Same issue here. My Prius just had its 1 year anniversary.(38k miles) and the very first tank of gas was 10.4 gallons. since then i have never put more than 9.45 gallons in it. And yes even when i ran it out of gas and had to drive almost 12 miles on ev, i only put in 9.3 in it. Weird...the way i look at it, as long as im still getting the milage, so what, ill just have to fill up a day earlier is all. Dean