Improving Hybrid Mileage Guide

The following article on how to improve mileage in a hybrid was written for Mapawatt by the Sustainable Living enthusiast Guy Marsden.


The Prius has become the ubiquitous symbol of the hybrid vehicle market, and recently I have met three new Prius owners who surprised me by displaying a complete lack of knowledge of how the vehicle worked. They did know that it got better gas mileage but not how or why. I have heard auto dealers refer to this as the “feel-good purchase”.

My wife and I have been driving hybrid vehicles for 10 years now, she bought her Honda Insight in 2010. That sporty little two-seater used to get over 60 MPG but with over 100,000 miles on the battery it is now getting about 53MPG. My 2006 Ford escape hybrid SUV gets 30 to 33 MPG, and I occasionally hypermile it up to 40MPG. So we are very familiar with the hybrid driving experience and strategies for optimizing the performance of our vehicles to get the best possible miles per gallon.

All hybrid vehicles have both an internal combustion engine (ICE) and an electric motor. The electric motor gets its power from a large 350 Volt battery in the back of the vehicle. In many hybrid vehicles such as the Prius the ICE is deliberately underpowered and designed to operate most efficiently at around 2000 RPM, this means that it's rather inefficient during acceleration. So the electric motor is used to augment the ICE during acceleration. This means that energy is being drawn from the battery to run the electric motor anytime you're accelerating, the faster you accelerate, the more battery power is used. Of course most hybrids can also operate entirely in electric mode for several miles and this draws a lot more power from the battery. (The Honda Insight is considered a mild hybrid and cannot run in electric mode entirely).

So where does all this battery power come from? Well it comes from two places, once the vehicle is up to speed and cruising along the ICE is running a small generator that will top up the battery slowly if it is low. But more significantly the electric motor is used as a generator anytime you take your foot off the gas or apply the brakes. This is called regenerative braking. If you take any DC motor and turn it, it will generate power, and the faster you turn it the more power you get. So in a hybrid vehicle most of the energy that goes into the battery comes from regenerative braking. This is free energy that is being recovered from the drivetrain while the vehicle is slowing. When you put your foot on the brake the vehicle's computer decides how much energy it needs to capture based on how low the battery is and uses a blend of the actual physical brake pads and regenerative braking. The physical brake pads and rotors in hybrid vehicles last nearly twice as long because they're actually used a great deal less. Understanding this key concept is essential to understanding how to drive a hybrid vehicle to optimize its performance.

Since regenerative braking is essentially free energy the more you can use it to maintain a high charge on your battery while driving, the less gas you will need to use while accelerating.  You will also benefit from longer EV range. With all electric vehicles and most hybrids on the road today you need to learn how to optimize your braking strategy to recover power and maintain an optimal charge in the battery. So you need to learn to think that you are rewarding the vehicle by braking. This means planning every stop as far ahead as possible and riding the brake for as long as possible until you come to a gentle stop. Hybrid vehicles all have a speed threshold at which they will turn off the gas engine as the vehicle is slowing, the sooner you can get the ICE to shut off the more gas you will save. If you just slam on the brakes right before you need to stop the ICE will not shut off until you have completely stopped, and you will lose the benefit of charging the battery.

You also need to pay attention to hills -- after you crest the top of a hill you can ride the brake pedal very gently to charge the battery without necessarily slowing the vehicle or wearing down the brake pads. For example when I drive into town in my Ford Escape Hybrid I cross a large bridge and in the middle of the bridge I start to ride the brake pedal on the downhill slope to top off the battery so that I can do all of my in-town errands (a round-trip of up to 2 miles) entirely in electric mode. If I had not paid attention to fully topping up the battery using this braking strategy I would not make it all the way through my errands in electric mode. So you need to become highly conscious of opportunities for regenerative braking and when the ICE is running. You also need to be conscious of the battery state of charge; there is usually a gauge or bar graph or some type of indicator showing how full the battery is. Knowing the battery state of charge is important so you can estimate how far you can travel in EV (Electric Vehicle) mode.

The flip side of all of this is to pay attention to conserving gas by reducing the use of the ICE. One of the simple tips that I offer to new hybrid drivers is to tell them to pretend that there is an egg under the accelerator and an egg under the brake pedal and to drive gently. This is not to say that you have to drive slowly at all times, it simply means accelerating gently and braking long and slow. Hybrids all shut off the ICE when you stop and most will take off entirely in EV mode up to a certain speed, the faster you try to accelerate the sooner the gas engine must kick in. By accelerating gently you optimize the use of the electric system to reduce your gas consumption because this delays when the ICE starts.

Another tip I give to hybrid drivers is to reverse your thinking from gas engine driving and see how high off the accelerator you can hold your foot at all times. Pay close attention to every hill and lower your foot to the minimum necessary to maintain speed on the uphill side, and lift your foot entirely from the accelerator on the downhill run so that you are coasting all the way down, or braking gently as I have mentioned before.

Ultimately learning to optimize a hybrid vehicle requires that you drive consciously; this is a real departure from the fossil fuel paradigm. With a gas vehicle the only time you really pay attention to gas consumption is when you fill the tank. With a hybrid vehicle you can significantly increase the performance by constantly paying attention to how you are using the stored energy in the battery and recovering it, when the ICE is operating, and when you are in EV mode. A number of electric vehicles are hitting the market this year and the same strategies will apply to optimizing EV range.


To recap, here are some of the key points Mapawatt has taken from Guy's excellent article:

  1. Think that you are rewarding a hybrid by braking
  2. Become highly conscious of opportunities for regenerative braking
  3. Be conscious of the battery state of charge
  4. Pretend that there is an egg under the accelerator and an egg under the brake pedal and to drive gently

Follow these steps to maximize your hybrid's performance!

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"pretend that there is an egg under the accelerator and an egg under the brake pedal and to drive gently" While these are all great tips, I've found this one to be less useful in my '10 Prius. After doing some reading of hypermiler blogs, and some real world testing, it seems that even with the reduced fuel consumption of slow accelleration, you're still using the ICE for a fairly long time to get up to speed; ie. still consuming fuel. As the author said, the engine appears to operate most efficiently at ~2k RPM. Unfortunately, there isn't an RPM gauge to stick to. As evidenced by the 'pulse and glide' technique, you reduce overall run-time of the ICE by accellerating fairly briskly, and make up the extre fuel use by using the electric motor and low rolling resistance to 'glide' the car down to a lower speed. Doing this, you can really boost the mpg. Of course you don't want to attempt this method on a busy road, but if you're on your own, pulsing and gliding from 55 to 65 mpg on at 60mph freeway can easily boost mpg some 20%. Using this technique, I've been able to get mixed driving, tank-average of 64.4 mpg. Also, depending on the cruise control and regen braking of the vehicle, it's much easier to set a speed when driving downhill and let the car's cuise control modulate the regenerative braking to maintain a speed. That way you aren't flashing your brake lights to the cars behind you, and you're maintaining a set speed.
Good points about pulse and glide (readers please see the link in the article), the effectiveness of this technique varies widely with road conditions, and seems to be best on flat roads in my experience. While hybrids will automatically use regenerative charging when going down hill in cruise control, you can get more charge by braking. I use this strategy primarily on rolling 2 lane blacktops at speeds below 50MPH, and use cruise control mostly on long clear stretches of freeway at 60+. I have also noticed that I often get better fuel economy with a heavily loaded vehicle on the freeway compared to driving solo. This seems to defy conventional wisdom and my only rationalization is that you have more inertia so the vehicle is not surging back and forth between charge and EV as much.

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