http://toyota.jp/86/001_p_001/spec/spec/index.html
Has different fuel economy depending on model and transmission but it looks like the G model (probably closest to scion FRS spec) will do 12.8 L/km (AT) and 13 L/km (MT) which converts to 30.1 (AT) and 30.6 MPG (MT) (US) combined cycle (urban and highway average).
so, fairly reasonable fuel economy.