FEH owner here who just got a job that requires lots of interstate travel.

It is not uncommon for me to have to drive 800 miles in one day, and I'd like to get that done in a reasonable amount of time. ( fast speed )

At 65 MPH I get mid-30's most of the time in my FEH. Up to 40 MPG if I can catch a tail wind. I'd love to cover miles quicker, but MPG suffers dramatically with speed.

So how does the Prius handle at high highway speeds? 65? 70? 75 MPH?

So how does the Prius do MPG wise at 65? 70? 75 MPH?

How does the Prius do with 10% ethanol in the gas? 20%? 30%?
( I do most of my travel in corn belt states )

I would like something that gets 40's or 50's for MPG at fast speeds, since now, I will cover about 4000 miles per month, almost all of it, over interstates with 65, 70, or 75 MPH speed limits.

I love hybrid cars, but am not very familiar with others besides Ford.
Thanks in Advance!
-John

BTW I'm open to models other than Pruis!

 

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 mph. 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 mph and 48 mph 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 mph, with efficiency. However, it requires manual speed controls and I find this fatiguing. My practice is to set the cruise control to 65-67 mph 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 mph 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

 

In case Bob's math and graphs intimidate you:

I get 4.9 l/100 km or 48 MPG at 110-115 km/hr or around 70 MPH. I have a new Prius and that was with the stock tires inflated to 40PSI front and 38 PSI rear (tire pressure affects mileage). Wind will effect the mileage.

I use the 10% ethanol fuel. Toyota says don't use any more than 10% ethanol. If you do, you will put a lot of very expensive fuel components at risk. Such as the fuel tank (around $1000 to replace - it has a rubber bladder inside).

 

I have measure the mpg-fill refill method, not the onboard display) on a couple of short trips-100 miles or so.I got almost exactly 50 mpg at 65mph-sea level.The AC was blasting the whole time(summer in south Louisiana).I see Bob's graph shows a sweet range from 60 to 69mph,and then it drops sharply at around 70 mph. I wonder if the Prius II is "clean enough" to delay that drop a couple of mph.Does anyone know how the total drag of the Prius I compares to the Prius II?The later Prius has a low coe, but the earlier one might actually have a little less frontal area-might be a wash in total drag.
The onboard display is always about 3 mpg to optimistic.
Luck,
Charlie

 

There is another discussion (with charts etc!) of Prius mpgs at high speeds here:

http://priuschat.com/lofiversion/index.php/t19900.html

 

Okay, here are my findings so far. Do these sound reasonable?

Assume 800 miles in one day.
Assume $3.00 per gallon of gasoline.
Assume 1.5 hours per trip for food, fuel, and potty breaks.

800 miles @ 75 miles per hour = 10.6h + 1.5h = 12.1 hours total trip time
800 miles @ 70 miles per hour = 11.4h + 1.5h = 12.9 hours total trip time
800 miles @ 65 MPH = 12.3h + 1.5h = 13.8 hours total trip time

Slowest vs. fastest delta t = 1.7 hours ( cost vs. savings )

800 miles @ 75 MPH and 27 MPG = 29.6 gallons FEH
800 miles @ 75 MPH and 36 MPG = 22.2 gallons PriusII
Delta fuel = 7.4 gallons Delta cost = $22.20

800 miles @ 70 MPH and 31 MPG = 25.8 gallons FEH
800 miles @ 70 MPH and 45 MPG = 17.8 gallons PriusII
Delta fuel = 8.0 gallons Delta cost = $24.00

800 miles @ 65 MPH and 34 MPG = 23.5 gallons FEH
800 miles @ 65 MPH and 50 MPG = 16.0 gallons PriusII
Delta fuel = 7.5 gallons Delta cost = $22.50

Assume 4 trips per month:
Total Cost Per Month @ 75 MPH = $355.20 FEH
Total Cost Per Month @ 75 MPH = $266.40 PriusII
Delta Cost Per Month @ 75 MPH = $ 88.80
Delta Cost Per Year @ 75 MPH = $1065.60

The delta cost for owning a FEH vs. a PriusII for ALL HIGHWAY USE
based on a very vigorous 3200 miles per month, or 38,000 miles per year is
$1065.
Most people will drive closer to 19,000 miles per year.
Based on a more reasonable 19,000 mile year, the FEH costs about $530 more fuel to use than a PriusII.

What I also walk away with from this data, is the delta cost of ownership was consistant with changes in speed. Thus, I conclude from this, neither the FEH nor the PriusII is better or worse suited for highway use. The delta cost stays the same between 65 MPH and 75 MPH, only the total cost changes.

The Prius' shape does not look that much better, and the FEH shape does not look that much worse from an aerodynamics fuel standpoint, which suprised me. If you see fault with my data, let me know.

Comments? Thoughts?
-John

 

Hi,

I think you've done an excellent job on the analysis. However, based upon my 2003, NHW11 Prius performance compared to yours:
My data suggests the NHW11 performance would be:

• 800 miles @ 75 MPH and 39 MPG = 20.5 gallons PriusI
• 800 miles @ 70 MPH and 49 MPG = 16.3 gallons PriusI
• 800 miles @ 65 MPH and 53 MPG = 15.1 gallons PriusI

I suspect the PriusII does no worse than my 2003, NHW11 PriusI, on the highway. I've assumed no AC on a standard day. It doesn't make a big difference but these are the numbers I drive by.

Excellent analysis and I'm glad you included the relative 'labor', the elapsed time including 1.5 hours of breaks. This is a very reasonable number and matches my practice.

Bob Wilson

 

Well, it's interesting math for sure. But all of this is based on the assumption that you would own a 48 mpg Prius over a 32 mpg Ford Escape mainly to save money on gas, when driving around 38 000 miles per year, at 75 mph? Or vice versa, if the cost difference is small, you might go for the Escape.

Hmmmm. Being green can indeed be a challenge at times.

 

Hi Battery Buddies - Just did several trips up to 300 miles - Headwind matters MUCH more than speed - 4 to 5 MPG. Also, hilly roads MUCH BETTER than flat interstate. That means - slower UPHILL @ 40mpg - HAUL A## down hill at 99MPG charging all the way! Love this car, but I don't baby it.

 

I've been telling people for 2 years my Ford Escape does better in hills than flat, and no one believes me!!!

( Do I have one believer now? )
-John