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You just spotted the two most stupid things about the Laguna trip computer. No instantaneous fuel economy, and the ridiculous refusal to tell you what your remaining range is when you most need to know.
Yes, you lose the estimated range feature the moment that the low fuel warning kicks in. Renault's excuse in the manual is that you should fill up ASAP on seeing the low fuel light anyway. It sounds like utter cobblers to me, considering that most other cars with trip computers will let you go right the way down to 0 miles range (at which point you usually still have a little bit left in the tank.)
But you learn to live with it. If I spot it just before it happens, I tend to do the following:
Flick onto the economy display and see what my current MPG is
Flick onto the range remaining display and see how many miles I have left
Do a quick bit of mental arithmetic to work out how much fuel that amounts to (e.g. 40mpg with 80 mile range means you have 2 gallons left.) Don't be afraid to round up and down a bit - it doesn't have to be spot on.
Flick onto the fuel used display, and either zero it or add on your fuel calculation. You can then count down to an empty tank in tenths of a gallon increments.
It's not nearly as tricky as it sounds once you actually try it.
Funnily enough, the Clio is similar but different. On all of my Lagunas, the blanking of the trip range display has coincided exactly with the low fuel light coming on and the voice synth saying "Warning, fuel level low." I'm not sure if this is the case on cars without the voice synth though. If they work like the Clio does, then you may be fortunate enough to get the warning light a short time before the trip computer gives up the ghost. That should give you a chance to make a note of the current range and do the calculations I mentioned above.
Currently in Horatio's garage:
2010 Laguna III Initiale.