Very low MPG across two states (38-40 MPG)

For 78 mph, that is about what to expect. See this post:
Updated MPG vs MPH chart

The EPA highway label is 48 mpg, not 50. The average testing speed is closer 48 mph (a legacy from the start of the National 55 MPH speed limit era), though the label is heavily discounted from the actual dyno test. Prius achieved just over 70 mpg on that test, before the EPA labeling fudge factors were applied. There are numerous posts pointing to articles with much more detail of the EPA tests.

The cold weather hurts, that can partially be addressed by grill blocking. Any wind (even crosswind) or moisture on the road would also hurt.

Your mpg will improve slightly as the tires break in, but not by enough to make up for your surprise.

 

On a trip a couple months ago, I got 39MPG in my 2005 doing 75-77MPH with cruise and many hills. I have the bottom grille blocked and 51PSI in the LRR tires. You're getting about what you should at those speeds. Winter gas is hurting you a bit. Climate control settings and temp probably didn't hurt too much because you only had to warm up a few times.

 

Was the M3 drafting the Prius to save fuel?

It isn't clear to me that there has to be any crossover point, unless they are deliberately flogging one to unnecessarily force its mpg down.

 

The Prius is more aerodynamic, its going to use less fuel when cruising until you hit the speed limiter. Speed doesn't have much to do with the mileage during the Top Gear test, its more the pretty much constant acceleration or braking. The BMW didn't even have to try to keep up with the Prius, but the Prius was being flogged. If you put them side by side with the cruise set at the same speed, the Prius would use less gas. The BMW will likely lose mileage at least as fast as the Prius as you increase speed.

 

I just did a trip from Los Angeles to Vegas and got about 38mpg going low to mid 70s the whole way. The section with the hills, the car got mileage in the teens. It seemed unusually low but there were lots of long Hills to climb.

SM-N900P ?

 

What you are primarily looking at is Physics, which will hold equally true for all engines/vehicles. If weight and CdA are the same, then the amount of mechanical force required to maintain a given speed will be the same. The difference in fuel consumption is then simply a function of how efficiently a given engine / drive line can convert the chemical energy released by igniting gasoline vapor into that mechanical force. Every engine has its own unique efficiency characteristics at different loads/speeds, which can be mapped in a chart that looks something like these, comparing the efficiency of the Gen 2 to Gen 3 Prius engines:



The contours plot out the engine efficiency in grams of fuel consumed per kWh of mechanical energy converted at that load/speed. On these plots the contour scale is I believe in steps of 10g/kWh so on the right plot for the Gen 3 engine the dark spot top left is the <220g/kWh region, and above the bottom-most contour would be the <260g/kWh region. From other versions of this plot I believe the Torque marks on the left equate to about 40Nm to 150Nm? On this plot its helpful that the constant power lines have been added, as well as the dark line that represents the Priuses "standard" operating line. IE if you "ask for" 40kW worth of power by pressing on the gas pedal, the controllers will try to set the rpm of the ICE and electric motors to the point on the contour where the standard operating line intersects the 40kW constant power line.

Playing with the numbers on the EV calculator, I'm estimating the Prius requires about 57hp (42.5kW) to maintain 90mph on a flat. Extrapolating from the data on the graph, I'm guessing this will still be around 230g/kWh efficiency.

Below is a similar BSFC contour which I believe is a 1.8L Mercedes Turbo.


The data is presented similarly, with the g/kWh efficiency contours overlaid with constant power lines. What we don't have is the top gear rpm vs speed curve. Based on the assumption above that the car with this engine has identical weight and CdA, it would also require 42.5kW at 90mph. Depending on what rpm this corresponds to at 90mph efficiency should be somewhere between 250 (2500rpm) and 300 (4250 rpm) g/kWh.

So at least for the case in question, the Prius still wins at 90mph. Driving faster kills fuel economy on all cars, the Prius just a bit less so due to its good CdA and unusually broad efficiency contours. Its not until you get it way out in the corners (like with the M3 track test) where it might be beat by more powerful engines, but these are well outside any kind of normal driving conditions.

The other worthy challenger is the small diesel, who like the Prius Atkinson cycle engine have an inherent efficiency advantage due to the design of the engine. From this curve, you can see that the 1999 VW TDI could certainly give the Prius engine a run for its money at certain sustained operating conditions. Depending on rpm, this engine could produce the same 57hp at between 200 and 250g/kWh depending on the final rpm at 90mph. On the other hand, diesel fuel also has about 10% more carbon/btu energy per gram which negates some of the difference but certainly not all.


Sorry, what was the question

 

KyleSTL:
1. If you can deal with the stiffer ride
To increase fuel efficiency increase tire pressure from 40/34 to 44/40.
2. Double check make sure oil level is correct.
3. To get better fuel efficiency drive at a lower speed
set change cruise control from 75-78 mph to 65-70mph and drive in the slow lane.
For super highway driving - (Time shift)schedule your trip around rush-hour when thesuperhighway has less traffic. When there is less traffic people are more tolerant
of slower vehicles.
4. Change ECO to Power mode. Power mode makes your ICE more responsive at hi speeds.
5. At any time while the Prius is in motion are any of the window open for ventilation?
Keep your windows closed while driving at high speeds to reduce aerodynamic drag.

6. How much cargo are you carrying in the rear? Avoid carrying too much cargo

7. Note that a Prius can only achieve its rated fuel efficiency if its engines are fully warmed up and that the initial cost of warming up the Prius means that to get the Prius to achieve the epa rating in good weather (no rain, +65 F degrees) most of the Prius' trips must last longer than 35 minutes and 8 mile. An Engine block heater (EBH) can be use to prewarm up the Prius' ICE to increase its fuel efficiency.

8. Top Gear is automotive entertainment and their test are fake. Top Gear's Clarkson hates alternative energy vehicles ( a 2nd gen Prius was destroyed with explosives on a Top Gear episode just to accent Clarkson's hatred). Top Gear also has fabricated test results for the Tesla and the Leaf (but Nissan telemetry caught Clarkson cheating). The Prius v M3 test involves a bit of trickery. Top Gear decreased the Prius Fuel efficiency on the racing track by pushing the ICE very hard into a curves to a dangerous speed to automatically engages the Prius Vehicle Stability Control which automatically slowed down the Prius to a safe speed while also lowers the Prius' Fuel efficiency.The Prius is a commuter car with a high center of gravity but it can't take curves at speeds as high as a BMW M3 which has a lower center of gravity . To pad the results, Top Gear also tried drafting the BMW M3 behind the Prius' Slipstream to get a bit more fuel efficiency albeit the increase in fuel efficiency wasn't that good because the Top Gear drivers are going too fast around Top Gear's test track.

If you drive a Prius at 90 mph - its fuel efficiency drops signficantly but it will still beat the M3 in fuel efficiency in the real world and in a fair test. Why? Because the Prius is using an Atkinson cycle engine which is more energy efficient than the M3's Otto cycle engine.

In the future, for +70 mph vehicles with +50 mpg fuel efficiency will morph into something akin to the VW single and double seat prototype vehicles - which from the outside looks like a thinner and more streamlined version of the 1999 Honda Insight.

hope this helps

Walter

 

Much of the MPG difference is likely to come from gear ratio changes, not merely BFSC improvements. For a very long time, American-market cars have been geared for performance, at the expense of fuel economy. But recently this focus has been changing rapidly.

 

I believe this is quite true. Making significant changes to the BSFC characteristics is hard, but changing the gearing to move around on the same chart is fairly easy. Its worth noting that ICEs in general are most efficient when fairly heavily loaded and at low rpms, in all the curves above peak efficiency occurs near max torque. A more powerful engine in the same vehicle will generally run more lightly loaded relative to its max output at a given cruising speed, unless it is also re-geared to push rpms down and engine torque up. So in general there is not an inherent advantage in cruising efficiency to a more powerful engine until you start talking about very high speeds that require high sustained output power and/or talking about underpowered engines being pushed well beyond their peak efficiency power. Its also worth remembering that better thermodynamic efficiency does not inherently equal better fuel economy. It takes more energy to move a vehicle of a given weight and with a given aerodynamic drag a given distance at a higher rate of speed than it does at a lower rate of speed. So even if an engine is running more efficiently at higher speed (load) it is doing more work and likely consuming more fuel than if it was run less efficiently at a lower speed. That's not always true, but generally so within certain bounds and particularly for steady state cruising at highway speeds.

You can kind of see it from the plots above, part of what is special about the Prius Atkinson cycle engine is not only that its peak efficiency is high, but that the peak region is very broad vs. rpm and load. Combine that with the Prius controllers ability to vary gearing to more or less follow the most efficient possible path through the bsfc contour for any given typical load and you have a pretty uniquely efficient drive system under a fairly wide range of typical driving conditions. The electric drive system compliments the Atkinson cycle engine particularly well, as the big downside to that engine design is pretty abysmal low end torque. This is exactly where the electric motor shines, making the ICE much more usable/acceptable. So even though its the engine design that makes the difference at sustained highway speeds, its still the electric drive system that makes using that engine much more feasible in the real world.

 

Diesel fuel has about 10% more carbon/energy per unit volume, but gasoline actually has slightly more per unit mass (gram is a mass unit).

Brake specific fuel consumption - Wikipedia, the free encyclopedia

 

KyleSTL;

4. PWR mode allows for a more responsive throttle which is necessary when going uphill and downhill and on a superhighway when the engine is almost always on. Cruise Control bypasses the PWR/normal/ECO throttle mode controls so if you are using Cruise Control it doesn't matter what mode you are using.

8. You can purchase vehicles which use a non-hybrid conventional gas engine (Otto cycle) that can get better MPG than a Prius - but it will be a smaller engine on a much lighter vehicle, i.e. motorcycle or a scooter.
You can verify this by going to Fuelly.com and look at the MPG ratings for Motorcycles. The Progressive X MPG winner -the Edison2 VLC- gets better fuel efficiency than the Prius but it never got to market [1].

An Atkinson cycle engine has almost the same energy efficiency in its power stroke as a diesel engine. Every internal combustion engine has a RPM range sweet point where it rus most efficiently. The sweet range of an Otto cycle engine is usually narrow at about 400 RPM (e.g. 1400 RPM to 1800 RPM) because the *burn* period is shorter in an Otto cycles - this is by design so that an Otto cycles engines can produce a significant amount of power in a very short time and give great acceleration and quick application of torque. As miscrms points out the sweet range of the Prius' Atkinson cycle RPM range is much wider, e.g. for a Prius it can be as low as 900 RPM and as high as 2200 RPM - but Atkinson cycle engine's longer power stroke means it takes time for it to accelerate and provide torque - hence, for the same displacement as an Otto cycle engine it takes much more time for the Prius ICE to produce same power. To offset this characteristic, the Toyota Prius uses two electric motor to provide better acceleration. The new 2nd generation Honda Accord Hybrid goes even a step further by using a much larger (displacement) Atkinson cycle engine than the 1st generation Honda Accord Hybrid. Diesels engines like the ones used by Mercedes and Volkswagen have poor acceleration characteristics - to offset this Mercedes and VW augment their diesel engines with turbo chargers and sophisticated fuel injectors to help boost accelerations. The complaint that I've heard in the past is turbo charger are not always reliable and that car shops often charge more to fix turbo charged engines than non turbo charged engines. Not sure if that's true today though.

I agree with fuzzy1 that many of the new vehicles with higher Highway MPG EPA ratings are using more sophisticated transmission to get there. However, it isn't exactly number of gears or whether one has a CVT but how high a transmission gear ratio can go. However, the problem with high gear ratios is that the higher it is the less useful it becomes.

The real world MPG boosting technology for hybrids is the stop start idle technology. Because almost every day my car is stuck in a traffic jam - it is really fantastic that my Prius will automatically turn its ICE off and save gas while its sitting there stuck in traffic. If I had gotten a BMW M3, it would be running its ICE that whole time while it was sitting there stuck in traffic getting 0 MPG. It's not that conventional gas cars can't benefit from stop start idle technology - its just most of them don't have it. From what I understand because of the higher CAFE standards - car makers are going to be putting stop start idle tech into the newer models.

Because I am often driving in hilly terrain one of the driving techniques I use to get better fuel efficiency on my Prius is I synch my accelerations with a downhill segment of the road and avoid accelerating on uphill segments. This is call *Driving with Load* in Hypermiling circles. The rocket science term for this type of driving is called: gravity assist , gravitational slingshot, swingby, or a gravity assist manuever [2].


[1]
Edison2 | Progressive Automotive XPRIZE
Edison2 - Welcome

[2]
Gravity assist - Wikipedia, the free encyclopedia

 

Thanks for clarifying!

 

Were there any items in the back seat that covered the battery air vent?