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I've owned a 2016 T120 Bonneville for three years and always felt the gearing ratio was a bit high for take-off, around town and countryside riding (low torque). Also, the axle in my bike was always near the back of the swing-arm slot like the OE rear sprocket was too small or the OE chain too long. I changed the rear sprocket from the OE 37 tooth to a larger OD aftermarket 39 tooth. Nice difference. The bike now has more torque "off the line". With the original OE 37 tooth sprocket I was hitting about 3,100 RPM at 70 MPH. The larger 39 tooth rear sprocket gives about 3,250 RPM at 70 MPH. Plus, the rear axle is now nearer the "center" of the swing-arm slot where it should have probably have been all along. No new chain required to go from a 37 tooth to 39 tooth rear sprocket. Acceleration is now livelier but comfort at highway speeds was not compromised. I used a JT Sprocket P/N JTR2014.39.
 

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Does the sprocket change affect the speedo? Did Triumph throw us a bone and have the bike measure its speed at the wheel? 🤞
 

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speed sensor is at the wheel.
So unless you change tire diameter or overall circumference then the speedometer and anti-lock features will still work.

Going up from 37 tooth to 39 tooth will increase your RMP 6-7% for the same speed and reduce the gas mileage by that much as well.

I ride mainly in the foothills and mountains. The change did help in mountain curves. But I notice the gear change as a negative thing when on the straight aways where I can get up to 65 to 70 MPH.

I am very glad I changed to a 39 tooth sprocket but the 41 tooth sprocket would not have been good. This is a T120 Bonneville not a Thruxton we are talking about. The bike geometry and seated position kind of limits what we can really get away with in the corners on the T120's.

But gears are cheap and easy to change with hand tools. I think I paid about $32 with tax and shipping.

Try a gear change and see if you like it. You can always go back to the stock gearing.
 

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I have a 40 tooth on my T120. I am not convinced it has any negative effect on the gas mileage. Sure makes the bike easier to ride at walking speeds though, and makes 6th gear get used a lot more.
 

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The modification to 17/40 greatly improves the driving pleasure and my consumption has not changed.
I explain this by the fact that we are more often on a higher gear with 200-250 less laps.
Small precision: the speed measurement is taken on the ABS and therefore does not change.
( Google Translate )
 

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speed sensor is at the wheel.
So unless you change tire diameter or overall circumference then the speedometer and anti-lock features will still work.

Going up from 37 tooth to 39 tooth will increase your RMP 6-7% for the same speed and reduce the gas mileage by that much as well.

I ride mainly in the foothills and mountains. The change did help in mountain curves. But I notice the gear change as a negative thing when on the straight aways where I can get up to 65 to 70 MPH.

I am very glad I changed to a 39 tooth sprocket but the 41 tooth sprocket would not have been good. This is a T120 Bonneville not a Thruxton we are talking about. The bike geometry and seated position kind of limits what we can really get away with in the corners on the T120's.

But gears are cheap and easy to change with hand tools. I think I paid about $32 with tax and shipping.

Try a gear change and see if you like it. You can always go back to the stock gearing.
My fuel mileage didn't change when going from 16/42 to 17/40, and others report the same. I did get a major improvement when I did Raisch stage 4 kit (cam,, x-pipe, Power Commander, air inlet opened). Mileage jumped from 46 to 53mpg.
 

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i am in the process of putting my 520 X ring chain with 17-41 sprockets on my 18 T120, glad to hear my speedo will still be fairly correct as it was OE. after pulling off the front sprocket cover i see theres xtra work to do compared to my traded 2012 bonnie, but i enjoy DIY + since weather is iffy at best i keeps me busy as being retired + living alone mostly theres lots of time to do whatever i like!!
 

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I've owned a 2016 T120 Bonneville for three years and always felt the gearing ratio was a bit high for take-off, around town and countryside riding (low torque). Also, the axle in my bike was always near the back of the swing-arm slot like the OE rear sprocket was too small or the OE chain too long. I changed the rear sprocket from the OE 37 tooth to a larger OD aftermarket 39 tooth. Nice difference. The bike now has more torque "off the line". With the original OE 37 tooth sprocket I was hitting about 3,100 RPM at 70 MPH. The larger 39 tooth rear sprocket gives about 3,250 RPM at 70 MPH. Plus, the rear axle is now nearer the "center" of the swing-arm slot where it should have probably have been all along. No new chain required to go from a 37 tooth to 39 tooth rear sprocket. Acceleration is now livelier but comfort at highway speeds was not compromised. I used a JT Sprocket P/N JTR2014.39.
I've got a 2019 bobber blk, up until I de cat'ed it it was jumpy torquey odd in 1st gear like it was way low geared ie tree climbing gear. Now with no cat it's really much nicer, I can even down shift into it at speed in tight corners. Like flying into my driveway:)
I want to go the other way on sprockets and push the top speed, but haven't heard good things about it. So I'll wait...
 

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Discussion Starter #10
Hi Jessica, it's your bike and your riding style. If you want to try higher gearing I'd say go for it. Sprockets are cheap and you can easily go back to your OE gearing if you want.
 

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I added a 39t rear to my Bobber. For me its a nice improvement. Acceleration is better and I haven't noticed any major change in fuel consumption. Its a cheap upgrade and easily reversible.
 

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Jessica, I am not sure what final ratios are on a Bobber, but if it is anything like a T120 I wouldn't expect an increase of top speed with raising the gear ratio. Theoretically you could, but the Triumph 1200 just doesn't make enough power to reach redline in top gear with any reasonable gear choice. De-catting sure isn't going to give it that much more power. So raising the gear ratio may make the bike rev less at high speeds, but it just wont rev as far and you will have a top speed that is pretty similar.
 

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I went to a 39T rear after the first month on my T120. I liked the change in feel too initially.

IMO, What really improved performance was when I loaded "Diva's tune". DNK TuneWorks It is the most worthwhile performance mod I've done so far.
 

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Added”boosterplug “ 1000miles ago. It is all I needed. Leveled out low idle, increased cruising power. Getting 53 mpg at almost any riding style. 2018 T120. Loving it!
 

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Just ordered a boosterplug mod, will try as soon as curfew allows, that should limit the need to change sprocket.
 

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For maximum user comfort in cities and built-up areas, the combination of the 3 modifications is perfect:
  • The rear crown of 39-41 teeth is essential and the effect is immediate (replacing the change of rear crown with a front sprocket of 16 teeth would not be a good solution: recoil of the rear wheel and greater chain fatigue )
  • Use of Rain mode.
  • Installation of a Booster plug for 1 month (little tried) = additional flexibility at low speed and delayed triggering of the fan.
Like that, the T120 is perfect.
( Google Translate )
 

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I swapped to a 41T rear sprocket a few years ago and love it. I couldn't ever imagine dropping back down unless I also made a change to the front sprocket too.

Sent from my Pixel 4 using Tapatalk
 

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i have a 41-17 520 combo ready but gotta get the stubborn dampers out of the old rear hub as i have a replacement sealed wire wheel to put on. rain + cold still so no hurry!
 
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