Replacement of one tire...?

So the car is in because the SRS airbag sensor failed (throwing a CEL)...took the car in last Friday. They diagnosed and got the parts in today. The dealer's technician went for a test ride and ran over something and blew the tire out (not repairable). They want to replace just one and the others have 18,000 miles on them...should I put up a fight? Would you replace just one with the tread wear differences? Let me know your thoughts. TIA

The tread on the other tires is likely pretty significant. If it is than there should be no problem replacing just one.

You don't give the specific tire type but 18,000 out of the typical 50,000-mile life is 36% expended. Assuming that the initial tread depth is around 12/32" , that translates to approximately 8/32" remaining (3/32" is considered worn out so it is the difference between the two times 0.36). The ~4/32" loss is 1/8", or 1/4" reduction in effective diameter. This results in some slight difference in rotational speed between the left and right sides, which constantly strains the differential, albeit slighty. Most diffs are okay with this minor difference.

The issue becomes more significant if you are equipped with AWD, of great significance if the new tire is mounted to one of the front wheels, and VERY significant if it is a different model of tire. It can severely affect steering and braking, especially in an emergency.

Indeed, there are some tire dealers that make you sign a waiver if you insist on replacing less than all four tires and/or replace two with a different model on AWD-equipped vehicles. I have an AWD Vibe with that very same situation.

If you have AWD, you have a good case to have all four replaced to minimize the strain on the AWD system...don't let them feed you BS that it can handle it, they're just pushing the problem down the road and the cost will be on you then.

If you don't have AWD, I'd insist on at least two new tires mounted on the front wheels...have them match the current make/model as well.

You don't give the specific tire type but 18,000 out of the typical 50,000-mile life is 36% expended. Assuming that the initial tread depth is around 12/32" , that translates to approximately 8/32" remaining (3/32" is considered worn out so it is the difference between the two times 0.36). The ~4/32" loss is 1/8", or 1/4" reduction in effective diameter. This results in some slight difference in rotational speed between the left and right sides, which constantly strains the differential, albeit slighty. Most diffs are okay with this minor difference.

They are OEM TOYOs. They don't last 50K unfortunately. They have 6/32 tread depth left (as measured by the stealer)...the new ones have 10/32.

The issue becomes more significant if you are equipped with AWD, of great significance if the new tire is mounted to one of the front wheels, and VERY significant if it is a different model of tire. It can severely affect steering and braking, especially in an emergency.

It is AWD, they replaced the front 2 tires (so they are both 10/32) and the rears are both 6/32. They replaced with exact same tires. Thanks for your response...I am still looking into it.

6/32"? That's more than half expended...57% to be exact. Hopefully they will do the right thing and replace the other two. But even if you kicked in $100, that wouldn't be a bad deal for alll new tires.

If not, then I'd definitely have them put it on the vehicle's service record that the dealer refused to change all four so they can watch for differential and transfer case problems in the future.

6/32"? That's more than half expended...57% to be exact. Hopefully they will do the right thing and replace the other two.

Yes they even said the OEM tires are poor for wear as they want them to ride smooth and be quiet to start...I had them document the fact they did not replace all 4 as you suggest. Thanks for your input!