000 feet).

However, Finns who bought ex-French Hawk 75´s from Germans came to much lower figures in their tests. In 6.7.1941 a test flight with Hawk 75 showed that the plane could achieve top speed of only 430 km/h (267 mph) at the altitude of approx. 4000 m.

My first thought was that this could be a result of the change of powerplant, but the powerplant was 1065 hp R-1830 - the very same used in the French Hawk 75 A!

So where does the difference of 44 mph in top speed come from? Are the American/English figures so-called "Curtiss factory figures" with a brandnew plane without ammunition load and other combat equipment and flying the stripped plane in very ideal conditions?

It would be also interesting to make some comparison between "true" technical performance of Hawk 75 A and Bf 109 E 3 (it´s main rival in Battle of France). And last but not least - was Curtiss Hawk 75 really a reliable plane easy to maintain and with "excellent handling charasteristics"?