Firstly, as a general rule reduced sidewall hieght means reduced sidewall flex & as you're looking for a more comfortable ride .....XP500FUN wrote:
A 120/70 14 is 84mm in height compared to a 80 at 96 so will loose abouit 12mm at the front, the speedo difference is negligable.
Secondly, the front tyre aspect ratio will have bugger-all effect on the speedo, SliverWing speedo sensor is on the rear wheel