Hi,
I have two MPG vs miles per hour charts that map out what the NHW11, 01-03 Prius does. Treat these as the low-end of what the current NHW20 Prius achieves because Toyota tweaked key elements of the NHW20 to make it a better hybrid, 52/60 versus 45/52 EPA (original, highway/city.)
NHW11 and NHW20 MPG vs miles per hour
This first chart consists of my earliest 2003 Prius data. When I flew to Texas to pick it up, I asked the flight crew for the number of passengers and fuel burn to calculate the end-to-end, MPG of flying out. On the way back, I broke up my trip on I-20, a nearly flat Interstate, into 2-3 hour segments and using cruise control, measured my MPG per segment as a function of miles per hour. From this and subsequent commuting trips, I learned there are two "MPG" hits:
- 65-70 miles per hour knee - at about 70 miles per hour, the engine has to spin faster than idle to keep MG1 from exceeding the 6,500 rpm speed limit. Even at highway speeds, downgrades and speed changes can result in the ICE throttling back to idle or even being spun by battery power. But above 70 miles per hour, the ICE has to run faster than idle and that really hurts higher speed performance.
- 42 miles per hour control law transition - below 42 miles per hour, the engine can stop and the car proceed on just battery power if the load and state of charge allows it. This ability to shutoff the ICE significantly saves fuel. Above 42 miles per hour, a different set of control laws apply that require the ICE to be spinning at all times. But transition between these two laws can lead to excessive ICE starting and stopping energy losses.
This chart also has a set of NHW20, current model, high speed data points captured a year ago last summer. The NHW20 has a higher MG2 limit, 10,000 rpm, which occurs around 85 miles per hour. Not shown but I've measured my NHW11 2003 Prius at ~21 MPG at 100 miles per hour, not quite as good as the NHW20.
The three red lines represents the 45/52 EPA rating for my NHW11 and the last one the 52/60 EPA rating (old style) for an NHW20. The average city speed under the old EPA rating was 28 miles per hour and 48 miles per hour on the highway test.
NHW11 Drag and Thrust Chart
Thanks to Ken@Japan pointing me to a Toyota drag formula, I have plotted the amount of power, the horsepower, needed to maintain speed of an NHW11 at any given speed, the 'red line'. If we assume the ICE is performing at 31% efficiency, the 'blue line' shows the expected MPG. Then if we factor in the vehicle overhead, the power needed for the daylight running head lights and control computers, we get the 'gray line.' These three curves represent the theoretical performance of an NHW11 Prius on a standard day. But then comes the question of how does this compare to experimental data.
I've added data points from my tests as well as open source reports from the 2005 Prius marathon, an NHW20. It is very difficult to get reproducible results at low speeds because such long test intervals are required. Worse, at speeds below 23-24 miles per hour, it has to be maintained manually and that is fatiguing. But to the best of my ability to measure and factoring in lower than normal temperatures, there is good agreement with these curves.
The one problem is ICE efficiency is not a constant 31% but varies across the rpm and power range. Toyota has reported the ICE efficiency has a maximum of 37%, not the 31% I used.
NHW11 ICE Efficiency
Using data recorded by a Graham scanner, I plotted the shaft power output as a function of relative fuel consumption. What this chart shows is the maximum fuel efficiency occurs between 1,400-2,600 rpm. From 2,600-3,300 rpm, it falls a bit and then really drops off above 3,300 rpm. Above 4,150 rpm, there is another drop. This has several implications for cross-country travel.
It is possible, at higher speeds, to adjust the speed under load to keep the vehicle in a high efficiency, power range. This means slacking off on the speed going up grade but making up for this on the down grade. This is called terrain driving and can lead to higher, average speeds, estimated to be a +5 miles per hour, with efficiency. However, it requires manual speed controls and I find this fatiguing. My practice is to set the cruise control to 65-67 miles per hour and drive relaxed.
In windy conditions, reduce the speed going into head and side winds but increase the speed with a tail wind. This can also result in excellent MPG at some cost of time.
Driver Fatigue
Driver fatigue is a real problem because it can leave to being unable to function effectively at the destination. I've done six, 700-750 mile trips taking 11-14 hours using cruise control at 65 miles per hour and following trucks at a safe distance. I found no problems doing more things when I arrived. This past summer, I drove for 20 hours to cover 1,000 miles and though I needed to sleep when I got home, I was perfectly fine the next day. The combination of low noise, vibration and comfort makes such long trips perfectly feasible and safe.
Fuel Quality
The quality of fuel, the energy per unit, does impact vehicle mileage. In the Huntsville AL samples, this ranges from 3-15% depending upon summer/winter blend and factors beyond our control. Attempts to use fuel density to predict energy density have not been successful with the 3% difference seen in summer blends. I have not found ethanol labeled gasoline in Huntsville to test.
CONCLUSION
It appears that 65 miles per hour is an excellent Prius cruise speed and delivers better than 50 MPG in a well tuned car (i.e., oil, tires and minimal AC demands.) At speeds of 70+ miles per hour, mileage will quickly fall to 40 MPG or less and adding additional loads, AC, can drop it even further. Curiously, one HCH driver also reports a performance knee at around 60-65 miles per hour. Of course the ultimate, high-speed, maximum efficiency machine is an Insight with a manual transmission but they are not easy to find and they have limited payload capacity.
Bob Wilson