(Single paragraph caused by unresolved obscure browser problem.) 150 is 100 hp, if older cylinders fitted, timing is retarded to 24 degrees BTDC, you'll lose about 3 or 4 hp which you really won't notice anyway. Later cylinders timing is 28 degrees BTDC, the AD note doesn't apply, you get all 100 (thundering) horsepower. 152 is Lycoming of 108 or 110 hp (depends on year), 28 volt electrics, flaps limited to 30 degrees. 70 lbs increase in MTOW. Overall performance is about the same between 150 and 152. Your 70 to 75 kts airspeed seems slow, go ahead and flog it a little, the O-200 likes to be run hard, 75% power all day long and it will lap it up. Variations: base engine is 100 hp O-200 on ALL USA 150s (there's no 80 hp option), 108/110 on all 152s. Rheims 150 (FR150) sometimes had a somewhat problematic Rolls-Royce O-240 of 135 hp (under license from Continental), few parts available any more, and they're expensive if you can find them. STCs for the 150 include 150, 160 and even 180 HP Lycomings, for the 152, there is a "Sparrow Hawk" conversion (prop & pistons) which takes it to 125 hp. If your airplane is really only going 70-75 Kts at cruise, either open the throttle some more or look for problems, cruise at 2,500 ft MSL on a 150 is right about 100 to 105 mph, or 90 KIAS. Check the ASI calibration, read the POH for 75% power at different altitudes. Best Regards, Echo Mike N150EM