Beware when buying tires for All-wheel drive vehicle. What you should know.
Recently we had a 2004 Chrysler Pacifica come into our shop for service. Two of the tires were worn beyond the safe point to drive, which is 2/32nds of an inch in tread depth (New is 10-12/32nds). The other two tires were measured at 7/32nds. The rule of thumb is to keep the difference in tread depth less than 3/32nds.
I know, this is confusing even for us sometimes. So, what did we do? We sold him two new tires that had a tread depth of 10-32nds affectively reducing the difference between the depths. It was a difference of 5/32nds, now it's 3/32nds. Well that turned out to be the wrong decision. When we replaced the tires, the model of tire that he had was no longer available, so we went with a similar but not exact match to the tread design.
The next day the customer was on a two hour trip, when his differential literally exploded. The differential is a component on all four-wheel drive vehicles that transfers the energy to each wheel from the engine/transmission. Once this happens, the vehicle will no longer move as was the case. Thank God no one was hurt.
So why did this happen?
Once the vehicle was towed in, the damage was obvious. We started assessing the vehicle,
- The tires were remeasured. Fronts were 7/32nds, rears were 10/32nds.
- We measured the circumference of each tire. Fronts were 91.5", the rears were 92.25". A 3/4" difference. Chrysler told us there should not be more than .25" difference. Uh Oh!
- Next we mounted the two original tires back on the vehicle and measured them. They were 90.25", a 1.25" difference. What?!?
So to be clear, by replacing the two tires we reduced the circumference differential by .75" and the tread depth by 2/32nds. This is supposed to be good. The speed rating was identical, but the tread design was different.
We concluded that the tread design was the culprit. We replaced the transfer case and installed two more tires to match. Car fixed.
So what should you do?
- Your tires all have to be the same brand and model. This can be a factor when you buy tires on special. If it's on special because it is being discontinued, Don't buy it!!! You may not be able to find one 6 months later if needed
- Rotate your tires every 5-6k miles especially if you drive an All-Wheel drive vehicle
- If you damage a tire beyond repair, and the tread depth is 5/32nds or half worn, then you can buy just one tire. There are services out there that will wear the replacement tire to any depth required, in this case it's 5/32nds
Let me have already made the mistake for you.
If you have run into this or a similar problem, please comment below. I would love to hear from you.
Brian Hohmann, owner of Accurate Automotive