Jose,

"why do some 172's have 40 degrees of
flap avalible and others only have 38?"


Dunno about 38, but the newer ones have 30, because full 40 degree flaps were
causing problems (which I can't recall) due to poor technique of some sort.

Might have something to do with slips with flaps, IIRC.

I'm always amazed at the amount of misinformation running around here.

Cessna switched to 30 degrees of flap deflection maximum on a heavier
model of the 172 because of rate of climb requirements during a balked
landing under the FARs. The earlier airplanes could meet the rate of
climb with 40 degrees, the newer ones could not, so flap travel was
limited to 30 degrees.

There is no problem with slips with full flaps on a 172, it's another
of aviation's old wives tales.

For an interesting descent rate, fly a Cessna O-1/L-19 (Model 305)
Bird Dog. It had 60 degrees of flaps available.

All the best,
Rick