There are two things preventing this from happening that I see:

• 100km isn't recognized as a unit by itself
• MPG and L/100km are inverses of each other and not the same dimensions

So the unit converter won't handle these kinds of queries naturally. This sounds like a great suggestion for how Wolfram|Alpha can be improved.

If you ever see anything that can improved like this, let the Wolfram|Alpha team know in the feedback form at the bottom of the query page next to "Give us your feedback":

http://www.wolframalpha.com/input/?i=2+100km

Am I missing something?

Yes, of course. Miles (a length unit) per gallons (a volume unit) should convert into 100 km (a length unit) per Liter (a volume unit). Doing so

33.2 mpg in 100km/L

the input interpretation is: convert 33.2 mpg (miles per galon) to kilometers per liter

and Wolfram|Alpha answers that by

14.11 km/L

or 0.01411 (100 km)/L

The input interpretation given by Wolfram|Alpha is not for Wolfram|Alpha, but for the problem owner.

Thanks Udo, I understand dimensional similarity. However, as I pointed out above, WA is smart enough to perform some reciprocal unit conversions. Try "33 mpg in L/km"