2012 Kia Soul 1.6L engine, automatic transmission.

In very hot weather my engine tends to ping. The only thing I have found to alleviate it is to use Premium (91 octane) fuel. I have no error codes and this issue has developed over time. The car has 108,00 miles on it. Most of my driving is on the interstate at 55 MPH.
any ideas of what I should look at to resolve this issue?