If you are driving a personal vehicle for work, I assume you will be compensated for mileage?
FYI, my manual transmission 2005 Toyota Matrix regularly gets 34 city/36 highway. Just another option. It's roomier than the Fit, too. Well, at least for driver headspace. DH sat in a Fit recently and his head was touching the top of the car; that doesn't happen in our Matrix.
You can calculate a price per mile assuming the same cost of fuel for your current car given what you paid, and then a price per mile for a newer option. Compare them and see if it is worth the change. I like to multiply the cost per mile by some large round number because it's easier for me to see the difference looking at $1,950 per 12K miles v. $2,160 per 12K miles rather than $0.1625 v $0.18 per mile.