Both, actually.

I use display to help me calculate how many more miles I can go before I have to fill up. For a long time I filled up after I'd used 10 gallons (e.g. 480 miles at 48 mpg) because that's when the light went on. Doing that showed me that the display was pretty close to reality-- I'd be putting in just over 10 gallons.

Lately I've realized I can go more than another gallon's worth and still have enough gas to get me to the station, but it is kinda nerve-wracking to drive so long with the light on and no bars left in the gas tank indicator.

I'm working on my own milage database, so I can see how close the readings are to the actual. Should be interesting-- so far I've only entered the first few records... many more to go!

--A