Rebel 300/500 speedometers are very accurate. I have compared my Rebel 500 speedo to GPS several times. My rear tire is stock and has 4/32 tread left. If your rear tire is a different brand, your speedo could be off.
My Rebel 300 speedo is accurate, also.
Motorcycle speedometer error examples (all tire sizes are stock)
1969 Honda CL-350 – 7% optimistic
1972 Honda CB-175 – zero speedometer error
1981 Honda CM400C – zero speedometer error
1986 Honda VT500C – zero speedometer error
2017 Honda Rebel 500 – zero speedometer error
2002 Yamaha Virago 250 – 6.8% optimistic
2019 Yamaha Bolt – 7% optimistic
2009 Suzuki C50 – 7.4% optimistic (65 mph indicated is 60 actual)
1999 HD XL883 – 3% optimistic (60 mph indicated is 58 actual)
1974 Kawasaki 650W3 – 11.7% optimistic
Rebel500 