Hi! It's very important when discussing speeds that you designate TAS,
density altitude and rpm. Typically your speed will decrease about
0.6%/1000' increase in altitude. Thus a plane with a normally-aspirated
engine that goes 200 mph TAS at sea-level and 2750 rpm will be down to
about 189 mph TAS at 10,000' density altitude and about 2600 rpm.
Indicated airspeeds, without a calibrated and properly-located static
port and OAT sensor, are pretty meaningless. Also, due to dynamic (RAM)
pressure effect on engine power at 200 mph TAS, you have to be at about
9500' density altitude to obtain 75% power at rated rpm, not
7500'-8000' as usually assumed. At that density altitude and TAS, IAS
is 173 mph. Sea-level power at 200 mph TAS will be about 105% due to
RAM.