Currently here in Southern California, it’s not uncommon to drive by the gas station and see some 6 or 7 dollar figures lit up on the sign, warning you not to wander off too far, unless you want to buy some Saudi oil prince a new Rolex. Let’s take a look at how we can maximize our fuel efficiency, so we’re only throwing in enough for like half a Rolex:
Utilize Your Prius Prime Power Options
Keep your car in Eco Mode everywhere you go. This keeps the climate control system from using as much power, but also dampens the Prime’s motive response to the accelerator, especially in the first half of its decent towards the floor, as you push down on it. This allows for slower accelerations towards a desired speed, and thus less power needed to get to such, which I’ll elaborate on, later in the article.
Contrary to internal combustion engine logic, the Prime gets better fuel economy in city conditions – stop and go traffic – so it might be best, for long distance highway driving, to charge up before your trip, then burn those electrons off on the highway in either EV Mode or EV Auto Mode, the latter of which keeps you mostly in electric mode but uses the combustion engine to assist in situation where more power is needed.
However, what you don’t want to happen on the freeway, is that your battery is fully charged, while you’re in hybrid mode, trying to save those electrons for later, when you are driving down a long slope, like a mountain, and miss out on that battery power regeneration; that particular condition can get you an extra 25-28 miles of free charge if you play your cards right. So be aware of the driving conditions on the horizon and drive accordingly.
Driving Speed Is Everything Regarding Fuel Economy
The equation that governs the power needed to push a Prius Prime forward against the resistance of the air in front of it, and gravity induced rolling resistance, per kilometer, on a perfectly flat road, is as follows:
P = Cr*m*g*d + (1/2)*p*Cd*A*d*V^2; where:
- P = power needed to push Prius Prime 1km on a perfectly flat road, in joules
- Cr = rolling resistance coefficient = 0.03
- m = mass of a Prius Prime = 1,530kg
- g = 9.8m/s^2
- V = velocity you are driving at in m/s
- Cd = drag coefficient for Prius Prime = 0.24
- p = air density = 1.2kg/m^3
- A = frontal surface area for Prius = 23.9 ft^2 = 2.22 m^2
- d = distance traveled = 1000m (equal to 1km)
P = 449,820 Joules + 316.8*V^2 Joules
The first thing you’ll notice is that you have to square the Prius Prime’s velocity in order to solve for the power needed to push it forward at a steady speed on the highway over 1 km. That means, as desired driving velocity increases linearly, the power needed increases exponentially.
| Speed on Highway (mph) | KJ of Power per Kilometer | Miles You Can Drive in EV Mode at Full Charge | Miles You Can Drive in HV Mode With 10 Gallons of Gas |
| 60 | 677 | 21 | 483 |
| 65 | 717 | 20 | 456 |
| 70 | 760 | 19 | 430 |
| 75 | 805 | 18 | 406 |
| 80 | 855 | 17 | 383 |
| 85 | 907 | 16 | 360 |
If you elect to drive 85mph instead of 60mph, you’ve cut your EV range by 5 miles and you HV range by 123 miles. (Based on a perfectly flat highway; 40% thermal efficiency of the HV; and 95% efficiency of the EV). Also, these are just theoretical numbers. Real world numbers are a little higher because of regenerative charging when you brake or go down a hill. My Prius Prime was logging in about 58mpg on my last camping trip, about 100 miles from home when I was driving 65mph on average. Just the main thing to note is the differences in the numbers.
Rate of Acceleration to a Desired Highway Speed is Important Also
The equation governing the power needed to accelerate to a desired speed is as follows:
P = m*a*V; where
- P = power needed in joules
- m = mass of a Prius Prime = 1,530kg
- a = acceleration = dV/dT
- dV = change in velocity in m/s (example going from 60mph (26.8224m/s) to 70mph (31.2928m/s) would be a difference of 4.4704m/s or 10mph)
- dT = time it took to change between the two velocities
- V = final velocity
Variables the driver can control here are dT, the time it takes to accelerate to a desired speed, and V final velocity desired. So it’s best to accelerate slow and steady, and target a slower driving speed.
Recognize When You’re In Battery Usage Mode Versus Combustion Engine Usage Mode On the Freeway
It’s important to know how your car is being powered at any moment in time, when you are in hybrid mode, or to a lesser extent, ev auto mode, on the freeway. Often if you’re driving on a newly flat area of the road, or at a small decline, you could be using the combustion engine a bit more than necessary, just from the act of holding the accelerator in a fixed position for long periods, as comes naturally when driving on the freeway. Basically once the car’s combustion engine is in use, the car won’t retest its acute necessity until you remove all pressure from the accelerator, telling it that conditions have changed.
To test, in the moment, if you can shift the motive burden to the battery, while in hybrid mode, rapidly take all pressure off the accelerator, which will halt the use of the combustion engine, then slowly reapply it, back to roughly where it was, without such triggering the combustion engine to restart again. Then see if you can maintain your speed, or gradually decelerate down to an appropriate speed, if you caught yourself going too fast. By using this trick over and over, you can save a little bit of gas.
Try to Avoid Installing a Roof Rack and Overhead Cargo Carrier at All Cost
I’ve covered which roof racks and cargo carriers work with the Prius Prime, in my Prius Prime car camping article here: How To: Prius Prime Car Camping at Reserved Campgrounds. Also in that article I show you how to avoid using one, by packing your car out fully with your camping gear.
Installing a roof rack and overhead cargo carrier will increase your drag coefficient and frontal surface area significantly. The Yakima Skybox 16 has roughly a 34×17 square inch frontal surface area, just from looking at its dimensions. This adds 0.3729 meters squared to the Prime’s frontal surface area. Also, based on this theoretical car’s drag coefficient, with and without a streamlined roof box, the Prime’s drag coefficient would probably go up to around 0.293 with a Yakima Skybox 16. The car in the model had a Cd of 0.254 when naked, and 0.307 when the box was added. Let’s look at a comparison of 65mph driving with and without the Skybox 16 on a flat, level highway:
| Prius Prime Driving 65mph | Miles You Can Drive in EV Mode at Full Charge | Miles You Can Drive in HV Mode with Full Tank of Gas |
| Naked | 20 | 465 |
| With Yakima Skybox 16 | 17 | 387 |
Again these numbers are just theoretical; real world numbers are higher because of regenerative charging when braking and going down hills. But just note that the Skybox 16 may shave off about 78 miles from your tank of gas, and 3 miles from your charged battery. So look for about a 8mpg reduction in fuel economy in HV Mode.
Don’t Run the A/C For Long Periods of Time in EV Mode When Stopped At High Ambient Temperatures
I started collecting data on this for people that want to sleep in the car at night when camping. Running the A/C even in Eco Mode at high ambient temperatures, especially in the day, when there is solar gain happening will eat your charge fairly quick. Check out my data collection on this subject here: Toyota Prius Prime Battery Discharge Rate When Sleeping In Car With AC Running. Based on the data I’ve collected so far, you can use up a full charge in 3 hours in daytime, solar gain conditions. Night time, low ambient temps fair much better, but don’t expect to be able to sleep for several nights on one charge.
Keep Your Tires at the Appropriate Pressure
The Prius Prime 2017 to 2022 requires 36psi for the front tires and 35psi for the back tires. This keeps your rolling resistance coefficient (Cr), in the equation I published above, as low as possible, minimizing energy needed to keep the car going forward. Also, if you can, try to keep the same model tires throughout the vehicle, and replace either all four, or either the two front, or two back tires in one replacement event.
More on Prius Prime Camping
If you found this article helpful, check out my other Prius Prime camping articles, which include anything from a Prius glamping gear haul to how fast your battery discharges when sleeping inside your Prime with the AC on.
Thanks for reading and take care!