Three conditions:
1. Age: tires that are too old shouldn't be used anymore, since they will become brittle and can blow. Find out the production date on the sidewall quoted in production week and year. I would say depending on intended use, no older than 5-7 years on a car or motorcycle, 10 years on a lil' red wagon or similar use
2. Cracks: if you see visible deterioration (cracks) anywhere it's time to chuck them
3. Use: under minimum amount of profile necessary, ditto. See the wear indicators being flush with the tire surface? Time to let go.
An old hand from Ford once told me: your car has road contact in four spots about the size of a postcard. Are you going to risk your life saving on those four contact spots?
The age is not a factor it is the mileage on the tires that matters. Go back to where you bought the tires and deal with them.
Your alignment could be off. Meaning your tires are either facing out a bit, or facing in a bit. This can cause your tires to go bad. Your local garage should be able to do a alignment on your vehicle to straighten things up.
Go on the high side, they wear the edges real bad. Go on the high side, they wear the edges real bad.
No, that is a very bad substitute. If you want to go with a 70 series tire go with a 195/70-15 which is a great substitute.
Bad tires and bad alignment can cause the car to shake at certain speeds.
My van has bad tires at front and better ones at rear, should wheel alignment be performed with bad tires?
A worn ball joint will cause tires to wobble or rotate in odd ways, causing uneven tread wear. If you suspect a bad ball joint get it professionally checked or changed ASAP.
This is not a question its just a statement.
Misaligned axles, bad mounts on the tires, improperly inflated tires.
You could, but it would be more accurate with new tires.
It is not. That is why a lot of tire shops fill tires with only nitrogen.
If you can have good winter tires fitted to them, there's nothing bad with having 20" tires in snow.