Stock front sprocket is 14 tooth.
Change to a 15 tooth for less revs and less vibration on the highway (change is 7.1%). With the 15T, the bike is more enjoyable to ride – you stay in the gears longer with less shifting required when starting out.
The new 5th gear will be more than halfway (~60%) between the old 5th and 6th gears. So with the 15T sprocket you now have two gears usable for the highway. If the bike struggles on a hill, just put it in 5th gear. If you need to, you can cruise forever in 5th gear. As expected, 6th gear does not climb hills as easily as the old 6th, but 5th gear is now more usable for the highway and climbs hills better than the old 6th gear.
A 6% hill on the highway that cars struggle with – the Acid Test – today my Rebel 300 climbed this hill very well even with the 15 tooth sprocket! It held 65 mph pretty good – I climbed 80% of the hill, dropping 2 mph before I downshifted to 5th.
Based on this, I would recommend also changing the rear sprocket if you use your Rebel 300 a lot on the highway. What I did (36/15) was +7.1%, 34/15 would be +13.4%, and 33/15 would be +16.9%. Rear sprockets for the Rebel 300 are much harder to find than front sprockets.
6th gear is 12.1% higher than 5th gear …. 6th gear is 29.4% higher than 4th gear
Rebel 300 gearing
36 14 2.571
36 15 2.400 1.071
34 15 2.267 1.134 …. The new 5th is slightly higher than the old 6th
33 15 2.200 1.169
This is the 15T I got for my Rebel 300: $36.48 USD shipped; it arrived in 4 days! https://sunstar-braking.com/products/361-520-powerdrive-countershaft-sprocket
INSTALLATION – You need a torque wrench. Loosen the chain first. You might need to leave off the metal part when you re-install the plastic sprocket cover (more below on this). Torque values: Front sprocket (2 M6 bolts) 7.0 ft lbs … Front sprocket cover 9.0 lbs/ft. …. I put a Sunstar 15 tooth sprocket on my Rebel 300. The Rebel has a steel half moon plate that goes on with the plastic sprocket cover. For some people it hits the chain, so they have to leave it off and some people have no problem (or they use a grinder on it), so they can leave it on. My front sprocket was wedged by it – I couldn’t even push the bike. It serves as a case guard in case the chain breaks, but that’s unlikely on a 300cc bike if the chain is well maintained. So I had to leave mine off. Maybe the Sunstar 15T sprocket is slightly bigger than the JT sprocket?
Rebel500 