Not very. You can check by filling up to the brim, writing the mileage on the ticket, then filling up to the brim again after 300 miles or so and writing that mileage on the ticket. Multiply the litres on the second ticket by 0.22 to get gallons, subtract the first mileage from the second mileage then divide the result by the gallons. Of course you then need to allow for odometer inaccuracy, but Mazdas are generally very close to accurate.