Tires: what is the cause of this?, firestone firehawk, inflation pressure

Question
QUESTION: on 2/23 i bought a 01 ford focus 4dr se, i noticed before i bought it that the car needed back tires so the used car dealership put on 2 tires which are p195/65/r15 1 tire is a firestone firehawk and the other is a dunlop sig and my front which are the stock pirelli p195/60/r15. since my back tires are different size can that cause any damage to my car?

ANSWER: Steven,

No, but you need to get that fixed.

First, vehicles tend to pivot around odd tires in emergency situations - and the more different the odd tire is, the worse the tendency.  You have probably the worse possible situation.  You have odd tires all around.

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QUESTION: i have noticed that on the highway around 70mph i have a vibration is the best way i can describe it. my steering whell doesnt shake but i double check to see if i needed a high speen balance but i only check 1 tire. the balance was perfect. i didnt think it would have been the back because there supposley new tires is there anything else that can cause this vibration which is not a strong vibration but i notice it.

Answer
Steven,

A vibration could also be caused by irregular wear - which is caused by mis-alignment and aggravated by insufficient inflation pressure and insufficient rotation practices.  For practical purposes, only replacing an offending tire will fix that.