Defects in manufacturing. Bold spots due to breaking or hard landings. Scars from debris.
tire balance or out of round tires.
drive shaft could be out of balance or tires could be out of round
Buy new ones! Out of round tires is a defect in the tire. Demand that the place that sold you the tires replace them. I am assuming you have not slammed on the brakes and slid the tires far enough to flat spot them. Tires can be shaved to make them round again but doing this causes a loss of rubber and they will wear out sooner.
Tires
out of round tires, needs balanced ,bad wheel bearings ,need more info; 65
If the trye has gone out of round (an MOT failiure ) Yes the car will shudder when driving.
All tires should be balance checked before being placed on a vehicle. If the tires are not balanced it will cause uneven wear and they will not last as long as they should.
Fling them away and get new ones.
Round Ones
Smaller tires should not cause the check engine light to come on. Smaller tires will cause the speedometer to be off.
Tires are made for the conditions that they are driven in. Therefore if you live in an area that has cold winters, that is it snows for a large portion of the winter, you probably should purchase winter tires. If you are living in Florida for example winter tires would not be required and year round tires would be sufficient.
Well I think you should change your tires to triangular blocks of wood and see how that goes.