Well there are a number of Scan Gauges IF you don't mind getting your MPG from the OBDII Port. I have two versions:
https://www.ebay.com/sch/i.html?_fro...cat=0....these are fine and well made. and as a cheaper version but seemly a little more complex is:
https://www.ebay.com/sch/i.html?_fro...kw=turbo+gauge
This is a new one, looks interesting.
https://www.ebay.com/itm/12V-OBD2-Ca...UAAOSwUrZd8ggE
The only need for a MPGunio is IF you tricking the car's PCM to improve its MPG and it some cases you might trick the MPG readings as well. Which is why I have them.
Rich