The modes are not explicitly intended for any specific type of road or traffic type, but enable to improve fuel economy by allowing more/less control on the request of power from the drivetrain via the gas pedal. For this reason ECO mode is better suited for city driving since there are frequent stops and accelerations, compared to e.g. highway traffic.
The modes work in the following way:
- ECO Mode: the gas pedal response is "toned down" offering the finest control possible on the Prius Gen 3; the A/C-heater functions are optimised for fuel consumption: in winter, this is achieved by keeping the ICE on less often, forcing the fan speed to be lowered to reduce the "wind-chill" effect, since cooler air is blown in the cabin; in summer, this is achieved by using aggressively recirculation, while throttling down the A/C compressor; these changes in the way the A/C-heater work, are to be seen, based on my experience, only if AUTO mode is on
- Normal mode: this mode is active when neither the ECO mode nor the PWR mode lights in the instrument cluster are on; the gas pedal is mapped in a linear fashion; no changes in A/C-heater functioning
- PWR Mode: the gas pedal is the most reactive; the ICE tends to stay on when releasing the gas pedal to improve response of the drivetrain when depressing the gas pedal again; no changes in A/C-heater functioning
- EV-only Mode: the drivetrain is forced to use the MG2 only until the SoC allows it; ECO mode/normal mode/PWR mode will still affect gas pedal response, i.e. if PWR mode is active when EV mode is engaged, the car will accelerate more promptly than if ECO mode were on; obviously this will discharge the battery faster since more powerful accelerations will require more current; EV-Mode, due to its limited range of action (2km/1.25mi at most, on a fully charged battery) is not really to be considered a "driving mode" as the 3 above mentioned ones
Additional details can be found for example here.
The following statements currently hold true regarding the modes:
- the same amount of power is available in all 3 modes, e.g. kick-down will provide the same performance in all 3 modes (there is a video showing exactly this here, where also a chart shows the gas pedal throttle response in all 3 modes); at constant speed the modes do not make any difference in the performance of the car (expect the A/C-heater changes in ECO mode, that might improve FE since less energy is required to keep the cabin at a set temperature)
- the fuel consumption might not be affected by using the 3 modes - fuel efficiency is greatly dependent on how one accelerates, traffic, type of roads, temperature, etc.; there are users on Prischat.com that claim very good fuel efficiency in PWR mode, others that cannot get a good fuel efficiency at all in PWR mode
- there are no measurements or objective tests/results showing that keeping always the same mode has any effect on the lifetime of the car as whole or on the battery/hybrid drivetrain
- there are no measurements or objective tests/results showing that the 3 modes have any effect on the way the cruise control responds to changes in road inclines
- there are no measurements or objective tests/results showing that changing the modes will change: the steering wheel response; the maximum voltage allowed on MG2 in ECO mode (e.g. 500V instead of 650V; there is such a claim for the Lexus CT200h, that shares the same drivetrain as the Prius Gen 3, between ECO/normal mode and Sport (PWR) mode); the ICE management (e.g. changes in the valve opening times, etc.) or the way the ECU provides current to MG2 (i.e. more in PWR/less in ECO) for the same acceleration/power request (same HSI bar length)
The modes are, except for the A/C-heater performance, in essence "psychological" ways to help the driver get the best FE, by affecting the way he/she accelerates the car to a certain speed.
Looking at the modes from this perspective, one could therefore say that:
- ECO mode is better suited for stop&go city traffic, where frequent stops and accelerations could lower FE if the driver accelerates aggressively (due to personal style, or traffic flow) several times in a row in short trips
- Normal mode is best suited for suburban driving with less traffic and stop&go situations, as well as for highway; good to excellent FE might be achieved in city traffic if a light foot is used, traffic flow allows for slower accelerations and temperatures do not force the ICE to be on longer than necessary to keep the battery charged;
- PWR mode is perfect to get with less "psychological" effort (i.e. no need to "stomp" on the gas pedal) more responsiveness from the drive train on winding-mountain roads (e.g. more fun drive), merging on an highway quickly without feeling that you need to kick down to get to speed, to get quicker overall response by keeping the ICE on more often
The Prius Gen 3 Owner's Manual describes the modes similarly as mentioned above (on page 173 of the U.S. Version manual).
In winter ECO mode does a better job in increasing the fuel efficiency by keeping the ICE more often off in city traffic (i.e. at traffic lights or in traffic jams) by reducing wind-chill while blowing less warm air (on average) and using recirculation more aggressively as per description above.