Here is a little illustrated tutorial!
Press the red button to go to the main menu. Select More.

Select Setup.

Scroll through the options to set your vehicle’s parameters.

The right tank size and engine size are important for mpg and miles remaining calculations.

Press the red button to get back to the main menu.

Next time you fill your tank, press the red button and select More. Then select Fillup.

Here it tells you how much gas it thinks you put in since the last fillup. If doing it for the first time, you can just Select Done to reset it. The second time you fill up, come here and match the amount of gas it thinks you used to the amount you actually used, displayed on the pump. The percentage below will change accordingly, as this is calibrating the Scangauge to your engine.

With the above completed, you will now have access to Miles Remaining!
Press the red button to get to the main menu and select Trip.

Next select Trips.

Scroll using the bottom 2 buttons until it reads Tank To Empty. Scroll using the top 2 buttons until it reads XXX Miles. It will remember your settings so it will open to the same page next time without scrolling.

Note, this will only work if you tell the scangauge that you had a Fillup every time you fill your tank.