Best Gear Ratio

Depends on what you're trying to achieve. 17/37 is also on my T120. I've read a number of posts on various forums about lowering by going to 39 or even 41 rear. Me personally, I am generally OK with the stock ratio. My T120 isn't a sport bike, and I like the very relaxed cruising rpms.
I also have a T100 which has 17/41 ratio. I am thinking of a higher ratio, maybe an 18 in front.

 

@BobberT and your priority is????

Acceleration.
Cruising.
About town.
Motorway/open miles.

 

I've read of folks going to 39 tooth on rear on T120 with good results. I would imagine this would really be a personal preference and that some experimentation may be required.

 

That unfortunately, to quote an English idiom, sounds like "to have your cake and eat it!"

You can't have both!

Gears are ratios so if you lower it the gears become closer, obviously the opposite is true likewise.

If you lower it then you'll pep up the acceleration but lose out in cruising overland both in revs/noise and economy so it depends on where your personal priority is.
The manufacturers have a wide audience to satisfy and so consequently standard will be a compromise so it's up to you to decide one way or the other or, maybe after further considerations, just stay as is?

 

I took one tooth off my ‘19 T120 Bonneville gearbox sprocket as it is ridiculously high as standard. I think the Bobber is already lower geared, as is the 1200 Scrambler, hence my lack of concern in doing it. Triumph do a sprocket anyway so that helps, if a bit pricey.

I went from never being in the right gear to being spot on for UK speed limits and brought 6th down to being a usable gear rather than a virtual overdrive that I never got to use much.

Pull away and response was hugely improved and, I would hazard, may even have a positive effect on longer distance fuel consumption.

 

Any check lights should not be affected by you modifying final drive ratios. It is pretty common for owners to lower the gearbox sprocket by one tooth from standard.

Larger rear or smaller front means greater acceleration. Approx 1 tooth front is the equivalent to 3 teeth rear, do the maths to clarify.

Interestingly, if you run your bike up on paddock stands the abs light may show up as the front wheel is stationary and it is seeing the difference so the ecu will cap the power/speed but will reset once status quo resumes.

 

One tooth down on gearbox won’t need a chain change but any smaller may not be so good mechanically.
Three teeth up on rear sprocket could well mean a chain change.
I was amazed at the different sprocket sizes and ratios on the various 1200 Bonnie variants.
The ‘ordinary’ Bonneville is definitely geared for loping, not overtaking. Mine feels transformed as I would have sold it otherwise. I am no speed demon but, in the busy south east, quick overtaking is more important than speed.