One objective was to achieve large temperature rises and high output temperatures. (builditsolar.com)
For the collector as is is now, there is a significant drop in efficiency and heat output to get the higher collector output temperatures. (builditsolar.com)
From my research, it appears that a vent output temperature of 40 °F to 45 °F (4 °C to 7 °C) measured after driving for 10 minutes at 45 + mph would be reasonable on a 85 °F (29.4 °C) day with 70 % relative humidity. (mechanics.stackexchange.com)