(I know there are a million fuel efficiency topics, so someone may want to merge this if they see fit.)

Basically I read that the Magna fuel tank is 71L. Yesterday my fuel light came on the center console and the gauge was at the very last thin line (indicating about 1/8th full). So I fueled up and the tank was full after pumping only 46L. This must mean that there was really 25L still left in the tank (71L-46L...).

So has the o2 sensor gone hay-wire and needs replacing or does the fuel guage and fuel light normally read like?

1. you might be on uneven ground which may give a temporary false reading, it happens sometimes when you are going up a hill or down a hill.

2. the fuel light usually turns on when approx 80% of the fuel tank volume has been used.

3. just reset your trip computer/metre everytime you fuel up, that would give you a rough indication on how much you have left and how far you can go, eg. if you know you can get 600km out of a full tank then you can fuel up when you get roughly there abouts

Im not sure if the fuel tank has a 71L capacity, i fill my tank to full every time i fuel up and it only goes to 55L

I was thinking of letting the fuel run out till the car stops but this could be a bad idea. I get about 350-450 km's till the fuel light comes on.

yeah, in my experience when the gauge was indicating empty and the warning light on, I'd have about another 15-20L of fuel to go. Based on whart I've heard from other magna drivers, this is typical.

Strangely enough, since I got a Walbro fuel pump, my fuel gauge reads quite differently, empty actually means almost empty now.

Btw your O2 sensor has nothing to do with how your fuel gauge reads. But a stuffed/misreading/slow O2 sensor will return worse fuel economy during lean-cruise and low load/throttle situations.

Unfortunantely with the 3rd gens you will not get a warning light on the dash if the O2 is stuffed, so the only way to tell is to test it with a multimiter, or watch how it reads with a ODB2 scan tool.

I wouldn't get too caught up in comparing fuel economy figures with other members, as driving style, transmission type, traffic conditions etc return mean vastly different figures between users. E.g one person's idea of "city" might be mostly cruising down arterial roads, for another person it could be creeping along in peak-hour traffic.