F-Type V6S First Impressions


(Gunnar Helliesen) #21

I’d just like to point out that at 70 mph, the hybrid drivetrain in a Prius is nothing but a heavy anchor. It’s not doing you any good as far as mileage is concerned.

At that speed, the gas engine is tasked with 100% of the effort of moving a 1.5+ ton steel box against the resistance of the air, the road, gravity (real world roads aren’t completely flat), and everything else. For an I-4, 1.8 liter, 121 hp gas engine to get 50 mpg under those conditions seems … overly optimistic, to put it mildly.

That’s just simply physics, as I’m sure you know. Toyota even tacitly acknowledges this, if you look carefully at the published data for the car (quoting from the 2017 Prius specs [1]):

EPA Fuel Economy Est - Hwy : 50 mpg
EPA Fuel Economy Est - City : 54 mpg
Fuel Economy Est-Combined : 52 mpg

What’s wrong with this picture? It’s the opposite of what we’re used to see, for a car solely powered by an internal combustion engine.

That’s because in city driving the hybrid drive train can really help. It lets you reclaim some of the energy spent by the gas engine accelerating the car between lights (for example), by using regenerative braking. Not so on the freeway at 70 mph. You are constantly pushing against resistance far outstripping the output of the electric drivetrain and hardly braking at all.

You see the same phenomenon reflected in the published range of the car:

Cruising Range - City : 610.2 miles
Cruising Range - Hwy : 565 miles

The Prius shines in the city, not so much on the highway. In addition, as Sandy pointed out, the EPA highway stats are measured at lower speeds, nowhere near the 70 mph we’re discussing here. To quote from Wikipedia [2]:

Under the assumption that the fluid is not moving relative to the currently used reference system, the power required to overcome the aerodynamic drag is given by:

P_d = \mathbf{F}_d \cdot \mathbf{v} = \tfrac12 \rho v^3 A C_d

Note that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome aerodynamic drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting 4 times the force over a fixed distance produces 4 times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, 4 times the work done in half the time requires 8 times the power.

(“Fluid” here being used to describe air. Emphasis mine.)

A large gas engine on the other hand, tuned to a high level of efficiency (by gas engine standards, anyway), turning at low revs and encased in a light weight sports car body with relatively low drag, can get excellent mileage under the same circumstances. That is, provided my lead right foot is kept out of the equation. I.e., you have cruise control engaged, a lock-up converter, fully electronic engine and transmission management, the works.

Would the sports car get better mileage than the Prius, even at 70 mph? That depends on multiple factors, such as aerodynamic drag of the two cars, head or tail wind in each measurement, the type of tires used, how those tires are inflated, weight of the vehicle, engine tune, and the driver’s driving style. But no, it’s not likely, even though it might be a closer race than you’d think.

Please also keep in mind that all the stats mentioned in this thread are anecdotal, with the exception of the published numbers from Toyota. “My friends says he gets 55 mpg at 70 mph” is the epitome of anecdotal data. Not only do we not know the exact circumstances of said measurement(s), but there’s also a lot of opportunity for personal bias to creep into the equation. If you bought a Prius to save gas, you’ll want to defend that investment by extolling its gas-saving virtues at every opportunity, including sometimes even fooling yourself. It’s just human nature.

[1] Toyota’s 2017 Prius specs:

[2] Wikipedia article on drag and power:

Regards,
Gunnar


(Paul Wigton) #22

Gunnar… in short, I thoroughly understand the physics you cite: Ive done many years and miles of automotive work and testing.

The assertion was made, “Prius got 17 mpg at up to 70 mph peak.”

I stand by my statement: a decent Prius WILL not, DOES not get 17 mpg, at 70 mph.


#23

Highway cruise is not what Top Gear tested. They tested actual driving at highway speeds.

One is steady state, the other is real world. But either way, they said it was independently verified.

It reminds me of when Top Gear claimed the Tesla Roadster ate a full charge in just over 40 miles on the road course, when it said it had well over 200 miles range at the outset. Tesla sued them and lost.


(Paul Wigton) #24

What the actual hell does that mean??

70 mph, on a highway, in the real world…is highway speed. PERIOD.


#25

Think throttle-steady cruise control vs. over the river and through the woods to grandmothers house we go. A steady state test point vs real world road course.


(Paul Wigton) #26

The topic was on the highway. Stop moving goalposts.


#27

No, the topic was your rejection of Top Gear’s claim that their Pruis logged 17 mpg … on a road course.

And… their V8 M3 averaged 19 mpg trailing it.


(Paul Wigton) #28

“Prius got 17 mpg at up to 70 mph peak.”

One more time (for this is my last on this post): Clarkson HATES Prius’, shows them in the worst possible light, and somehow, his “independent” numbers were jiggered.

If this were really the case with the Prius, there would be MILLIONS screaming it at the top of their lungs. NO Prius, in decent nick, is going to get 17 mpg ANYWHERE. It’s bullshit.

Now, play by yourself. I’m tired of the game.


#29

I posted the Top Gear video. There should be no confusion.


(Mike B) #30

Bringing this back from the dead…

Seems to me that they had to beat the living crap out of the prius to maintain that pace on the track.

That causes it to operate WELL outside its designed efficiency range, where it could very well produce terrible results.

The M3 is having a much easier time of it because it’s engine is not expected to put out 99.999% the whole time.

You could argue that it is “rigged” in that they probably found the pace at which the prius would do its worst, and the M3 would obviously not be overtaxed by that pace.

Makes perfect sense to me that the prius would turn in terrible numbers when driven that harshly. I have no bias or love, for either car nor the host, just using some logic I think.


(Paul Wigton) #31

A possible and likely scenario, one that, if so, would show how intellectually bankrupt the idea is, that a Prius is not a fuel-saving vehicle, in standard use.

That would be jiggering.


(Mike B) #32

Sure but these are two different things.

The point of the show is to be absurd, obtuse, and funny.

I don’t think anyone with an ounce of sense walked away thinking that an M3 in normal use is more fuel efficient.

We’re just considering how it is completely plausible for the results to be correct, without the need to outright lie.

Here it’s fun, and everyone with a sense of humor knows they they were being silly by picking the worst case scenario for the prius.


(Paul Wigton) #33

I must know or have communicated with a LOTTA folks with only a dram of common sense: not only do they stubbornly insist that the example is why hybrids are “bad,” the now-recused OP of the article did just that.

QED.

The point was not about torturing the data: it was presented as a relevant and accurate description of how an M3 is better for the environment.

It is not.