I ran a 19 toothed front against the factory rear sprocket for around 8,000 miles.
While it certainly does lower the RPMs, my lil ol 2 cents is it works the engine too hard around town and on a grade, even if you keep it tached up. I can't say for sure, but I would want to guess & believe a proper Triumph driveline engineer might say the 19 is probably not a great idea most of the time. Then again, I'm not exactly what you'd call a light load on the poor little seat. (In fact I could swear I heard it say "git off me ya fat bas****," just the other day.)
Your mileage may vary.
