a thought experiment:
Let's say you and I work in the same office. Every morning you are intrigued to watch me lug a 25 pound insulated metal cube -- about 18 inches on a side -- into my cubicle, with some effort. Every evening, with another grunt or two, I lug the darned thing out again. It looks to you something like a portable mini-safe.
Eventually your curiosity gets the better of you. You peek over the partition one day and say, "I'm so sorry to bother you, but I'm just dying to know about this heavy metal box you bring to work every day! This isn't the Pentagon, you know -- what on earth are you carrying in the armoured case?"
Let's say I look at you blankly for a moment, then heave the mysterious box up onto my desk and open it. Inside several inches of foam padding you see the big secret: one apple, one sandwich. "I usually bring my lunch," I say. "Happy now?"
You stare for a few seconds at the mini-safe lying open on my desk, at its contents, and at me. Shaking your head, you go back to your desk. You can't figure out quite what is wrong with someone who feels a need to carry around 20 ounces of lunch in a 25 pound metal box, and you can tell by the blank expression on my face that I don't understand why you're asking me silly questions about my lunchbox. You decide that it takes all kinds, etc.
Now let's say you and I both go shopping. You arrive in your car; I arrive on my bike.
When I see you get into your car with your bag of groceries, this is what's going through my mind:
You weigh somewhere between 130 and 180 pounds. Your groceries weigh about 30 pounds. Your car weighs over 2000 pounds. If it's a SUV or minivan, it might weigh over 3000 pounds. On average, your car weighs at least 10 times more than its payload -- maybe 15 times more, but to keep the math simple, let's say 10.
So if we accept this optimistic figure, then your car's engine devotes only 1/11th of its total energy output to transporting its payload, and 10/11ths of its energy transporting only itself. To me this seems as crazy as our thought experiment above, in which I lugged my lunch to work in a box that weighs ten times as much as the lunch itself!
But it's actually worse than that. Of the energy released by burning fossil fuel in your car's internal combustion engine, only about 10 percent actually goes to moving the wheels around and moving the car forward. The remaining 90 percent is lost as heat: friction which heats up bearings, heat dissipated by the radiator and other engine surfaces, and heat vented in the exhaust.
So for each gallon of fuel you're burning as you drive your car, only 1/10th of it actually moves the car. The rest just warms up the air around the car. And of the 1/10th of a gallon that's not wasted as heat, only 1/11th actually goes to moving you and your cargo, and the rest goes to just moving the heavy body of the car. The resulting percentage (how much fuel is actually used to move you and your cargo) is an attractive repeating decimal: .00909090909... or about .91 percent. In other words, not quite one percent of the fuel you burn in the car is devoted to moving you and your cargo.
If I apply this ratio back to the lunchbox model, I would have to use a lunchbox 100 times heavier than my lunch in order to approximate the "overkill" represented by your car. If I bring about a pound and a half of lunch (24 ounces), I'd have to carry 2400 ounces, or 150 lbs, of lunchbox.
This is one reason why I think the word "efficient" doesn't apply to cars.
One reason people like cars is that cars travel fast. People often say that walking, biking, or taking a bus is "so much slower", "wastes my time," etc. The whole reason people like cars is that cars shrink distance by compressing time -- by raising travel speed. In terms of miles per hour, the car is hard to beat! It's so efficient. It saves us hours and hours of precious time.
But there is another way to look at miles per hour. The obvious way is "how many hours does it take me, door to door, to travel N miles to my destination" or "how many hours did it take me, in a year, to travel the total number of miles I drove in that year". This is the way that most drivers calculate miles per hour.
The other way of calculating miles per hour is to figure out how many hours of your life are consumed, total, by travelling N miles. One sobering statistic is that the average middle class American family spends 20 percent of its income on the family car(s). When we total up the car payments, insurance, registration, repairs, parking fees, and gasoline, the working car owner seems to be spending one day out of every 40 hour week just paying for the car. In many cases, the worker can't get to the job without using a car; but the cost of getting there is one fifth of the wages earned at the job.
If this is the case, then we have to add the time spent working to pay for the car, to the hours spent in transit: in order to travel N miles using my private automobile, I have to work H hours to pay for the car. If H is 20 percent of my annual salaried labour, and I work 2000 hours in a year, then I spend 400 hours a year paying for the car.
To this I should add any hours I spend waiting in line at the DMV, taking the car to the mechanic and picking it up again, washing the car, parking the car, filling out insurance forms, etc. I'm not the only person to try to figure this out. Check out this True Miles Per Hour Worksheet by Dehan Michael Davis. Several analysts pursuing calculations of this sort have concluded that the real average speed (in terms of miles travelled per life hours invested) of the private automobile in America is somewhere between 10 mph and 20 mph.
It gets worse if you live and drive in an urban area. Dehan points out, on his site, a statistic often remarked upon in transit studies: "The average speed of horse drawn vehicles through Manhattan in 1907 was 11.5 mph; today trucks and cars creep along at an average of 3 miles per hour." My own experience is that on a crowded weekend afternoon in summer, I can get across town faster on my bike than a driver can in a car. I routinely ride past long, long lines of traffic backed up in near-gridlock.
Even under ideal conditions (where cars can travel freely at or near the speed limit), the car owner invests many hours in the car which are not counted when the speed or efficiency of a trip is computed. Under congested urban conditions, the equation gets worse and worse.
One reason why I built an electric car a few years ago was in the hope of investing fewer "life hours" in my vehicle. A gas car (especially if you don't drive it often) takes a fair amount of maintenance. Things rust. Carburetors gum up with disuse. And lots of parts are disposable: air filter, fuel filter, oil filter, points, plugs, antifreeze, engine oil, tranny fluid, brake fluid, brake pads... and the beat goes on. I used to do all my own basic tune-up work, but it took a certain number of hours per year. I figured an EV would suck up less of my time in maintenance, and I'd be buying and discarding fewer disposable parts. I was right :-)
But what became clearer and clearer over the lifetime of that EV was that a bike would suck up even less time than any full sized car or truck, no matter what the method of propulsion. I now own no cars, and I spend very little time repairing my bike. Whenever it takes me twice as long (in trip hours) to go somewhere (say, by transit, or by bike, or by a combination of the two) as it would take a friend with a car, I remember all the hours I am not spending in maintenance, and all the hours I am not working to pay for a car.
One related problem is that time spent driving may, for many people, be more truly "lost time" than time spent travelling by other means. Time spent driving really can't be safely spent on other interesting activities. People do realize that the time spent trapped in their cars during long commutes is wasted time, and so they try to do all sorts of things while driving: chat on their cell phones, shave, read the newspaper, brush their hair (or teeth!), eat breakfast, have shouting arguments with their loved ones, and so forth. Some people actually have fax machines and even televisions in their cars.
Unfortunately all these distractions make them much less effective and attentive drivers. Police in the US are starting to assert that cell phone use by drivers is as conducive to driving error and subsequent collisions, as driving drunk! If we really pay adequate attention to our driving, we can't be doing other entertaining or relaxing things at the same time. Driving time is subtracted from our leisure time for other activities.
A transit user obviously can't do just anything he or she feels like on the bus or train. Shaving and having shouting arguments with loved ones are not really acceptable, for example. But one can definitely chat on a cell phone; have a lively conversation (including eye contact) with one or more friends; play cards; read a book, magazine, or newspaper; play a pocket video game; do a little text editing on a laptop; listen to the ball game on the radio; knit or crochet; and many other things (including a short nap) which would be irresponsible and dangerous to do behind the wheel of a car. The transit user can make double use of their travel time -- to get from A to B, and to do other amusing, entertaining, or productive things meanwhile.
In our natural rebellion against the restrictions imposed by the "driver's seat", we are doing more and more inappropriate and distracting things in moving cars [and if you're thinking about some salacious possibilities, yes, people have tried those too -- and several have crashed as a result, killing or injuring themselves or others]. The result has been a decline in road safety for everyone. The fact that people court disaster by attempting to do something (anything) useful or entertaining to relieve the tedium of driving, indicates that we are on some level painfully aware of the consumption of our time, and are trying to "have our cake and eat it too."
The bottom line is that the car subtracts enough hours from our lives that even with its occasional bursts of high speed, the lifetime average miles per hour is just not that impressive. Not enough "time efficiency" is gained to justify the extraordinary inefficiencies of weight and energy use.