A fuel efficient car in the roads as of now typically gives about 40 miles per gallon of gas. The fuel economy or how efficient a car is now is measured by the distance it can go with a gallon of gas plainly referred to as miles per gallon or MPG. A hybrid car such as a Toyota Prius gives about 52 to 57 miles per gallon but this, like with every other car, depends on how one drives, the speed, terrain, traffic, weather conditions and the state of the car.
Measuring car efficiency: It’s not entirely about gas
The physics that determine a car’s fuel consumption as very complicated but we as humans do not like too many variables so we just work on assumptions – the assumption that the driving conditions are uniform, the weather is perfect, the drivers drive the same way with the same speeds and everything else is uniform.
Today, we see all sorts of cars entering the market that do not typically run on gas alone. Some cars even run on electricity alone – which means that they are not refueled with gas. Based on how we rate fuel economy, can we say that these cars offer infinite mileage per gallon of gas? Certainly no because this would imply that it costs nothing to run them yet electricity costs money. Therefore, to accommodate hybrid and electric vehicles, there has to be a new system of measuring car economy that will accommodate gasoline powered, hybrid and electric cars for easy comparison.
Has MPG been effective? Can’t we stick to it?
First off, the use of miles per gallon (mpg) as a unit to measure fuel economy has been wrong ever since it became too mainstream. This is because drivers typically fuel their cars with a destination in mind, they get x gallons of fuel depending on how far they are going. MPG would imply that a driver would put x gallons of fuel in the car and drive to see how far the fuel would take them. It would therefore have been more accurate to use miles per gallon to give the drivers a more accurate and realistic figure to work on when they are fueling their cars.
What measure would work for cars that run on gas and electricity alike? The best approach would be to measure the power or energy that a car requires to travel a particular distance. Using a universal energy unit and applying it to cars is the best way to go. The most common energy units are Btu, Therm, Joules, Calories and kilowatt hours.
The numbers behind Kilowatt Hour units
Kilowatt hour unit is the most practical to use in such machines as car engines. Converting ordinary mpg mileage to kilowatt hours is not that complicated. 1 gallon of gas would produce about 36.6 kilowatt hours of energy when combusted. This means that a car with fuel efficiency of 40 miles per gallon requires 36.6 kilowatt hours of energy to drive for 40 miles. This translates to 0.915 kilowatt hours per mile or 0.915kWh/mi. Since bigger numbers are easier to work with, the American standards should go the European way and use kWh/100-mi units. This would give our sedan above 91.5kWh/m efficiency.
Electric cars and hybrid cars’ efficiency will be easy to calculate because their power consumption is already in kWh battery. For an electric car with a 10 kWh battery, if it can go for 25 miles, the consumption would be 40 kWh per mile or 40 kWh/mi.
The use of kWh/mi as a unit of measurement is more accurate and covers hybrid, electric and gas powered cars. In this age when hybrid and electric cars are becoming so popular and the world needs a simplified way to compare power consumption to calculate running costs, MPG is being rendered obsolete and a new unit of measurement has to be adopted.
About the Author
James McDonnel contributed this guest post. James is an automotive enthusiast and a freelance writer. He writes for wish.co.uk and he enjoys rare driving experiences like the ones they offer on this website