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>> After research found out that if you have some mileage on your tires, you can't just change one You go played by the mechanic. Even if the tread depth thing

>> After research found out that if you have some mileage on your tires, you can't just change one You go played by the mechanic. Even if the tread depth thing was an issue, you could have swapped out two rather than four. Unless you are racing, your car will be fine with slightly different treads rear and front. The reality is that most cars are like that anyway. Fronts often wear more than rears these days. So unless you are rotating your tires every month, tread depths will be different no matter what you do.

legitster 3y ago
If OP has an AWD the mechanic has to swap out all four.
- sandworm101 3y ago
  The AWD argument is one of those things that sounds logical but really isn't. Lots of AWD vehicles, I would say most AWD vehicles, are driving around with slightly different sized tired front-to-rear. Fronts just wear faster than rears. So within a few miles even a brand new car will have slightly smaller fronts than rear. The diff will be fine. Think of this scenario. What happens when someone sits in the back seat of a car? That compresses the rear tires slightly, imparting a slightly a different radius. Many cars will ask you to increase rear PSI to compensate, but nobody ever does. Are diffs failing for this? Nope. AWD cars and trucks manage carrying a random number of rear passengers just fine without any adjustment to tire radii.
  - legitster 3y ago
    I am not an expert on the sorts of computer chicanery that goes into AWD systems. But there is quite a big difference between the normal discrepancies that happen while driving and throwing in one or two brand new tires. Especially given that AWDs are already supposed to account for things like wheel slippage and power distribution. Besides, regardless if it's a myth - doing it will void your warranty, it's a requirement in every AWD service manual, and any tire guy caught doing it will be held liable if your diff blows up later. No mechanic or tire shop will agree to swapping out just two.
    - sandworm101 3y ago
      Everything voids warranties. If you are driving a new car, ie one still under warrantee, then you have to do whatever they want you to do. But there are lots of little myths about fixing cars, stuff leveraged by mechanics. Telling people that a diff will "blow" should the tires be 0.1% different in size is one of those myths. The entire purpose of a diff is to allow wheels to turn at different rates, hence the name differential. Only when you are getting towards differences enough to trigger anti-slip mechanisms during normal driving would it become an issue. The manual probably also says that aftermarket oil/air filters will void warrantees, or that changing your own brake pads is dangerous. Total bunk.
      - legitster 3y ago
        Most AWDs use a central diff for all four wheels. It's for distributing different amounts of torque front and back. It's not quite the same as a 2WD or 4WD differential(s) that only handle left/right. It's not at all the same as just a big open differential. A central diff compensating for front/back distribution while also handling a constant mismatched load on one side is definitely going to mess it up. If it's your car feel free to try it and tell me if you get a bunch of dash lights from your AWD computer. > The manual probably also says that aftermarket oil/air filters will void warrantees, or that changing your own brake pads is dangerous. If your manual actually says this I will eat my hat. It's literally illegal in the US: https://en.wikipedia.org/wiki/Magnuson%E2%80%93Moss_Warranty_Act https://en.wikipedia.org/wiki/Magnuson%E2%80%93Moss_Warranty...
  - KennyBlanken 3y ago
    If you have a true full-time AWD vehicle such as a Subaru or Audi with longitudinal engine, then mismatched tire wear will cause increased wear and tear on the center differential, which is expensive and difficult to get to. This is especially true if the differential is limited-slip. If you have a torsen or similar FWD car that only engages the rear wheels when slip is detected (most Audis with transverse mounted engines, VWs, etc), then you're fine. Tire diameter in passenger cars does not change with loading.
    - deleted 3y ago
      [deleted]
  - creato 3y ago
    Such cars also have rotating the tires as part of the maintenance schedule. If you rotate the tires, I don't think the difference in size will be significant. > Think of this scenario. What happens when someone sits in the back seat of a car? That compresses the rear tires slightly, imparting a slightly a different radius. The tire radius changes, but that doesn't matter. What matters is the rate at which the axle turns relative to distance covered, and I doubt that changes significantly with loading.
lsllc 3y ago
Definitely got played. Unless your daily driver is a train and you only go in straight lines, it won't matter about differences in the tire circumference. Every time you turn the steering wheel (even a little, let alone make a turn) all 4 wheels are rotating at different speeds anyway -- it's why we need differentials! An AWD has an additional center diff or transfer case that allows the combined rotation of the front (trans)axle and rear axle to be different (because unless you are steering dead straight on a flat non-cambered road they will always be slightly different). A 4WD doesn't have that center diff and you have to manually engage 4WD to use it (that difference between 4WD and AWD). You will get transmission wind-up if drive in 4WD in anything other than a straight line on a normal road surface -- fine in the mud or snow because the wheels will slip easily enough to reduce the windup. In fact if you get really stuck, you want a diff-lock or limited slip diff so you can prevent the power going all to one wheel (or two in an AWD/4WD config). It's a myth used to sell more tires.

legitster 3y ago

If OP has an AWD the mechanic has to swap out all four.

sandworm101 3y ago
The AWD argument is one of those things that sounds logical but really isn't. Lots of AWD vehicles, I would say most AWD vehicles, are driving around with slightly different sized tired front-to-rear. Fronts just wear faster than rears. So within a few miles even a brand new car will have slightly smaller fronts than rear. The diff will be fine. Think of this scenario. What happens when someone sits in the back seat of a car? That compresses the rear tires slightly, imparting a slightly a different radius. Many cars will ask you to increase rear PSI to compensate, but nobody ever does. Are diffs failing for this? Nope. AWD cars and trucks manage carrying a random number of rear passengers just fine without any adjustment to tire radii.
- legitster 3y ago
  I am not an expert on the sorts of computer chicanery that goes into AWD systems. But there is quite a big difference between the normal discrepancies that happen while driving and throwing in one or two brand new tires. Especially given that AWDs are already supposed to account for things like wheel slippage and power distribution. Besides, regardless if it's a myth - doing it will void your warranty, it's a requirement in every AWD service manual, and any tire guy caught doing it will be held liable if your diff blows up later. No mechanic or tire shop will agree to swapping out just two.
  - sandworm101 3y ago
    Everything voids warranties. If you are driving a new car, ie one still under warrantee, then you have to do whatever they want you to do. But there are lots of little myths about fixing cars, stuff leveraged by mechanics. Telling people that a diff will "blow" should the tires be 0.1% different in size is one of those myths. The entire purpose of a diff is to allow wheels to turn at different rates, hence the name differential. Only when you are getting towards differences enough to trigger anti-slip mechanisms during normal driving would it become an issue. The manual probably also says that aftermarket oil/air filters will void warrantees, or that changing your own brake pads is dangerous. Total bunk.
    - legitster 3y ago
      Most AWDs use a central diff for all four wheels. It's for distributing different amounts of torque front and back. It's not quite the same as a 2WD or 4WD differential(s) that only handle left/right. It's not at all the same as just a big open differential. A central diff compensating for front/back distribution while also handling a constant mismatched load on one side is definitely going to mess it up. If it's your car feel free to try it and tell me if you get a bunch of dash lights from your AWD computer. > The manual probably also says that aftermarket oil/air filters will void warrantees, or that changing your own brake pads is dangerous. If your manual actually says this I will eat my hat. It's literally illegal in the US: https://en.wikipedia.org/wiki/Magnuson%E2%80%93Moss_Warranty_Act https://en.wikipedia.org/wiki/Magnuson%E2%80%93Moss_Warranty...
- KennyBlanken 3y ago
  If you have a true full-time AWD vehicle such as a Subaru or Audi with longitudinal engine, then mismatched tire wear will cause increased wear and tear on the center differential, which is expensive and difficult to get to. This is especially true if the differential is limited-slip. If you have a torsen or similar FWD car that only engages the rear wheels when slip is detected (most Audis with transverse mounted engines, VWs, etc), then you're fine. Tire diameter in passenger cars does not change with loading.
  - deleted 3y ago
    [deleted]
- creato 3y ago
  Such cars also have rotating the tires as part of the maintenance schedule. If you rotate the tires, I don't think the difference in size will be significant. > Think of this scenario. What happens when someone sits in the back seat of a car? That compresses the rear tires slightly, imparting a slightly a different radius. The tire radius changes, but that doesn't matter. What matters is the rate at which the axle turns relative to distance covered, and I doubt that changes significantly with loading.

lsllc 3y ago

Definitely got played. Unless your daily driver is a train and you only go in straight lines, it won't matter about differences in the tire circumference. Every time you turn the steering wheel (even a little, let alone make a turn) all 4 wheels are rotating at different speeds anyway -- it's why we need differentials! An AWD has an additional center diff or transfer case that allows the combined rotation of the front (trans)axle and rear axle to be different (because unless you are steering dead straight on a flat non-cambered road they will always be slightly different). A 4WD doesn't have that center diff and you have to manually engage 4WD to use it (that difference between 4WD and AWD). You will get transmission wind-up if drive in 4WD in anything other than a straight line on a normal road surface -- fine in the mud or snow because the wheels will slip easily enough to reduce the windup. In fact if you get really stuck, you want a diff-lock or limited slip diff so you can prevent the power going all to one wheel (or two in an AWD/4WD config). It's a myth used to sell more tires.