Source: Comcast, on-demand, Top Gear, #36

Claim: BMW M3 vs Toyota Prius in 10 lap race on what appears to be a flat track. The BMW M3 drafting the Prius for all 10 laps.

BMW M3 - 19.4 MPG
Prius - 17.2 MPG

"It isn't what you drive, it is how you drive it." Clarkson

ANALYSIS

So let's see how to do it. The first clue is to look at MPG vs MPH for maximum vehicle acceleration:

So this offers a number of options:

• maximum acceleration - up until 75 mph, this always has lower than 17 MPG because the vehicle energy is going into acceleration
• maximum braking - defeats regenerative energy recovery, it simply heats the brakes

This driving style makes sense on a flat race track racing for speed, a common Top Gear test. It also ensures the Prius stays in an inertial energy region -- either burning fuel to accelerate or braking hard to warm the pads. But remember we are dealing with two vehicle of different weights and one is a BMW M3. So one could 'equalize' the vehicles:

• a significant mass added to the Prius brings the vehicles to identical weights
  • 3,700 lbs - BMW M3
  • 2,700 lbs + 1,000 lbs equalizer - Prius

There are a few ancillary things that could reduce mileage:

• using "B" - this keeps the engine running at all speeds to minimize hybrid operation
• maximum AC or defrost - another energy waster

One could also have a rubbish pit crew:

• lower tire pressure
• over-filled oil

PRIUS ENGINE EFFICIENCY

So when looking for how to drive a Prius inefficiently, you need to defeat the mechanisms that make it efficient such as the Atkinson engine cycle. To generate maximum power, the engine has to run as fast as possible with the valves moved to maximum power. This finds the worst case, brake specific fuel consumption (BSFC):

Here is another chart, a homemake BSFC chart:

The key to an inefficient Prius engine is to keep it in the far right side of the power curve. Keeping the accelerator on the floor achieves that trick but puts the energy into kinetic momentum. The next challenge is what to do with this kinetic energy.

PRIUS BRAKING

The easiest way is to get rid of kinetic energy is to heat the brake pads. It also helps to use "B" to also warm the engine. Braking hard enough to trigger the ABS would be a perfect threshold.

TIRE DRAG ON TRACK

David Kelly of Huntsville AL posted this note in the YahooGroup, "Prius Technical Stuff," on December 14, 2008, message #32398. He pointed out that tire drag of a vehicle driven 'Formula 1' style is significantly higher:

ANALYSIS

Now how to reply to Top Gear is easy. Use the facts and data, mileage data from every legitimate testing lab:

USA EPA

City/Hwy:Combined
48/45:46 - Prius
14/20:16 - BMW

Find the equivalent UK, Japanese, and in particular, the German mileage testing reports. Don't argue with the Top Gear fool, smile and say:Take your willing skeptic to a place where you can repeat the Top Gear test with maximum acceleration and maximum braking. Think in terms of 'wind sprints', a physical training technique that involves sprinting, stopping and sprinting back, over and over again. BTW, the maximum speed is unimportant but the test area should be away from other traffic. Hand them a sick bag and continue until they throw up.

Bob Wilson

ps. Other nonsense from Clarkson:

• Nickel mining suffur dioxide and acid rain - lie
• Shipping cost of nickel to China and Japan - nonsense compared to oil shipping costs
• Quiet hazard to pedestrians - no empirical accident data, 4,700 USA pedestrians die each year and hybrids have the same rate as ordinary cars
• Radical environmentalist owners - nonsense, not supported by any empirical study

pps. I just learned, 2008/12/15, there was an earlier thread on this subject at PriusChat based upon a U-tube extract. However, this U-tube extract has been removed due to copyright violation. Regardless, another discussion of merit:
http://priuschat.com/forums/prius-ma...tml#post651065

 

I don't agree with Clarkson (who I confess I see as utterly juvenile) that it is not what you drive but how you drive it. It is obviously both.

To have been at all fair, his test should also have included someone trying to get the highest mpg possible from the Prius. It would have been interesting to see how close the BMW could get then...

 

I hear the arguments but it's one of the few shows that I record every episode. I love it. Have you seen the rocket episode...I could not believe they got the car off the ground.

 

The only reason for my write up was a member of the "Prius Technical Stuff" explained that some folks in the UK take "Top Gear" seriously. Sad to say, I hadn't seen this episode until this weekend and that is when I took fingers to keyboard. I wanted to provide solid engineering data showing how the Prius could get worse mileage than the BMW. In fact, if you look carefully at the data, it is possible to get even worse mileage by simply accelerating and maximum braking ... no race course or other excuse is needed.

A joke in which every laughs is good fun. My write up makes it possible to explain the trick to those may have been taken in by 'Top Gear.'

Bob Wilson

 

Well done Bob, but I think you missed the point. What THEY were trying to do was keep their TV ratings up, NOT measure efficiency or anything else.

So you're analyzing junk TV. Good luck with that.

 

I think you have something that I had not thought about before:

• How does Top Gear get so many super-cars to test?

Each week during the production run, Top Gear gets one or more super-cars to test. Super-car manufactures have no fondness for the Prius and nothing in their inventory that remotely competes.

Good call!

What I love most about Top Gear is they take some 'junkers' and torture them with a Laurel and Hardy stunt. It is like seeing Junk Yard Wars meets the Marx Brothers.

Bob Wilson

ps. http://www.youtube.com/watch?v=jeR9znRIVWA

 

I think the message they were trying to convey is that the prius is not as green as people think and to prove that EPA numbers are not necessarily real world, not that racing around a track is real world. That is to say that Person As driving habits yield better fuel economy in Car A than in Car B, that does not mean that Person B's habits driving habits will yield better fuel economy in Car A.

As for your analysis, it is true that the prius was not driven in the most efficient manner, nor was the M3. The prius however was driven to the limits which are almost always among the least efficient for any car while the M3 was not at its limits. As for the tires, as a percentage of total energy consumed racing around a track I doubt the claimed efficiency difference accounts for much. In fact the M3 tires are wider, probably heavier and a greater contact patch with the road resulting in a higher rolling resistance. There might be something to the tires though depending on how they raced. They seemed to keep roughly and equal distance, but it is possible that the m3 could be more efficient going around a corner since it would not need to use the brakes(waste kinetic energy) as much to get around a corner.

My belief is that the M3 achieved a higher fuel economy due to the fact that the prius was racing at a low efficiency and M3 engine is far more efficient at producing the 100 horsepower or so to go around a track in the time it takes the prius, the M3 was probably able to keep up with the prius in 6th or 7th gear.

As for something more real world.. How about cruising down the highway at 80mph or at the prius top speed of 100-110 will the M3 get better fuel economy? The M3 really is not focused on fuel economy so how about a BMW 335D?

 

Yet curiously, the GreenHybrid database has over 1,700 NHW20 drivers who have achieved a median of 47 MPG in the NHW20.
Actually the EPA, Euro, Japanese or other standard mileage tests are there to help folks spend their $20-30,000, a years salary, on a vehicle that they can afford to operate, especially when fuel costs go up. It keeps car makers and sellers from making false claims ... like "Top Gear" or the current nonsense by the VW dealers pushing the Jetta TDI.
Between scraping the rubber off of the tires and heating the brake shoes by maximum braking, both are simple techniques to convert the Prius kinetic energy into heat. They might as well have driven the course with the parking brake set. Of course the second "Top Gear" technique is to use maximum acceleration of the Prius, to keep it out of energy efficient regions:

The "green" line (as in dollars spent at the pump green,) is the MPG at each point during the test.
Actually on a straight or round track at a constant 100 mph, the older, NHW11 Prius returns 22 MPG. The newer, NHW20 gets 25 MPG:

It really comes down to trading off speed versus fuel. Pick a cruising speed that matches your budget and angst.
I've always preferred using a Cherokee 140 or Cessna 172 to go 100 mph or faster or limited access roads free from other traffic. But to each their own. M3 and BMW 335Ds travelling at 100 mph or faster in Alabama would soon solve the problem:

My understanding is the first 100 mph ticket is called 'reckless driving' with not only a fine but an increase in annual insurance. The second such ticket loses the license along with a fine and loss of insurance. The third stop leads to jail time and possibly confiscation of the vehicle. Then there is the fast track:

Folks are free to make their choices.

Bob Wilson

 

The point of the show was that if your driving a prius at autobahn speeds and claiming your green someone should smack you.

The pet peeve expressed by top gear perfectly is the "green" who is anything but. Why exactl;y can you speed in a prius anyway?

 

I'm sorry but this question is a little confusing. I'll try to state what I think you are asking:For my engineering records, I needed to know what is called the inertial cost, the fuel burned to reach a given speed. The only way to get a good set of numbers is to do a maximum acceleration run recording the fuel burn and speed. So I instrumented my Prius and used a GPS to record the altitude.

Interesting it turns out the 17 MPG corresponds with a speed of ~75 mph. To replicate the "Top Gear" result:

1. maximum acceleration to some speed above 75 mph, say 85 mph
2. maximum brake (aka., ABS) to an equal speed under 75 mph, say 65 mph
3. repeat as many times for as long of a period as you can stand

This will give you an average MPG very close to 17 MPG. If you apply the same protocol around any given target MPG, you can dial in your result. For example, 30 mph has 10 MPG:

1. maximum acceleration to 40 mph
2. maximum brake (aka., ABS) to 20 mph
3. repeat as many times for as long of a period as you can stand

Now this is not "green$" driving but rather "Top Gear" driving. Anyone can do this, a trivial process. But if this is too boring, add some power slides, using the tires as your brakes by traveling sideways on them. This totally eliminates regenerative braking.

So let us assume you have a year's salary to spend on a car. "Top Gear" would have someone buy the BMW because it gets better mileage than the Prius. So that person would be condemned to paying for 52/19 ~= 2.5 times as much volume of diesel that today costs ~25% more than regular. Their annual fuel bill will be about 3 times mine and I approve!

It is their money and I would rather they burn it up than have it for more practical purposes ... like electing politicians they favor.

Bob Wilson