Chuck wrote:

I fly a CAP C172P that has also been upgraded with the O-360 180HP
engine. But ours stuck with a fixed-pitch prop. On our 172, it
brought our gross up to 2550. I wonder why the difference???

Chuck

In this conversion, the prop and prop governor are a bit heavier than a
fixed-pitch prop, so that extra weight may account for part of the
difference.

A second potential factor is that in this conversion (STC by Del-Air of
Porterville CA), the STC holder wanted to have full use of flaps (40
degrees) instead of the 30 degree restriction characteristic of other
conversions. My understanding is that FAA certification required that
the aircraft be able to climb at gross weight with full flaps deployed.
Those extra 50 pounds may have been significant in meeting that
specification.

The aircraft used for certification flight was loaded up with fuel and
sandbags and did indeed climb with full flaps on a hot day at
Porterville (442 MSL) to the satisfaction of the FAA inspector.