Hahaha. Thank you for posting this! I’m struggling to get my 172 up to FL16 when doing IFR flights. 
I did a 2 hour flight last night and requested ATC to lower my flight level to 8,000. I’m still pretty new to some of these concepts, but I couldn’t get my plane to the cruising speed of 124 knots. I had my throttle at full and my mixture to about 30%. The max I got out of it was about 100 knots.
I’ll have to look at the POH. Thanks again for posting about this.