what mpg rating speed is
MPG For Speed .com. Unfortunately, it's true. Your car's gas mileage decreases once it gets past its optimal speed. For most cars, this is around 55-60 mph. This means that every time you go over this speed, you're essentially wasting gas and money - and creating unnecessary greenhouse gases.
According to studies backed by the department of energy, the average car will be at its advertised MPG at 55 mph. But as the speed increases: - 3% less efficient at 60 mph. - 8% less efficient at 65 mph.