The older models of 172 have up to 40 degrees of flap, which is quite interesting to experience... On a hot day with full power, 2 people & full tanks you will have difficulty holding altitude. I'm told that Cessna changed the flap setting to max 30 degrees because of problems with the flaps getting stuck at 40 degrees. For this reason I personally never use the full 40 even though I've got the option. The POH also says don't side-slip with 40 degrees of flap.