Do I need an alignment after replacing one tire?
 

My wife's 2006 Mini has a low tire pressure warning light. The only time it has come on, all tires were fine and equal. Recently I noticed the left front getting low kind of often. It was down 18 psi when I checked Sat., filled it up and was down 10 psi after 100 miles later that night. Yesterday we looked at it closely and the lower sides are trashed. Looks like it was from driving too long with too low pressure.

Car is still under warranty. Mini says the low tire pressure warning light will only come on if one or two tires are low, not if all four are equally low. Well, that's the exact opposite of what has happened. At minimum, the left front tire needs to be replaced. Dealer wants $190 per, recommends both fronts and an alignment too. Discount Tire wants $110 per. Car is at the dealer now having the low tire pressure light looked at. If it's bad, do I have a case that they should replace that tire for free, since it got beyond repair as a result of the light's not coming on? I never drive the car, so I only recently started looking at the tires when my wife said something about it.

If the dealer won't do anything for us, we'll go to Discount Tire. But do we really need an alignment after replacing one or two front tires with 20k on the car? Also, can't a shop check with their alignment rack whether it's really necessary before doing the work?

no

Changing tires has nothing to do with alignment.

You can need an alignment w/o having changed tires.

Tires and alignment are two separate issues although one does affect the other and not the other way around.

And no they won't check it for you as setting it up to get the 1st reading is work.

Several ways to tell if you need the work, check the tire wear, does it pull, does it crab, etc...

I always get an alignment with new tires.

Not because the car needs it, but because I want the tires to last--and "checking" the alignment is not far off from doing an alignment.

No. And it's best to replace tires in pairs for consistent handling. Some shops will check alignment for free if you buy tires from them, our local Firestone factory store does.

I know I won't be able to convince the wife to do this. But I was thinking it would be a good idea to replace both fronts and use the one good old tire for a full size spare. The wheels are cheap on eBay. Still haven't heard from the dealer.

Just do it as it's a sensible idea.

At 20k miles per tire I would not replace just the one either.

Just make sure they get you new valve stems. Almost everyone does it but they can and do forget and you could end up getting 2 failed stems a mile apart and of course on the second one you'll no longer have a spare. Ask me how I know.

Mini service rep. just called and said the low tire pressure sensors were working fine. I asked why the light didn't come on when my wife drove many miles while 10-20 psi low. He said they're gonna put it back in the shop and look at it again. WTF? What good is this warning light if it doesn't come on when one tire is so low?