I would assume the car you drive and its miles/gallon capability is more important than the price of petrol for keeping prices down. A small Renault Clio will go above 50 miles/gallon but a lot of much heavier (and more expensive) cars can be below 20.
That's true. My Camry gets around 30 mpg, but most trucks and SUVs get more like 15 mpg. Some SUVs get as much as 20 mpg. Hybrid cars and trucks get better milage. Chevy as advertizing a large SUV which gets over 30 mpg, but I would bet on it doing that.
Another distinction is city driving vs highway driving. If you are in the city, starting and stopping, you will get much worse mileage than when you are on the highway and driving at a constant speed.