About changing sprocket (preferably rear) for improving top speed/cruising speed/mileage. Can anyone suggest me a good replacement for the current rear sprocket of size 42T for my needs that fits perfectly and does not cause any other trouble? Do tell if anyone has changed the sprockets on their uni for improving top speed and if they have faced some problems due to that... and also if they got the desired result. So please anybody feel free to suggest a good replacement....