The engines in modern cars can last for more than 200,000mi. Heavy-duty trucks usually don't have life spans as long as cars. Going up steep hills often could shorten your vehicle's life as well. However, repairing is usually much less costly than buying a new vehicle. Your vehicle's gas mileage will go down over time, but regular maintenance can keep it as high as possible. Your mileage will also decrease when you tow a trailer or use the heating or air conditioning.