Sms segment length (140 vs 160)
A single-segment sms can be 160 7-bit characters, 140 8-bit characters, or 70 16-bit characters.
In a multi-segment sms, some space in each segment is used to store fragmentation information, so each segment can be (approximately) 153 7-bit characters, 134 8-bit characters, or 67 16-bit characters.
If there are any extended, foreign, or emoji characters in a message, that will force the use of 16 bit characters.
If a block of text is copy and pasted into a message, sometimes invisible formatting characters are pasted as well. This may also force the use of 16 bit characters.
To try to get rid of these characters, manually delete the spaces between sentences and type the spaces again yourself (This may or may not work).
Foreign keyboard app and ROMs
There have been some situations where combinations of foreign language keyboards and foreign language ROMs give unicode input to 8sms, resulting in the use of 16 bit characters and shorter segment lengths.
If you encounter this situation, please test by changing back to the default Android keyboard on your device.