Question about replacing tires
#1
#3
It can depend on the milage on the tires! If you ride a lot, 8000 miles a year and your putting a new tire on one side to match a 3/4 warn tire on the other expect the new tire to wear down quickly. This is more of an issue on cars and trucks that tend to have higher milage.
#4
#5
I'm assuming the other tire is probably at least pretty worn out as well. If those are the original tires, its probably time to simply replace all 4 of them. The only time I've just replaced 2 tires was when I had a 2wd machine and only bought new rear tires as the front tires weren't that important for traction. Stock tires are usually pretty crappy in the first place, replacing all of them will help a ton with traction
#6
I'm assuming the tire on the other side isn't worn out or s/he wouldn't ask in the first place. We'll see. The difference in tread depth doesn't seem to be a huge factor. One of the guys I ride with replaced one front tire that was bald but the other tire was still good. It took a long time to get that way from riding on the shoulder of the road but he kept riding it without any problems.