Uh-Oh…. A Fuel Economy Plateau


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Model 3 Unveiling Invite – Click To Enlarge

Has ICE fuel economy maxed out? At first glance, this chart does suggest it has plateaued, at least in the US, after years of consecutive progress. But TeslaMondo doesn’t like statistics or charts, not even this one. The overall product mix for the last couple of years undoubtedly has shifted to piggier vehicles, as cheap gas invites consumer complacency. So a lateral move on that chart, instead of a reversal, is a miracle.

However . . .

This does raise a question. Where exactly is the ceiling in ICE technology? The diesel subset has already bumped its head on the ceiling. Ask VW. And the torch-bearing hybrid, the Prius, seems saddled by its ICE innards. Sure, it has improved its mpg from low 40s in 1997 to low 50s in 2016 while also becoming a bigger, safer car. That’s great, but is it repeatable? Probably not, hence the Mirai.

And so . . .

Assuming gas prices stay down, and complacency up, the only way to kick-start that chart is by rolling out a new generation of electric cars. These have to be cars you want because they’re exciting, not because they’re fuel-efficient — akin to foods that taste so good, you forget they’re good for you. The automotive equivalent of grapefruit. All eyes on March 31. We’re about to get an automotive grapefruit. If the rumored 2.o drag coefficient is true, it might even look like a grapefruit.

*Editor’s Note: This and countless other Tesla-related posts appear on TeslaMondo. Check it out here.

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56 Comments on "Uh-Oh…. A Fuel Economy Plateau"

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Here’s hoping the Prius gets a good PHEV option.

+1, I am hoping for ok range and for it to be priced the same as the higher trim models (i.e. have the PHEV standard at the higher trim), I’d also like a shock announcement that they are going to do the same with all of the other Toyota hybrid models.


But because of overall plug-in market impact rather than efficiency improvements. Prius incremental improvements aren’t going to make much difference to gasoline consumption. It’s replacing guzzlers that would really make a big impact.

Sure, I think we are in the twilight of the ice, reaching/nearing maximum efficiency gains are just another indicator, that such is the case.

With the diesel I would use an analogy like they were using a smokescreen to conceal that their efficiency gains could only be achieved by turning off their pollution controls. So diesel is mostly a dead duck, or certainly a ruptured duck.
(WW II allusion).

Don’t listen to the haters. I *LOVE* your articles and writing style.

Possum Peckers Forever! 😉

“Where exactly is the ceiling in ICE technology?” I don’t think that’s the right question, at all. Almost from the beginning of the age of the internal combustion engine, it’s been possible to engineer them for greater fuel efficiency. That simply makes them more expensive. With governments around the world mandating higher fuel efficiency, auto makers have responded by making cars with more efficient and also more costly engines. That isn’t a matter of technological improvement; it’s a matter of choosing a different “sweet spot” on the cost/benefit analysis. Now, that’s not to say there haven’t been real technological advancements in fuel efficiency within the past few decades. Electronic fuel injection, replacing the carburetor, was a significant improvement. So is stop/start tech, using an oversized starter battery and a beefed-up alternator which doubles as a starter motor, to prevent fuel losses from idling. But the basic flash-bang technology of exploding vaporized fuel inside a cylinder to push on a piston hasn’t advanced in well over a century, and isn’t likely to. We’ve seen attempts to improve the basic tech: the Wankel rotary engine, and the OPOC (Opposed Pistons Opposed Cylinders) piston configuration, which like the Wankel, looks more efficient on… Read more »

Start/stop technology stinks in my opinion, unless it is part of a full hybrid system. Here in Texas we are using air conditioners 3/4 of the year. So the start/stop systems are useless because the A/C compressor has to keep turning. In a full hybrid, it isn’t a problem because the battery pack is large enough to keep the cold air flowing while the car is stopped.

Also, everyone I know that has tried driving one of those did not like the car’s behavior when the start/stop was working.

Thanks for the input, David.

The only first-hand experience I have with stop/start is a few occasions driving a first-generation Honda Insight, quite similar in operation to a (non plug-in) Prius, and of course the very sort of “full hybrid” you describe.

Many batteries in early stop/start systems were too small and would shut off the AC at a stop light. Many of the new start/stop systems use larger batteries that keep the AC running for a couple of minutes at a stop light. The batteries in the first stop/start systems were deep discharge lead acid batteries. Now they use Li-ion batteries in start/stop systems.

I try never to read your posts but I made the mistake of reading this one. ICEs haven’t progressed in 100 years? The same displacement that could only achieve 20 horsepower 100 years ago can now achieve 200 horsepower. Do you actually think about the things you write?

The Ford Model T had a 177 CID engine and produced 20 horsepower. The current Ford 2.7 liter (164 CID) ecoboost engine produces 325 horsepower.

I’m certainly no expert in the engineering of the internal combustion engine, not by a long shot, but the first reference I find to a compression ratio in a production car from the 1910s (a century ago) is a 4.5:1 ratio in a 1910 REO Model G:


Modern production cars typically have a compression ratio between, what? 10:1 to 12:1 or 13:1?

Rather far from a ten-fold improvement in compression ratios you’re claiming, Texas FFE.

Now, if you can find an authoritative citation to back up your assertion, then please do continue the conversation.

Compression ratio is not in anyway a measure of horsepower. It is the ratio of the volume of an ICE’s combustion chamber from its largest capacity to its smallest capacity.

Anyways, ICE automakers are trying hard to make HCCI engines (gasoline sparkless ignition) that are only now possible with the advances in computing power in automobiles. HCCI should give about a 15% bump in an ICE engine’s fuel efficiency.



Based on your comment, I don’t think you actually read the article. Nobody denied any improvements in 100 years. What they said was that there is a fundamental limit to the level of fuel efficiency in ICE motors, and, in the blogger’s opinion, we as a civilization are reaching that limit.

This article is so narrow minded. 70% of all the energy that goes in as gasoline gets lost as waste heat. There are a lot of technologies to capture and use this waste heat that have never really been explored. I’m an advocate of electric vehicles because they are not dependent on any one energy source and because I think EVs are much less complicated than ICEs, not because I think ICEs have even come close to their technological limit.

Though there is the nagging scientific discipline called physics.

Yes and there are a few thermodynamic processes called the ranking cycle, the otto cycle, the carnot cycle and the diesel cycle. We also have the zero, first and second laws of thermodynamics. There are also a few industrial process like combined heat and power, topping cycle and bottoming cycle.

Yeah so with each conversion you lose efficiency. So what’s your point? That by recapturing lost heat you could make an ice more efficient, sure, but not enough to make much of a difference with efficiency gains.

Ev’s have regenerative braking. No matter how many laws of thermodynamics you allude to, you can’t beat that with improved waste heat recapture which requires an extra conversion process thereby losing more in efficiency.

Evs win ices lose. Plain and simple.

Actually power plants are very successful at capturing waste heat. In an industrial scale capturing waste heat makes a lot of sense, it’s a lot more difficult in a car but it can and has been done. It’s hard to beat regenerative braking though, it’s truly a great technology that cannot easily be duplicated using a thermal engine.

“It’s hard to beat regenerative braking though, it’s truly a great technology that cannot easily be duplicated using a thermal engine.”

ICE hybrids use regenerative braking.

The electric part of a hybrid has regenerative braking. The ICE part still loses converts 70% of its fuel to waste heat.

Regen braking recapture velocity energy. No matter from which creating device it come.

ICE simply can not use it as it’s in the form of electricity 😉

True. I just wonder about the added weight of a heat capture system, maybe making a mini steam turbine connected to the water jacket to generate electricity would be an area of interest.

What does your number mean??? Surely you remember the theoretical limitations involved in the Carnot cycle. The source and sink temperatures dictate how much heat MUST be dumped to the environment…even if the universe were imaginary (like heat transferring without a temperature difference).

The combustion temperature is about 3000 degrees F absolute and rejection temperature is around 500 degrees F absolute so the theoretical Carnot efficiency would be around 83%.

And what is the limit of Carnot efficiency for a steel engine? Or are you suggesting using a ceramic one, and if so, then what is the limit of Carnot efficiency for that?

It’s all very well to say that in theory, you can achieve 83% efficiency in an internal combustion engine. But after well over a century of development, the best we’ve ever done in a relatively small motor (like an automobile engine) is less than 45% using diesel, not gasoline, and that’s in a bench test with the motor running at its most efficient speed. Put that into a real car in real-world driving conditions, and you’ll be lucky to achieve as much as 33%, even for an optimally engineered car, let alone a mass produced one.

The gasoline ICE in the 2016 Prius has a thermal efficiency of 40%, the world’s highest for a mass-produced gasoline engine.


I’ve seen claims that large marine (as in an ocean going ship) engines have achieved as high as 51% Carnot efficiency. But they achieve that partly due to advantage of scaling up to larger size. Again, that efficiency isn’t possible in a smaller engine. Or at least, nobody has yet achieved it.

* * * * *

sven, thanks for your comment above about HCCI engines. It’s always interesting to see what innovators are working on. But they need to actually demonstrate that they can achieve a higher efficiency. A few years back there were claims that OPOC engines could reach higher efficiency, describing them as something “new” altho they’ve actually been around for awhile. But so far as I know, that went nowhere. We’ll hope that HCCI will prove a more successful innovation, but I certainly wouldn’t bet on it.

“In theory, there is no difference between theory and practice. But, in practice, there is.” — attributed to Jan L. A. van de Snepscheut

Aaaaand you just demonstrated why we don’t have an average MPG over 35 in today’s pure ICE passenger vehicles. Diesel simply gives you better returns due to being more hydrocarbon rich, but the emissions (and the costs of managing them) are worse.

PHYSICS! COST! And ultimately, you’re still burning carbon-based cr.p in something that’s directly descended from a STEAM LOCOMOTIVE.

Time to move on… Electrons efficiently move the Universe. Why not make them move your car? Why this resistance?

Being more hydrocarbon rich is not why Diesel engines gives a better return. Diesel engines give a higher return because they have a higher compression ratio which produces a higher combustion temperature. Remember the Carnot cycle.

You can also increase the efficiency of a gasoline engine by increasing the compression but there are two very distinct problems with increasing the compression ratio in a gasoline engine, one the engine starts knocking and two engine creates more NOx. Diesel engines have a big problem with NOx, that’s why VW engine without NOx controls got VW in so much. It’s ironic that something that does so well at increasing ICE efficiency also increases air pollution.

Yes and no.
Compression ratio help efficiency only because it put combustion at higher temperature.
But indeed diesel fuel contain more energy per volume than gas by about 20%.
That flush down the drain the alleged superior efficiency of diesel.

It’s not more efficient, it just use a more dense hydrocarbon (mainly)
Extract from http://large.stanford.edu/courses/2012/ph240/ramos1/
“Relating to energy, another important metric to keep in mind is the energy density of diesel and gasoline. Diesel fuel is heavier and oiler than gasoline and its takes less refining to create, its chemical compound is C14H30. Gasoline on the other hand is C9H20. [4] When burned, these chemical compounds correspond to energy densities of approximately 155 million Joules per gallon for diesel and 132 million Joules per gallon for gasoline. Thus, in terms of energy density, diesel is clearly chemically ahead.”

Sorry, but no. Diesel engines can achieve a higher energy efficiency, or Carnot efficiency, and that means using a larger percentage of the potential chemical energy contained in the fuel. The fact that it’s denser, that it weighs more per gallon, is a separate issue. That gives diesel fuel a higher MPG rating, but that has nothing to do with the energy efficiency of the engine, which is expressed as a percentage.

Texas FFE is correct on this point: That diesel engines achieve higher energy efficiency, or Carnot efficiency, because the high temperature developed by exploding diesel is hotter than the high temperature developed by exploding gasoline. Greater temperature difference (between the hottest and coldest parts of the engine) equals greater Carnot efficiency; and assuming all else is equal, that means higher energy efficiency.

I am not contradicting this, that’s what I wrote!
Higher compression produce hotter combustion and do provide better efficiency, but it not the main reason of better mpg.
Having a denser energy fuel is the one. (Mainly)
Consider the difference between the most efficient gasoline engine and diesel one and it’s minor, if it exist.
Some gasoline engine goes up to 40% efficiency, just about the same as a good diesel.
Saying” good diesel” is just an expression, not a fact.
Bottom line, there a limit of efficiency in ICE and some engine are close to it, and there’s no way to best it really.

Just to add something about.
If some sophisticated expensive ICE engine can achieve 40% efficiency, it has to be said, that it’s only at a certain rpm, temperature and condition that does not occur a vast majority of the time.
Compare this to an electric motor that has 90+% efficiency 90+% of the time is what this article is about.
ICE cannot even come close.

“Why this resistance?”

Ohm my god. I got a charge of that so I’m amped up now.

As per “Why this resistance?”, as on nice trains, to get from ‘A’ to ‘B’ efficiently, you need a good ‘Conductor’, and to get people to move from ICE to EV, you also need a good conductor: Actual Test Drives, with some introduction training, and preperation, then about 3 to 5 days in the EV Drivers Seat!

Other good conductors are: Workplace Charging, Visible and Accessable 24x7x365 Public Charging, HOV Lane access, Free Parking, Reduced Insurance Costs, Special Overnight Electrcity Rates for EV Charging, etc

All of these conductors serve to reduce resistance! So does direct EV purchase rebates.

The plateau is due to cheap gas prices causing people to fall back into their old habits of buying gas guzzlers.


But this is only for USA…

EU buy electric gass guzzlers, and enjoy benefits. There where non available on the market for USA. Tesla and Mitsubishi wants to change that.

Yes. Cheap gas moved people from sedans to SUVs. NOT from electric to gas engines. And thus pretty much every analysis from past 6-12 month is invalid as soon as those electric SUVs apear on thearket in significant numbers.

grapefruit 🙂

One year is a bit short to confirm a tendancy. We can assume that the plateau in average sales mpg can be easily explained by the drop in fuel prices.


Look, everyone knows that consumers have moved to heavier, larger vehicles (on average) as gas prices have come down over the last 2 years. The fact that the average fuel efficiency was retained tells us that the average ton-mile/gallon metric continues to improve. There are additional opportunities to improve ICE fuel economy. Both ICE and electric propulsion systems are getting better – which is wonderful. Then the consumer can choose what works best for their needs, preferences and desires.

Still does nothing to reduce the atmosphere’s level of carbon being dumped into it. More trucks and SUV’s = more carbon being output from burning fossil fuel

If you’re into having the thermal trapping capacity of approx. 400,000 Hiroshima Class Devices worth of heat; only because your fuel is cheap– that seems incredibly short sighted to me. This is why carbon needs Taxed.

We could also talk about direct deaths globally, due to gasoline & diesel emissions, and the siphoning of our country’s money (including oil subsidies) spent on burning foreign oil… But that might upset (Exxon) people. 😛

Your ‘fuel’ Cost is FAR higher than the dollar amount displayed at the pumping station.

I’m sure there were people in the early 1900’s who claimed the steam engine could be improved… after all steam had been the best choice for over a century!

Physics, economics and environmental concerns go against the ICE.
It will improve 5% while already more efficient electric drive trains improve 25%.

Watch it fall:

I really hope for drag coeff of 2.0 or lower. That’ll be drooling worthy car!

I sincerely hope the Model 3 isn’t an automotive equivalent of grapefruit.

(I’m with Randall Munroe on grapefruit.)

I am seriously confused but this article and the following discussion. We are no where near the theoretical limit of the ICE cars on the road, anyone with half a brain can see that even if you have exactly the same mix of cars on the road but replaced all of those cars with the most efficient version of those cars we could add 5-10 mpg to the average. The same could be achieved by putting the best technology mix in each car, not many hybrid pickup’s floating around. You could get exactly the same performance in terms of power and acceleration with a hybrid pick up as a V8 but no one would buy it. It is also not just a cost thing. People generally pay extra for cars with low mpg’s , often for no significant benefit in performance – alloy wheels on cheap cars being the classic example, normally the only metric affected is the mpg. If you wanted to see an increase in mpg ration fuel within 5 years we’d be at 40+ mpg.

It could be better, but I wouldn’t bet much on it.
People aren’t so much interested in saving when they save so little.
Like you said, put a ration or put it at high price would do a lot more.

Wow, pull back for a second and notice the trend lines in the industry. People are buying crossovers and SUVs and Trucks. The crossovers have a 1-5mpg penalty against their car platform-siblings. Combine this consumer preference for bigger suv-style vehicles with record low gas prices AND cheap credit; it’s no wonder that the average is down. Consumers can suddenly afford the payment on that $35,000 SUV/Truck and the gas isn’t so bad when it now gets 20-30mpg instead of the 17mpg it did in 2008.

Grapefruit that tastes good and is good for you? Okay, it is good for you, but they are SOUR!

Also, if the Model 3 is shaped like a grapefruit, it would have a very high coefficient of drag due to the low pressure vortices that would develop due to the round back end. Remember, the ideal shape is much closer to a teardrop, not a sphere.

I love grapefruit!

The Mirai is Toyota’s ZEV credit car…

Look at GM, they were able to create the Malibu Hybrid that gets 48MPG…Direct Injection, use of turbos, exotic materials such as Aluminum in F150 and carbon fiber into the BMW i3, EU has mandated 48 volt systems which supposedly can run an A/C super efficient…Improvements to areo and lightening will continue to improve…

Everybody, please keep in mind that low gas prices are temporary and there was a ton of worn out SUV’s that needed replacing, from the 2009 era, and there is a GLUT of higher MPG vehicles that were purchased between 2010 and 2015. As soon as those cars start to wear down, you’ll see it cycle HIGHER again on the mpg chart. I will say, that the cheap oil has made for an interesting cycle, and enabled auto manufacturers to sell reasonable amounts of both cars and SUV’s.

RE: “Has ICE fuel economy maxed out?”

The answer is YES … peak MPG occurred a couple years ago (2013/14).

To see true ICE MPG, just remove PEVs from the average over the last 5-6 years!
There are ~200,000 BEVs with a MPGe of 90-120 MPGe; and ~200,000 PHEVs with a MPGe of 40-90 MPGe. It is not to hard to calculate average PEV MPGe for BEVs & PHEVs (all PEVs) each year … just need to reference InsideEVs annual model sales numbers and EPA’s Fuel Economy data for each model. Since we now total vehicle sales each year and average fleet MPG we can reverse calculate ICE-V only fleet MPG.

Hint: while PEVs have only accounted for 0.4-0.9% of US sales over last few years their effect is to raise fleet MPGe values by 1-2 MPG points! (ie: in 22.x range for last couple years)

50 MPG is the approximate upper limit for a normal size car.