C172 performance
There are also many other factors to consider such as mountain waves etc. Are you familiar with the machines performance? Not all same models performe the same or anywhere near book figures. Why don't you load it up to the same weight and take it to the same altitudes and see what sort of clumb rate you get. I was once in the mountains with a lightly loaded early model C210, and we were just holding altitude. Try and get yourself the Kiwis handbook on mountain flying, well worth a read.