Subaru wrong tires

subaru
tires
forester

#1

Mom’s car is about 10 yrs old and she had the tires replaced about a year ago. About a month ago or so I drove the car and noticed that it was making some odd sounds on the front side of the car. Mom took it to the dealer and then said that the wrong tire size was put on the car and that is what is causing the noice. I called the tire store where we go to get our tires services and he agreed that the wrong tires where put on but that he had not heard that less than 1/4 of inch in size would make a difference. Can you help? with some guidance as to what we should do about this wrong size tire drama?


#2

Assuming this car has all-wheel drive, the most important thing is that all 4 tires be the same size, same model and tread pattern, and same amount of wear. If one or two tires were slightly different size than the others, this can wreck the transmission, and even a quarter-inch difference is enough.

How many tires were replaced a year ago? Are they the “wrong size” because you mixed old and new tires, or did you buy 4 new tires that are now supposedly the wrong size. If you bought 4 tires, what did she buy and why are they “wrong”?


#3

SDWH is correct. As long as all 4 tires are the same size/model/tread pattern and have the same amount of tread wear, it is really immaterial whether they are the exact size specified by the car manufacturer.

Problems occur when the tires are not matched closely enough, and this can happen when a car owner does not rotate the tires on schedule, or if only 2 tires are bought at a time, rather than 4.

We need more details on this situation, but based on the sketchy details presented so far, I don’t think that these “wrong” tires are the source of the problem.


#4

hi, wow thanks for the quick reply. All 4 tires were replaced at the same time and they are all the same model. According to the dealer rep the size that is recommended for the car and what was installed is the wrong size – about a 1/4 inch diff. Can that small amount cuase damage to the front end?


#5

The tire stores enjoy the business generated by such a claim but I personally can’t see the need for replacing tires in sets of 4, or even in pairs. If ‘comparable’ tires of the same ‘nominal’ size are on all 4 corners so that the car handles as it is designed and the wheels turn at very near the same speed what more can be needed. If the inflated diameters of 2 tires differed by 1/8 inch how many revolutions difference would that accumulate to in 100 miles? And that is the issue here.


#6

If all 4 tires were installed together and are the same size, they should not have damaged the transmission.

What size tires did you buy and what size are specified?

For example, a 2000 Impreza uses P215/60R16 tires. If you installed P215/70R16, the tires would have a higher sidewall and therefore would be a little bigger in diameter and a little bigger in circumference, and the car will ride a little higher off the road. On the other hand if you bought P215/50R16 tires your sidewall would be lower, the tire smaller, and the ride a little lower.

These differences would cause your speedometer to be off and might put the suspension a little closer to its travel limit (because the car is starting out a little too high or too low). Your tires might be out of alignment (too much or too little camber) and wear out sooner. Maybe the suspension would make a bit more noise, but I’m not sure why. I am not aware that it would cause permanent damage to the front end. Maybe someone more knowledgeable than me knows something different.


#7

There’s another discussion from a day or two ago where someone replaced two tires on his Subaru and lost the transmission a few thousand miles later. It could be a coincidence of course. The owner’s manual specifically advises keeping the tires matched. If one tire has new tread and the others are worn down by 4/16th, the new tire makes around 20 fewer revolutions per mile (depending on the size).

But, this question seems to be about “front end” noise, rather than transmission noise.


#8

I believe Subarus use a viscous fluid coupling separate from the transmission to power the rear wheels. A difference of 1/4 inch in the diameter of the tires would amount to less than 4 revolutions per mile on that coupling. How fragile is it? On a highway exit with a 200 foot radius what is the resulting front to rear disparity in revolutions?


#9

Maybe you and VDCdriver can have a steel cage match showdown. I’m sure the disparity is quite high in a sharp turn or even an off-ramp but that is occasional, rather than constant.


#10

Mom took it to the dealer and then said that the wrong tire size was put on the car and that is what is causing the noice.

Assuming your Forester is AWD, then the most important is the circumference of the tyres. All four tyres need to be very close in circumference. If they are not, you can get noise and expensive drive train damage. That maybe what you are hearing. Yea, even a small difference can do damage.

Tyres may be all the same size when you buy them, but they do wear and get smaller. That is why you may need to replace all four tyres if one of them wears out. If you have AWD there should be information in the owner’s manual.

If you need to buy a single new tyre, you may be able to have it shaved down to the size of the other tyres.

Now there is another issue you should remember. On any car the tyres should be closely matched side to side and front to back to assure safe handling under emergency conditions. You will not likely notice any problems in day to day situations, but in that once in a life time emergency, it will be too late.


#11

Subaru’s could care less on size listed or tread depth on tire. They care about rolling circumference which is the distance around the tire at a given pressure.

The viscous coupling does allow for slippage in manual transmission Subaru’s and more than 1/4" that Subaru specs. My long experienced mechanic states 1/2" is okay. 1"(difference in circumference) he has seen broken internals of transmissions. The automatic Subaru uses an electronic clutch pack not a viscous coupling. It is more sensitive to problems. If you notice very few AWD problems are every posted here about manual Subaru’s.


#12

If you compare a new tire and a tire the is worn down by 4/16, or 1/4 of an inch, that means the diameter of the tire is 1/2 inch less and the circumference (diameter x pi) will be 1.5 inches less.


#13

Exactly
While the disparity is indeed quite high on a sharp turn or an off-ramp, the duration of that disparity is mere moments and represents probably a fraction of a mile each time. Compare that to someone driving…let’s say…500 miles or more with mismatched tires and you get the idea of the difference in those situations.

In the other thread that is currently running, the OP tells us that he drove for 8 months/10k miles with new tires on the front and old, worn tires on the back. Truthfully, I am surprised that his viscous coupler lasted for that length of time, but it gives you an idea of how that component can withstand disparities in revolutions–but only up to a point.


#14

I will again challenge anyone who is truly curious regarding the effects of minor deviations in tire size to mark their tires at the 6:00 position and drive straight for 1/4 mile and compare the relative positions of the marks. In fact, have someone stand nearby and video the short drive. You might then let the air out of one tire to make it 1/2 inch lower, i.e., 1 inch rolling diameter, and retry the test. The proof is in the pudding.


#15

With the exception of same make/model/size tire there is not consistency in rolling circumference of a tires. It has to be measured.


#16

The fragile, POS Subaru driveline is failure prone. The dealers will go to great lengths to lay the blame on tires or anything else they can to avoid taking responsibility. If the tires are ALL THE SAME SIZE and that size differs from stock by less than .5 inches, (like going from a 60 series to a 65 or 70 series) then the TIRES are NOT the problem. NO CAR, not even Subaru, is THAT fragile…