Question: The speed of your automobile has a huge effect on fuel consumption. Traveling at 65 miles per hour (mph) instead of 55 mph can consume almost 20% more fuel. As a general rule, for every mile per hour over 55, you lose 2% in fuel economy. For example, if your automobile gets 30 miles per gallon at 55 mph, the fuel consumption is 21 miles per gallon at 70 mph. If you take a 400-mile trip and your average speed is 80 mph rather than the posted speed limit of 70 mph, what is the extra cost of fuel if gasoline costs $4.00 per gallon? Your car gets 30 miles per gallon (mpg) at 60 mph.