Tires
Tires can be checked with a simple visual inspection and a penny. Tires are worn out when the tread is down to 2/32. As a general rule, if you insert a penny with Lincoln’s head upside down into the tread of your tire and you can see his entire head, it is time for new tires. There is also a high spot molded into the rubber called a wear indicator that indicates when the 2/32 mark has been reached.
Tread depth is not the only consideration when evaluating your tires. It is also important to frequently check the tire pressure and visually inspect them for cracks, bulges and uneven wear. If your vehicles seems hard to handle or keep going straight on dry roads, imagine how much worse it may be on ice or snow.
If you do need new tires, be sure to get a good quality tire from a reliable shop that is willing to stand behind the product they sell if a warranty situation occurs.

If your car is still hard to keep straight after new tires, what could possible be the reason? Please give me some advice. Thanks!
1The first thing I would have checked is the alignment. If things check out ok there then letting a professional look at it would be the next step.
2