I have been driving my s2000 for about a week and it feels like it loses gas pretty fast, I drive 50miles round trip for work, I had a full tank Monday before work and on Wednesday it is at half a tank. Just wondering if that is normal?

 

Wednesday after work, or before?

13.5 gallon tank, so if you've done 150 miles, yeah a half tank is about right.

 

And out of that 13, it only takes about 11 from almost empty. I think if I refil when there is only one bar left it is 11 gal.

But the car averages 26 mpg, so less in city stop and go traffic. It is not a great fuel economy car in terms of a 2L 4 banger.

But the numbers you gave is not a good way to tell much really. Fill it up till it clicks, drive for a few days, fill it back up till it clicks and see how much fuel it took. Use that and the mileage between the two to see what fuel economy you are actually getting. Then do that a bunch of times and average them. A single tank is not a great sample size as mileage varies quite a bit really.

 

You can also use the Fuelly app. to track your mileage if you don't want to have to calculate things yourself.
I get about 20 around the city which is fine for this kind of high revving car. Mine has 426WHP so I think it's very reasonable with that considered.
Highway I'll get high 20's. I think best I've gotten was 29mpg on a road trip... and that was after I had supercharged the car.