Definitely sounds like your vent tube dropped or something....unless you are actually not as empty as you think. In that case it would be a sending unit or gauge problem. I know the 1999 and up models are computer controlled and can be calibrated...I don't know of an easy way to figure out how empty you are though without just running out of gas.

The 2000 and I think 1999 4Runners had a problem with this from the get-go. The vent tube was actually too long and would cut the pump off before the tank was full. I have run out of gas 3 times with this problem, and it has only taken 15 gallons each time....it is a 18.5 gallon tank...like yours is. I discovered this problem on my 2000 (one of the first ones off the boat in Sept 1999) and Toyota had no fix for it. Then they later issued a technical service bulletin about the problem...which I was never informed about. Now they want $400 to fix the issue...it should have been a recall...but that's good ol' Toyota for you!

While I understand this is not the problem you are having...it is similar and might be a vent tube issue.

 

I usually put in 16-16.5 gallons in every fill up. Anywhere between 230-270 miles per tank. The most I have ever stuffed in there was 17.1 but the needle was WAY below the E at that point.

 

Yeah, mine will only take about 14 after the light has been on for a while. I really like the earlier 4Runner front bumpers better, but I wouldn't trade my 2000's cup holder for anything!