Take a look at the "Compare" tab for the Prius II experience. Mileage vs temperature is available in the data.
I have an 03 Prius and am very happy. But our other car is a Toyota Echo and our Prius was bought to replace our 91 Camry. Still, you're taking the right approach on test driving.
In 2001, we rented a Prius for two days to get a real handle on it. But the price-performance didn't make sense so we bought the 01 Echo. Since then, used Prius have became available which is how we got our
03 Prius.
Bob Wilson