FS 2004:
...just tried the Beech C-23 Sundowner out...(changed some of the instruments on the Panel to suit me)...
...and found that she is infinitely/unbelievably slower than my speedy little Cessna 150...painfully slower!
Paul....going nowhere very fast in a Sundowner!...

...!
VSI at 2 feet per Minute = Stall....

..

....!
have you tried copying over from the default c172 to the sundowner folder?
1. The Cessna172SP.air file and remaning it the same as the original air sundowner air file?
2. the aircraft.cfg and just keeping the headers from the sundowner, i.e. for example this is the 172s
[fltsim.1]
title=Cessna Skyhawk 172SP Paint2
sim=Cessna172SP
model=
panel=
sound=
texture=2
kb_checklists=Cessna172SP_check
kb_reference=Cessna172SP_ref
atc_id=HB-MES
ui_manufacturer="Cessna"
ui_type="C172SP Skyhawk"
ui_variation=" White,Green"
ui_typerole="Single Engine Prop"
ui_createdby="Microsoft Corporation"
description="A stable and trustworthy plane, most pilots have logged at least a few hours in a Cessna 172, since it's the most widely available aircraft in the rental fleet, and is used by most flight schools. Since the first prototype was completed in 1955, more than 35,000 C172s have been produced, making it the world's most popular single-engine plane. One of Cessna's first tricycle-gear airplanes, the 172 quickly became the favorite of a growing class of business pilots. Its reliability and easy handling (along with thoughtful engineering and structural updates) have ensured its continued popularity for decades."