In all seriousness, hopefully OP got his question answered.

21MPG is like...normal for a S2000. Especially one being driven short trips in "cold" weather.

so there's nothing wrong with your car. that's probably why the check engine light isn't on. it run fine.

If you wish to improve it further, just take the correct basic steps.

I'm confident that any person who has a "I GET LIKE 80 MILES FOR 2 BARS. WYD?" question will search, find this thread, and ask more pointed questions.