This may sting a little, but it is key to getting an accurate reading. Before even pulling away from the pump, write down your current mileage. We will call this Mileage A. To get as accurate a reading as possible, drive until the tank is less than half full. The longer you can go before getting gas, the better. Try to use the same station using the same pump as the first fill up, as pumps may be calibrated differently. This time, pay attention to how many gallons it takes to fill up the tank. This is usually shown at the pump.  We will call this Gallons. This number we will call Mileage B. The formula for determining your MPG is this:  MPG = (Mileage B - Mileage A)÷Gallons. Subtract Mileage A from Mileage B. This will give you the number of miles you drove since your last fill-up. Divide your answer by the number of gallons (Gallons B) it took to fill up your tank. This will give you your car's MPG.

How would you rephrase that briefly using Chinese?
到加油站加油，给油箱加满。 记录下里程表数。 正常驾驶汽车。 再一次给车加满油。 再一次记下汽车的里程数。 然后我们要做数学题了。