Don't know about the temperature thing.
My MPGs are all over the place with city and highway. I am not a right lane driver, I like 8 to 10 miles per hour over the limit. I Have done some aero mods and others to come.

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1999 Mercedes E300TD daily driver sold at 238K miles 106K miles were mine, rust worm got it :-(
2006 Mercedes CDI new daily driver! 56,000 miles May 2016 now 85,625 Apr 2018 and Apr 2019 101,000 miles Apr 2020 109,875. March 2024 135,250, Dec 2024 145,000 miles