Driving for Fuel Economy

The EPA ratings for the Prius are 52 m.p.g. city, 45 m.p.g. highway and 48 m.p.g. average.  In most cars, simple economical driving techniques allow you to exceed EPA ratings.  Since the Prius already embodies economy, you may have to try harder in this car to exceed the ratings.  Worse, you may find that you get poorer economy than the ratings imply.  Before the first grumble leaves your lips, ask yourself this - what fuel economy was I getting in my previous car, as compared to EPA figures?   Don't know?  In this case, the Prius may just be giving you a gentle hint that your driving technique could be improved.

Techniques Applicable to Any Car

The next few paragraphs describe economical driving techniques applicable to any car, not only the Prius.

Staying Off the Brake

The braking system of a car is designed to reduce its speed.  Since a car travelling at speed stores energy in its motion (kinetic energy), to reduce speed, that energy must be converted into some other form of energy.  Neglecting regenerative braking for now, brakes use friction to convert that energy into heat.  The heat is lost into the surroundings.  So, the brakes take energy that came from your fuel and throw it away as heat.  To the exact extent you can avoid using the brakes, you can save fuel.

Of course, slowing down and possibly stopping is necessary for safety, conformance with rules of the road and comfort when arriving at your final destination.  Even so, simple strategies exist for reducing how much slowing down you have to do and slowing down other than by braking.

First, anticipate the need to slow down and get off the gas pedal immediately.   The gas that enters your engine after you've identified the need to slow down is for the most part wasted.  So, look ahead for red lights, stop signs and the brake lights of cars in front of you.  Release or ease up on the gas pedal so as to lose speed by coasting.  While coasting, you're still converting motion to heat using the resistance of the air and your tires.  However, this is energy you'd have to use up anyway to get where you're going.  The point is to minimize additional wasted energy by avoiding or delaying applying the brakes.  Your goal is to coast gently to a stop or have the reason for slowing down clear itself before you get to it.  The opposite of what you're trying to do is staying on the gas until just before you need to stop or slow and then braking hard.  Do you do this to get to the front of the line at red lights?  Or to indicate that you want someone to move out of your lane?   Consider not doing this.  True, others will do it and to some extent you'll have to play along for safety.  However, you can make a big difference if you can separate your self-esteem from your road speed.

Second, try to avoid routes that require a lot of slowing down and stopping, such as roads with stop signs at every block, lots of traffic lights or congestion.  The repeated slowing down is one of the things that makes the EPA rating for city driving lower than for highway driving in a conventional car.  Try to find a route along which you can drive at a steady speed.

Finally, bear in mind that braking is the enemy, not acceleration, even though the fuel is actually drawn into the engine during acceleration.  This energy ends up as motion and it is only when you destroy this motion by braking that the energy is wasted.  To replace the motion energy, you'll have to accelerate again and if this could have been avoided by not braking, it's wasted fuel.  So, economical driving doesn't deprive you of all the fun.  By all means, accelerate hard when you need to increase speed.  Just don't get yourself in the position of having to brake again soon afterwards!

Avoid Excessive Speed

Auxiliary Power Drains

Air conditioning.

Preserving the Vehicle's Aerodynamics

At steady speeds on a level highway, most of the energy used to move a car is spent against aerodynamic drag.  The air in front exerts pressure as the car forces it out of the way.  Surprisingly, a greater force is exerted by the air trying to come back together behind the car and this tries to suck the car backwards.  Manufacturers try to reduce drag by careful design of the car's shape, avoiding bumps, etc.  A measure of their success is the "drag coefficient", which will be smaller for a more successful aerodynamic design.  The Prius does quite well with a drag coefficient of 0.29.  (The Honda Insight puts it to shame with a figure of 0.25, but the two-seater format gives the designers more choices than the five-seater Prius.)

Changes you make to the shape or surface of the car are likely to increase the drag coefficient (unless you've designed them to decrease it, such as removing door mirrors).   Even worse, protrusions both increase the drag coefficient and the cross-sectional area of the car that has to be propelled through the air.  This will reduce fuel economy and should be avoided when not necessary.  So, don't keep a rack or carrier on the car when you're not using it, don't open the windows needlessly and don't let the dog hang its head out.  At speed, running the air conditioning with the windows closed is thought to be better than having the windows open.

Unnecessary Weight

During acceleration and going up hills, most of the energy used to move a car is converted to kinetic energy (speed) and potential energy (elevation).  Both increase with the mass of the vehicle.  Since all this energy will be eventually lost when you reach your destination, it pays to minimize it.  Therefore, don't carry around unnecessary weight in the car.  It takes fuel to accelerate it to speed and to raise it over hills.  Take it out of the car and leave it at home instead.  And look at you!  You could do with losing a few pounds, as well!

Correct Tire Inflation

Because tires are not rigid but flex to absorb minor bumps, it takes energy to roll them along.  How much energy depends on several things, but at least one of them you can control - the tire pressure.  The higher the pressure, the stiffer the tire and the less energy it takes to roll and the better will be your fuel economy.  Now, tires have a maximum pressure rating and the manufacturer of your car will tell you what they consider to be the correct pressure.  So, you might not want to put more pressure in than is recommended.  But, you should definitely avoid having less pressure!  Not only is this bad for your fuel economy, but it is bad for the tire and dangerous too.  By the way, the maximum pressure of the OEM tires on the Prius is 50 p.s.i., which is much higher than Toyota's recommendation (35/33 p.s.i.).  Many Prius owners run with higher pressure such as 40/38 or 42/40 and get better fuel economy as a result.  Some find the ride a bit stiffer, but not objectionably so.  Some people use pressures in the high 40s.  No one has reported a problem resulting from higher pressure in the tires (other than the dealer's service departments letting out the extra air).  Unless it makes you uncomfortable, I suggest you try 40/38 or 42/40 in your Prius, assuming you have the OEM Bridgestone Potenza tires.

Proper Vehicle Servicing

In additional to tire inflation, there are many aspects of a car that can degrade economy if allowed to fall out of specification.  An example is wheel alignment, which, if not properly done, can cause the tires to "scrub" the road, increasing fuel consumption as well as tire wear.  Not to labor the point, keeping your vehicle properly serviced is important to fuel economy as well as peace of mind.  I'm not saying that it's easy to find someone to "properly" service your vehicle and I'm aware that improper servicing ruins your peace of mind.  We have to hope that, in the case of the Prius, at least, "Toyota Cares".

Special Measures for the Prius

The next few paragraphs describe economical driving techniques that are applicable to the Prius and similar cars.

Regenerative Braking

The Prius, Honda Insight and pure Electric Vehicles have regenerative braking systems.   When the brake pedal is pressed, the drive motor is used as a generator to create electricity that recharges the batteries.  The force needed to drive the generator slows the car.  Motion energy is converted into electrical energy and stored for later re-use, instead of being converted into heat and wasted.  However, this is not a perfect process and some energy escapes and does not reappear later to help drive the car.  Also, regenerative braking is always supplemented with conventional friction brakes for heavy braking and for use at low speeds when the generator does not work so well.  So, staying off the brake is good advice even if you have regenerative braking.

To maximize the use of regenerative braking and therefore minimize wasted energy, you should try to brake gradually rather than forcefully.  This avoids the mechanical brakes coming on and provides an energy flow that the re-charging system can use efficiently.  So, in addition to getting off the accelerator when you're going to have to slow or stop, you should try to immediately apply the right brake pressure to do the job.  Without this consideration, you might coast for a while and then brake forcefully.  This would be fine in a conventional car.  With regenerative braking, it is preferable (for fuel economy) to brake evenly over the remaining distance to the obstacle.

Combining Short Trips

After you turn a Prius on, it runs the gasoline engine for a while.  The purpose of this is to get the catalytic converter and the engine up to operating temperature.   During this time, you won't get much better fuel economy than a similarly sized conventional car.  If your driving consists of lots of short trips, taking five or ten minutes, with hour-long pauses in between, you will be disappointed with the Prius fuel consumption.  If you have the option, combine short trips and minimize the waiting time between trips when the engine and catalytic converter cool back down.

The off time that begins to affect economy depends on the air temperature and is shorter in winter.  In hot weather, cool down takes a long time and warm up is fast so stopping for half-an-hour will make little difference.  In cold weather, stopping for ten minutes will likely cause five minutes of poor economy when you get going again.   Of course, you may be stuck with this pattern of driving.  You won't do worse than a conventional car and the Prius has many advantages other than economy (e.g. SULEV).   Just be aware of what is happening and don't give other people the wrong impression.

Pulse Driving

Pulse driving is an advanced (probably extreme, definitely eccentric) technique that may (or may not) increase fuel economy (at the expense of comfort and safety). You can get some details by searching for the phrase "pulse driving" in the yahoo discussion group toyota-prius, but you probably won't want to after reading the following.

Suppose you want to get from A to B on a clear road at an average of 60 mph.  A normal person would get the car up to 60 and more-or-less stay there for the journey.   A pulse driver would floor the accelerator until the car reached 70 mph and then release it completely to stop the ICE.  Then he'd tickle the pedal trying to get the car to coast with a bit of electric power but not starting the ICE.  When the speed drops to 50, he'd floor it again back up to 70 and repeat.  He averages 60 (assuming he doesn't get stopped by the police) just as you did, but arrives at the destination having used (he claims) less gas.  Of course, he may also be totally stressed out from concentrating hard and his passengers may hate him.  You may be able to tell that personally I'm not in favor of this kind of pulse driving.

Hard Acceleration

Some advice for economical driving includes recommendations against hard acceleration.   Personally, I can find no justification for this other than the fact that drivers who tend to accelerate hard also tend to drive fast and wait until the last moment to brake.  I do not think that gradual acceleration improves fuel economy.  To get to any given speed, you need a certain amount of energy from the engine.  The total amount does not change with how long you spend getting up to this speed.  The energy you get is the amount of fuel you use multiplied by the engine efficiency.  If the efficiency is constant, therefore, you need the same amount of fuel to get to any given speed whether you accelerate gradually or hard.  In actual fact, the engine efficiency is quite likely to be slightly better at high power demand.  This would result in better fuel economy for hard acceleration.  However, my advice is to accelerate at whatever rate makes you comfortable.


Last edited February 7, 2004.  All material Copyright  2001 - 2004, Graham Davies.  No liability accepted.