Typical summer highs here are mid to upper 90s. This time of year, that means Speedster drives start around 7am, and I like to be off the road by about 10.
'All-day' driving weather usually doesn't get here until late September.
Warmest weather I've done any extended Speedster driving in was about 95. The cooler handled that OK, with the temp needle just slightly above half-way (maybe 205-210 degrees). It 'controlled' to that temp - the needle stayed there, without continuing to rise, on some pretty steep grades in the mountains.