In 1.1.1 the unit system has been shuffled around quite a bit, I hope for the better! Now, instead of having to manually
create buttons for units, I have done that hard work for you. To get to the unit screen press the 
button. You'll then see:
If you don't, hit the "UNI" button in the upper left hand corner. Now you're looking at the jumping-off point of unit manipulation.
There are a few types of units here, length, time, etc. Click on one of these to zoom in, giving you unit options in each category.
There are more under "P2". Pressing on a specific unit pushes it onto the stack, and then hitting
will apply it to the top of the stack. You can manipulate units just like numbers (in general a "1" is assumed), to create complex units,
before applying them to numbers. The main reason to keep units around is to convert between them. So, the way this is done is by using
, which takes the number & unit in the Y slot (second from the bottom), and
converts it to the units in the X slot (on the bottom).
Let's have a practical example: converting miles to kilometers. Your german friend tells you it's about "300km" to her city from where you live.
What's that in miles? Type "300", "km" (by pressing, "S", "len", "km"), .
Now 300km is on the top of the stack. Now press "mi" (by pressing "len", "mi"). You should see something like:
Now press , giving you the answer, 186.416 mi. Now for a slightly more complex example!
Converting miles to kilometers, liters to gallons, etc. is pretty easy for me to do in my head. When it gets more complicated, I start to get confused. For example, converting miles per gallon to kilometers per liter, there are two conversions happening simultaneously, and I can't as easily convert in my head. Luckily, TouchRPN can help here too! Let's get l/km in terms of mi/gal. Press "len", "km", "vol", "L", "÷". Now you have km/L on the stack. Now to get mi/gal on the stack, press "len", "mi", "vol", "gal", "÷". You'll see:
You'll notice that TouchRPN displays all units with exponents, negative ones meaning that they are in the denominator.
Now hit , and you'll see 2.352 mi/gal. Which means that 1 kmpl is 2.352 mpg.
So, when you see a car that gets 10 km/l, it gets 23.5 mi/gal. Not bad!