One of the four tires has been losing its air regularly and it has been almost an year. I didn't really bother to fix it since the pressure inside maintained quite well once I filled it (I have a portable electronic pump in the trunk).

For the first six months, It used to take about 3 months before it went completely flat. And then it became 2 months basis, 1 months, and finally daily now. I didn't find any object or damage on the tire, nor can't really tell with my eyes whether the rim is bent or not.

I can't fill up the tire everyday and want to fix the problem right away. I don't want to buy both the wheel and the tire (poor college student ). How can I be sure which is causing the flat tire?

BTW, it is 19 RJ Inspyre with 245/35/19. Sorry for the bad English.

 

Take it to a Firestone (or similar) nearby. Tell 'em you've got a slow leak and want to get it plugged. If it's the tire and repairable, it'll cost ~$25.

If it's the tire and not repairable, it'll cost at least 1 (preferably 2) new tires. Or daily filling with air.

If it's not the tire, they'll tell you.

 

If you're lucky, the problem will be a bad valve stem or a nail/screw puncture where the head of the object is embedded into the tire tread so you can't see it.

Replacement of the valve stem or repair of a repairable punctured tire costs $10-15 in SoCal.