In case Bob's math and graphs intimidate you:
I get 4.9 l/100 km or 48 MPG at 110-115 km/hr or around 70 miles per hour. I have a new Prius and that was with the stock tires inflated to 40PSI front and 38 PSI rear (tire pressure affects mileage). Wind will effect the mileage.
I use the 10% ethanol fuel. Toyota says don't use any more than 10% ethanol. If you do, you will put a lot of very expensive fuel components at risk. Such as the fuel tank (around $1000 to replace - it has a rubber bladder inside).