Mpg to l/100km conversion, fuel economy problem?

Question

We have two vehicles
A:old truck that does 17 mpg (13.84l/100km)
and
B:old car that does 47 mpg (5.00l/100km)
We are looking to replace one of these vehicles with a new one (of the same size)
C: new truck 19 mpg (12.38l/100km)
or
D: new car 61 mpg (3.85l/100km)

So which vehicle should we replace to save fuel? The old truck or the old car to end up with.
1). AD (old truck+new car)
2). BC (new truck+old car)

Read bellow once you have your anwser:



The way I solved this:
A:Old truck 13.84l/100km + D:new car 3.85l/100km = 17.69l/100km,
B:New truck 12.38l/100km + C:old car 5.00/100km = 17.39l/100km
Combination BC uses less fuel.(0.31l less)

Now using mpg:
17mpg+61mpg=78mpg
19mpg+47mpg=66mpg
Making the combination AD a better choice.

Why the exact opposite answer? What am I missing here?

Answer 1

Take a look at the way the fuel information is presented.  For the mpg values the number represents the distance travelled on a certain amount of fuel.  In the other values you are seeing the amount of fuel used over a certain distance.

miles (distance)/gallon (volume of fuel)

liters (volume of fuel)/100km (distance)

 

Hope this helps.