It's certainly a better deal than not getting reimbursed for mileage, that's for sure!
In general the way to optimize these things is to get a vehicle that is as inexpensive, efficient, and reliable as possible. Honda Civics are pretty efficient and reliable. A car that new is still going to have a reasonably high value that will be decreasing with every trip you take. You could probably do a bit better by selling it and buying an older vehicle, but otherwise you're doing pretty well.