What is this stat?
 

What do the red and green positive and negative numbers mean?

For example my best mpg is 37.4(-16.1)

What does the (-16.1) mean?

Looks like your Average MPG is 37.4. The number under your Average MPG (-16.1) is the difference between your current average and the average before adding your last fuel-up. It gives you a sense of how your average mileage is trending.

Your vehicle has a fairly wide swing of 16mpg because you have all partial tanks right now. This should even out over time as you add some full fuel-ups.