Well unless your measuring the exact diameter of the tire it's all BS

Reality
Go into a parking lot and find me one car that has all of the tires filled to the exact same PSI. Every one that doesn't but has the same tread depth would be OK then right. It's all right there- ON THE POSTER.

Someone needs to actually look at the tires for her. end of story.

 

i stand corrected. durometer it is.

i can guarantee mine wouldn't be the exact same PSI. cold PSI is 35 on the fronts, 32 on the rear. but other than that they are within 1lb every time i check them. i mean we can go back and forth over variables and this and that. there are a number of reasons why, with few exceptions, you don't replace a single tire. always in pairs. most of them have nothing to do with diameter or drivetrain problems.

but i agree, someone needs to actually look at the tire.

 

Look up Volvo V70/850 Cross Country AWD's. Look for posts on damaged bevel gears(1200.00 USD). Ask those owners who drove their Volvo's with a spare tire with a different diameter for less than 5 miles what happened. Now Audi and Volvo do not use the same type of AWD setup, but the possible damage is the same.

Are the rules on when all 4 vs 1 tire should be changed set in stone...of course not. But there are expensive consequences to not taking precautions.

 

Because they will have less tread. A typical new tire has a tread of 10/32. For saftey sake a tire should be replaced if the tread is down to 4/32. As a result, the usefule life of a tire is the 6/32 that falls in between. Since the all wheel drive tire replacement rule of 3/32 falls exactly in the middle of the 6/32 tread life of a tire, tires that have already passed half their mileage expectancy will be incompatable with a new tire. In my experience a typical high performance tire only lasts about 20,000 miles. Accordingly, if the other tires have more than 10,000 miles they should be replaced when you replace the bad tire.

 

What? this is, What???????? Man I hope my Michelin energy tires are Not considered high performance. Cause I have 30K on one set and I think I'm going to go for another 20K just because I like to live on the wild side.

ARE YOU WRITING THIS ALL DOWN> LOOK AT WHAT YOU HAVE STARTED> Just look at it! OMG.

 

I started to write about this but it got very long. Thus I decided to break it into three parts...

Tire discussions are always fun because there are many opinions, wives tales, and bull crap but few actual facts.

Let’s look at actual dimensions on a 205-55-16 tire:

According to Tire Rack the 206/55/16 has an outside diameter (OD) of 24.88 inches. Using simple math this gives us a circumference (Cir) of 78.194 inches and a revs per mile (rev/mile) of 810.289. Taking the same tire and removing 3/32 of tread depth the OD would be 24.789 inches (24.88 minus 0.09375 times 2) and the Cir would be 77.900 inches with 813.354 rev/mile. The difference being 3 rev/mile.

Now comes the fun part –

If you take a vehicle with identical front tires and make a 90 degree left turn with a radius of about 60 ft, the inside tire travels 94.28 feet while the outside tire travels 102.14 feet for a difference of 7.86 feet or 94.3 inches. 94.3 inches is close to 1 ¼ times the circumference of the tire so the outer tire makes more than 1 revolution more than the inner tire in a distance of 100 ft (1/4 of a 60 ft radius circle)

There are 5280 feet in a mile so it would take 13.2 revolutions to drive a mile in a 60 ft radius circle. We did 1 ¼ additional rev in a ¼ circle so 1 ¼ times 4 (a complete circle) times 13.2 gives us the additional amount the outside tire would travel. If you’re doing the math you’ll find that it is 66 more revolutions.

Compare 66 added revolutions to 3 and you’ll see how ridiculous the 3/32 comments are. It would be stupid for an engineer to design a vehicle that will not consistently make turn with a 60 ft radius. Thus vehicles have differentials.

 

Still with me? OK!

Next we need to look at manufacturing and operating variances.

We have been told that the 3/32 measurement is based on tread depth. This assumes that the “root diameter” (diameter at the base if the tread) is the same for all the tires being measured. This might hold water if all the tires came from the same mold and the curing process was exactly the same, but that is highly unlikely. Normal statistical analysis is based on variation – and I can assure you that tires have lots of variation. Tires are not perfectly round and tread depth can vary. In fact the tire manufactures have specifications with tolerances for many of there attributes. The key word is tolerances – meaning they are allowed to vary. The real issue is rolling diameter.

I was especially amused at the 10,000 mile comment. Typical new tires have a tread depth of 10 to 11/32s, but some are 12/32. But let’s use 10/32 and assume that it is exactly the same all around the tire and the 3/32 is valid. A tire is considered to be worn out with 2/32 remaining so we have 8/32 to play with – or 7/32 if you want to stay on the safe side. Tires typically run between 20 and 80K miles. So if the wear is even and linear, a 20K mile tire would be over the 3/32 limit in 750 miles, and an 80K tire would be over 3/32 limit in 30K miles. So what does mileage have to do with the fictitious 3/32 limit? They are not connected.

 

Asleep yet?
Really Po'd yet?
At least wait until you get to the end before jumping up and down! LOL

Air pressure

In racing (yes it is NASCAR) we always measure the OD of tires to make sure we know the real size, and check them often. We can easily find differences of and inch in OD and sometimes see up to 2 inches on un-mounted tires. (We do not measure in the center of the tread before they are mounted.) Once they are mounted we can do some adjustment with air pressure (these damn things are just rubber balloons with a few cords for strength) but the difference in diameters are still there. Getting a “matched set” means having the correct “difference in size” on the four corners of the vehicle for the best performance, so we do a lot of work.

Street tires are somewhat different because they are radials and there is less air pressure induced OD differences but it is still there.

So what do I do?

I try to keep a matched set of tires on my Quattro, but if I have a problem tire I change them in sets of two and keep the best ones on the front. I also begin looking for new tires when they get close to 3/32 left.

This last winter I trashed a front tire (Mich Exalto) in a New England pothole. The other tires were worn but in good shape so I got two new tires for the front, put the best of the rest on the rear, and put one on the shelf in the garage. Since they are directional tires I had to dismount all but one. I’ll watch the rears tires and if one begins to get worn I’ll use the garaged tire. When both are worn I’ll get two new front tires and swap the fronts to the rear. This leaves me with tires that are 50% worn on the rear and I expect that to be in the same place when I move the fronts to the rear.

I don’t get overly **** about tires and usually take my Audi’s to 300K before moving to the next one so I not hurting things.

Oh YA -- I almost forgot to repeat the answer to the question about needing 4 tires on an Audi. You might get by with three but I hope you have a strong girl friend if you plan on moving the vehicle.

 

What he said ^^^. Heh. Glad to see someone actually put up the numbers. NH USA, you have too much time on your hands! FWIW, I'm actually following the same practice as yourself. Two new ones on the front, two half worn ones on the back. All same make, model, size.

 

Brilliant!