i bought my maxima with 2 different tires, in the front are continental all season tires. in the back are michilan sports. its kinda hard to get these 2 tires to hold the same PSI for very long, due to the fact that the front are all season and back are sport. could this be why my gas miliage is so horrible??

 

depending on how low the tire pressure is, yea, that could throw your gas milage off.. but exactly how horrible is it?

How fast do they drop pressure?
It is reccomended to check your tire pressure like once a month
it's not a big deal to check it and inflate them..
if they loose air way too fast..
you need to get some new tires

either way, i don't see why they'd deflate faster being different tires...
By "sport".. do you mean, Summer performance tires? They too could be all season tires for all you know. check tirerack.com and find the same model tire and see what it says..