just by looking at the code, i would guess you'd have to change the minimum and maximum rotation values to negative and possibly every other part in which these values appear
Code:<Rotate>
<Value Minimum="3" Maximum="35">(A:Prop1 RPM,RPM) 100/> <-- possibly here
</Value>
- <Failures>
<SYSTEM_ENGINE Action="0" />
</Failures>
- <Nonlinearity>
<Item Value="5" X="23" Y="96" /> <-- and here
<Item Value="35" X="116" Y="107" /> <-- and here too
</Nonlinearity>
<Delay DegreesPerSecond="15" />
</Rotate>
</Element>
- <Mouse>
i don't know much about gauge programming, but i would guess that these entries are those that control the movement of the needle, right now the inversed prop rotation might be pushing them past their minimum value, so the gauge will stay still at 0, so by inversing the rotation values of the gauge, they might work correctly again... (maybe)
i'd backup that file and then go making controlled changes and noting their effects in FS, so that when you get it to work you'll know what you did
BTW, there is a 'refresh aircraft' command in the control assignment list, which is unmapped by default... map that to a key and you won't have to reload FS every time you make a change
hope this helps
Cheers
EDIT: one more possibility i just saw, and which actually seems more correct is changing the Prop1 RPM parameter there to -(prop1 RPM) or something like that... you're inversing the value of the prop rotation, so that it passes a corrected number to the gauge... of course, you should leave all other values untouched when you try changing this one
Code:<Rotate>
<Value Minimum="3" Maximum="35">(A:-(Prop1 RPM),RPM) 100/> <-- here (the A:Prop1 RPM bit)
i don't know much about XML syntax as well, so those parenthesis might not work well, so you could try without them as well