a tonne of stress

A spelling change can make its way haphazardly through our English.  Just because we now spell the weight tonne rather than ton, we don’t necessarily want to change the spelling of the word in an idiomatic expressions like a ton of stress.

 Google Ngram shows that the notion of tons of stress took off in the mid 1800s, peaked in the early 1900s and remained relatively constant throughout the century.  Tonnes of stress followed later, beginning in the early 1900s and peaking around the 1980s.

 It seems that the generation which knows only the tonne and has forgotten the ton is beginning to have an impact on tons of stress, turning it into tonnes of stress.  They are still very much in the minority (1 in 20) but they are evident and could have an effect in the future.