POH, it is all there
Look in your aircraft's POH, you will find several charts and it will be easy to photocopy them and construct some examples to illustrate the point at different speeds and power settings, etc. Backed up with the appropriate theory (which is well covered in Trevor Thom and ATPL Principles of Flight/Performance manuals), your audience will find this approach most credible/persuasive.
For a typical complex MEP type you should find charts for Economy/Long Range Cruise and Normal/Fast Cruise. Even for the most basic fixed pitch prop single engine types you should find quite a bit of data.
Hope this helps.
George