The BIS gauges must be added via a unit's config. It would be possible to have them for cars (or people for that matter), but you would need to make a new addon, or else edit the main config.
I actually like how the cutresources fade away. Since they only have limited positions, it would look funky if it stayed up all the time, and "jumped" positions every so often. That said, it would be nice to have as many gauge positions as possible.
Looking inside the script, I'm a little confused about what the _fuel variable is for. Why cant you just compare _realFuel to fuel? Okay, I think it is so that the gauge only displays when a certain amount of fuel is used, but why not just check to see if the difference between current and last fuel is above a certain amount?
I also see a bug in the script. The second @ condition doesn't seem to care if the player leaves his vehicle. It seemed to work fine in-game, but it looks fishy...
I'd suggest running this script via a "getin" eventhandler, added to each vehicle. That way, the script only runs when the player gets into a vehicle.
Looks good though.