Solution 30571: Creating User Defined Units For Unit Conversion on the TI-89 Family, TI-92 Family, and Voyage™ 200 Graphing Calculators.
How do I create my own user defined units for unit conversion on my TI-89 family, TI-92 family, or Voyage 200 graphing calculator?
If the appropriate unit cannot be found in the units menu a user may define a unit to store in the units menu.
For example, to convert cubic feet into gallons or vice versa, it is known that 7.48051948 gallons is equal to one cubic foot. This value may be saved as _CUBFT in the units menu by following the steps below:
1) Press [ON] [HOME] [CLEAR].
2) Press  [.]        .
3) Press [2ND] [UNITS].
4) Press [DOWN ARROW] [DOWN ARROW] [DOWN ARROW] [RIGHT ARROW].
5) Select _gal and press [ENTER].
6) Press [STO►] [◊] [_].
7) Press [C] [U] [B] [F] [T].
8) Press [ENTER].
The unit will now be stored under the Units menu.
There are four rules for naming defined units:
1) Can have up to 8 characters.
2) First character must be a _ (underscore).
3) Second character can be any valid variable name character except for a _ or a number.
4) Remaining characters (up to 6) can be any valid variable name character except a _.
Please see the TI-89 family, TI-92 family and Voyage 200 guidebooks for additional information.