How can a poorly tuned engine affect fuel consumption?

• A poorly tuned engine can decrease fuel consumption by up to 50 per cent.
• A poorly tuned engine can increase fuel consumption by up to 30 per cent.
• A poorly tuned engine can increase fuel consumption by up to 40 per cent.
• A poorly tuned engine can increase fuel consumption by up to 50 per cent.
  Explanation:

  "A poorly tuned engine can increase fuel consumption by up to 50 percent."