When I bought my GS the light would come on somewhere between 1/4 and 1/2 a tank. I ended up filling it up and running it down to E three times. Now the light doesn't turn on until it's almost at E.
Even running it all the way down I haven't put more than 17 gallons in it yet.

 

I've had 2 gs's a 400 and now a 430. I've ran my car till the needle was below E to the point where there was a space between the E and the actual needle. I actually filled my 400 with 19.2 gallons before, and the 430 I have never put more than 18 gallons into it, no matter how empty I ran it. So there must be a good reserve tank. When my low fuel light goes on, I can't put more than 16.4 gallons into it.

 

I was heading to Vegas 2 weekends ago and the light came on. So not knowing this car to well (since I recently bought it) I reset the trip meter as soon as the light came on and went for it (piece of mind since I have AAA ). It took me another 26 miles to get to a gas station at which I filled her up Through the time when the light came on my girlfriend had the A/C blasting the radio was on and we had about 5 pieces of luggage in the car and 2 others in the back seats. Oh I was also cruising at 75 mph, forgot that fact.

 

For the lexus' with nav, if you hit the menu button there'll be a mileage left indicator in the upper right hand corner...I just found this out not too long ago after having the car for two years....

just something to take notice if anyone hasn't seen it...i thought that was cool....

 

I have heard over the years that most car manufacturers design the fuel system in there vehicles to have at least 1/4 tank left after the needle hits the "E". I have tested this on a few occasions and so far no problems. Not recommended though..............for the most part, I fill mines up when its at 1/4 tank left.