To get an accurate estimate, you need to know which engine your specific Impala has (e.g., 3.4L V6, 3.8L V6). Once you know the engine size, you can search online (e.g., using Google, fueleconomy.gov - if US based) for the EPA estimated MPG for that specific model and year. Keep in mind that real-world MPG will often be lower than the EPA estimates.
