Total noob, if I get a 42T rear do I need to replace the chain. I am sorry to ask to be spoon feed, but this is my 1st bike. I am 225 and most roads here are 55 mph (traffic at 65-70)and the bike screams at 45 to 50. The freeways are 70 so there's no chance. Is a larger front just as effective, say a 15 or 16t.
Your stock sprockets is 15/46 - so stock gear ratio is 0,3260869565217391
In case of replace only the front spr. to 16 gear ratio will be 0,3478260869565217
in case of replace only the rear spr. to 42 gear ratio will be 0,3571428571428571
So the replacing rear sprocket is more effective to get top speed instead of replacing front.
Replacing both more efficient of course.
Please note you can't install bigger than 17t to front due the physical sizes.