Hi I only got me 2003 Magna 3.5 series 2 a while ago and was wondering who accurate the fuel usage in the modes digital display is.
I had the mods display set on how many ks I could go on the remaining fuel. It showed 170 km. then the fuel warning light come on and the digital display showed nothing.
Does that mean there is still 170kms worth is is still in the tank?,
or I have to reset something to make it accurate,
or they are just unreliable.

It shows nothing when the fuel light comes on, thats what its designed to do.
As far as accuracy, I wouldn't take its word over the gauge. When the fuel light comes on, I take it as saying theres about 50km in the tank, but I'm sure theres a little more than that.
You wouldn't want to run it bone dry anyway, you'll pick up all the shit at the bottom of the tank, and eventually stuff up the fuel pump.

From what I've read, the trip computer uses your current average fuel consumption to
"guess-timate" the remaining distance that could be travelled with remaining fuel.
If you travel along the freeway for 500km then do some city driving (as an example) you can find yourself looking for a petrol station 100+km before it says you need to.
Re the idiot light (fill me up now light) I've been able to go a further 35km (highway) beyond that...but I keep a 5L fuel container in the boot at all times, so don't take my word for it unless you do likewise !

If I am not mistaken the car holds 71L of fuel.
And from filling up it seems to me that my fuel light lights up when I have used some 57L of fuel or so, giving 14L remaining.
So 100+ after the light goes on is theoretically possible but as suggested it's not a good idea to run it so empty.

Either way, I've noticed on every car I've driven that these "remaining km" are always completely off. Same on the new expensive German cars too.

I filled mine up with 69 litres once, over 900km of hwy driving. Was still running good. When the tank is really low the engine will start to cut out when going over speed bumps or around bends.