It will depend on the situation or personal preference. If you are lightweight with no serious modification of tires enlarged and your road there is flat go to 43 to 39. But seriously, If your heavy and your rims and tires are large go to 45 above. Less stress for the engine. Below are some things/knowledge I knew about sprocket change results.
45 below
advantage:
- Increase speed
- 39 is ideal for speed devils
- Great for flat roads
- Ideal for light rider, or stock rims and tires
- For light rider/load, slight decrease on fuel consumption
disadvantage:
- Slow start from full stop
- Slow on uphill
- Stresses the engine in full load
- For heavy rider/load, slight increase on fuel consumption(specially on uphills compared to higher than 45)
45 above
advantages:
- Great on start up run from full stop
- Ideal if the rider is heavy
- Ideal if your changing to bigger tires and rims-heavy
- Great on uphills
- For heavy rider/load, slight decrease in fuel consumption(specially on uphills compared to lower than 45)
disadvantages:
- Not ideal for speed devils
- Not ideal for flat roads-long runs
- Stress on engine if you want to speed-up constantly
- For light rider/load, slight increase in fuel consumption
