Why is the word "mileage" used for vehicles, instead of efficiency?
Is there any difference between mileage and efficiency?
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Fuel efficiency was long ago typically measured in
miles traveled per gallon of fuel
= miles per gallon
Nowadays we tend to use litres per 100 kilometre (BUT we don't say litreage).
Both measures of efficiency have merit - the old way (mpg) stated what you could achieve with what you had. The new way (l/100 km) states what you need to achieve what you want to do. Either method is good enough once people are used to it.
A vehicle's efficiency of fuel use is usually stated in one of two ways
Both are legitimate means of conveying differences in efficiency but one is proportional to the inverse of the other so not only does one increase as the other decreases but the way that inverse proportionality works, one "squashes the results together" at one end of the measurement range and the other does it at the other end.
As others have noted, distance in the UK was traditionally measured in miles and fuel volume was measured in gallons. Efficiency statements were usually of the form in (1) above ie distance per standard amount of fuel.
The resultant logical units are miles per gallon or mpg.
"Mileage" is just a "short hand" way of expressing mpg efficiency.
A modern small car may achieve mileages of 30 to 50 mpg. A truck may achieve a mileage or fuel economy of 5 to 10 mpg, the heavier the truck the lowert the mpg and a 60 tonne/ton + main battle tank in enthusiastic may approach the gallons per mile range. So the mpgs between a very carefully driven small car and a medium car driven at motorway speeds may be 50 and 25 mpg - with a difference of 50-25 = 25 mpgh difference. However a tank transporter may achieve say 4 mpg but the main battle tank it carries may achieve 2 mpg. The ratio is again 2:1 but the difference is 4-2 = 2 mpg.
So a 2:1 difference may be 25 mpg or 2 mpg. This is OK as long as it is understood.
In my country at least (New Zealand) we used to use method 1. above but it is now more common to use form 2. As distance units are kilometres and fuel volumes are measured in litres you MIGHT expect that we would measure economy in litres per kilometre. We don't. A very economical small car may achieve efficincies of 0.05 litres per kilometre or 50 ml per km. A heavy truck may achieve 300 ml per km. These are perfectly sensible units to use BUT mls ( = millilitres, = 1/1000 th of a litre) are not what people are used to thinking oin when buying fuel. and, while a typical fuel tank may contain 50,000 ml of fuel, most people are not overly aware of this. So it was desired to have a unit that used litres rather than ml. And so, the measure used is litres per 100 km. So the 50 ml/km car now achieves 5l/100km and the 300 ml/km lorry now achieves 30l/100km.
We do NOT generally talk about "literage" instead of "mileage" even though it would make as much sense to do so.
Note that we still get "compression" of results at one end of the range - it's just at the other end. A medium car around town may achieve 10 l/100km. A small car on the open road may achieve 5 l/100 km. The ratio is 2:1. The difference is 10l-5l = 5l/100 km difference. But a tank transporter may achieve say 100l per 100 kilometre and a Main Battle Tank 200 l/00 km. The ratio in duel consumption is again 2:1 but now the difference is 200 -100 = 100 l/100 km difference.
So 2 cars with 2:1 difference in fuel economies may achieve 5l/100km difference in fuel consumption. 2 heavy vehicles with 2:1 ratio of fuel efficiency may have a 100 l/100km difference in consumptions. Again these results are not a problem as long as people understand them.
One interesting change occurs with the change away from mileage (= mpg) to l/100 km. It was common to talk about "fuel economy" -the result was in miles per unit of fuel used, and went up as economy improved. We now till talk about fuel economy with the measure being litres per special-unit of distance and the result goes DOWN as economy improves. If we are talking about fuel economy it arguably makes sense to have a measure which is expressed in terms of fuel used. Arguably :-).
Curious about the question, I did a little bit of research on mileage and efficiency. Efficiency of a vehicle is measured in various different forms and context matters. Efficiency in motor sports is quite different from efficiency as relates to general consumers. Fuel efficiency or mileage is what most consumers interested.
Mileage as it is related to fuel efficiency most likely has root to 1973 oil embargo. The PEW article on “Driving to 54.5 MPG: The History of Fuel Economy” tracks “Average estimated fuel economy by model year" starting 1975, which most likely has some relationship to the 1973 oil embargo. The Corporate Average Fuel Economy (CAFE) standard passed by USA congress in 1975 in response to the early 1970’s oil price shock also supports the above claim.
Needless to say these days, automotive manufactures use mileage as a marketing tool to promote new model automotive, which is an efficient method to promote new vehicle models.
Adding to the discussion, new hybrid vehicle platforms lead by Toyota Prius I believe has redefined fuel efficiency or mileage to higher level, which brings me to a question. With the introduction of electric cars lead by Tesla Motors is how long will mileage (mpg) be relevant? Is miles per watt (mpw) the new efficiency barometer for automotive?
Strictly efficiency is a ratio of the energy you put into a system compared to the useful work you get out so for example an electric motor that was 90% efficient might draw 100W of electrical power and produce 90W of mechanical power at the shaft.
The drawback with using efficiency as a measure for vehicles is that it doesn't directly tell you want you need to know since it is, by definition a ratio of power and doesn't in itself tell you the amount of energy required to travel a particular distance.
The actual amount of energy used by a car to travel a particular distance is determined not just by the efficiency of its engine but also its mass, aerodynamic drag (plus other resistive forces) and any other power used by sub-systems (eg air conditioning, lights etc).
With this in mind you can have a heavy car which is very efficient (in the sense that it converts a lot of the energy from burning fuel into mechanical power) but still has poor fuel economy because it needs more energy to accelerate its mass.
So overall quoting mpg or litres per km gives you more useful information about how much fuel a vehicle actually uses. Just saying that a vehicle is xxx% efficient doesn't actually tell you much.
Alternatively you could quote the power use by a vehicle in standard conditions in kW but that's still less useful as fuel is generally sold by volume and distance is a better approximation of the energy requirements of a given journey than the time taken.
Miles per gallon or km per litre is probably more easy for consumer to comprehend.
Efficiency requires a unit of kg x meter / joule where the mass is the payload of the vehicle and meter the distance traveled on 1 joule worth of fuel. E.g. Passenger km per kWh and Cargo load km per kWh are measurements of this but is seldom used other then by professionals.
Here's some example data estimates in kWh/ Passanger km for city traffic:
Light MC 0,40 Car 0,65 Bus 0,30 Tram 0,12