Tire Pressure in Phoenix
I have been running 44/42 on Michelin HydroEdge tires for the last 35,000 miles. Your 42/40 should be fine here, but I wouldn't let any air out.
Ford SUVs and Firestone got some bad publicity here in Arizona the last couple of years due to tire failures, but it was mostly caused by low tire pressure in the summer. Dealers were recommending lower pressures to make trucks ride like cars. The heat destroyed the tires.
Higher pressures help tires run cooler; less flexing, like ChasSuitt says.
Check your tires first thing in the morning, before you start driving on them, and fill them if necessary. If you pump them up in Phoenix, you may be able to deflate them slightly when you get home.
By the way, our highs have cooled a few degrees to under 110 the past week or so, but the monsoons are here and our relative humidity has gone from 5-10 percent in June to 40 percent and higher. 105 and 65% feels worse than 112 and 10%. Bring a hat and some cold drinks, and try to stay indoors or at least in the shade!