QuestionQUESTION: I recently bought two new front tires for my 2003 Sorento 4x4 and went on a 100 mile trip. On the way back we noticed a sporadic vibration noise coming from the dashboard between 35mph and 55mph. Once home, now it doesn't seem to be doing it. One mechanic said it sounds like a speed sensor problem. The other mechanic said he's certain that I need to buy two matching rear tires. Something about if the tread wear is uneven (different tire diameters) between front and back that it makes the trans slip or lose grip somehow. Do either of these sound right?
ANSWER: Chris,
All AWD's and 4X4's should have their tires replaced in sets of 4. Tires of different diameters can cause problems with the transfer case / center differential. There has been many cars / trucks that have add these types of failures because of mismatching tires.
So at the very least get 2 more identical tires. If the problem continues, it's either the speed sensor (and I don't know why that would be the problem) or the transfer case has already been damaged.
BTW, new tires should always be put on the rear positions. That way in an emergency maneuver, the rear end will always stay behind the front end. It's bad when the rear end leads.
---------- FOLLOW-UP ----------
QUESTION: Just to make sure I'm understanding, even though there's plenty of tread on the rear tires and the new ones are the exact same tire (P245/70R16), there's enough of a diameter difference to cause this problem? Wouldn't the fact that the front tires' tread didn't pass inspection but the rear one's did, mean that the differential shouldn't been making this vibration noise before I got the car inspected? (I'm also kinda bothered that the mechanic didn't mention the need to buy 4 tires).
AnswerChris,
Even though the tires may be the same "size", unless they are EXACTLY the same tire, there is a possibility that they are slightly different diameters - and even if they are exactly the same tire, the difference in tread depth means they are slightly different in diameter. That can be enough to cause a problem.
This is one of the reasons why regular rotation is important, especially for AWD and 4X4's. The wear can cause differences in rolling diameter and rotating the tires regularly prevents the wear differences from becoming too great.
The fact that the front tires didn't pass inspection only means the tires were worn enough to REQUIRE replacement without any other considerations. That doesn't mean the tires couldn't have eventually caused a problem or that it was causing a problem that long term usage would reveal.
And it also bothers me that the mechanic didn't mention the 4 tire recommendation. But it is possible the mechanic either doesn't know about this, or that he wasn't aware that the vehicle was a 4X4. But some folks will only do what they are told - and if the customer doesn't want to purchase 4 tires, he may take the path of least resistance.
