Rotating Tires

Depends on the tire and driving conditions. Some people rotate their tires religiously - some don't. Some cases, you cannot rotate the tires, or the tire manufacturer recommends not to rotate (usually sports cars).

Tire gauges or even the coin trick can be used. More info on sites like Tirerack - lots of articles on tire tread and measuring tread depth. http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=51

You also have the tire's built-in tread wear bars to tell you when the tires are down to the minimum legal limit. Be pretty obvious when you have the wheel off the car.

You should measure the tire in several points to make sure you are getting a good sampling of tread depth. If the depth varies across the tire, could have another issue to worry about, like alignment.

Myself, I rotate them every 10K or so - when I have the wheels off to check on the brakes. But I have let some rotations go due to time constraints - not a deal breaker for me, as I change the tires before they drop past 50% tread life anyways. As I find that only a small fraction of my tires need to be changed due to tread being totally worn out - most of the time its tire alignment issues, road hazards, punctures, and rubber degradation kills the tire before the tread wears out. I think I only got two of three sets of tires that I completely worn down the bars in my lifetime.

I think it is more important to look at how the tire worn out instead of going by tread depth - but like they say, your mileage may vary.

Lots of info out there - as echarleswhyte alluded to - depends on the tires you have.

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=43

I usually follow position "D" in the link - moving the tires front to back / back to front, keeping them on the same side of the car. I usually don't include the full size spare tire on the Rav4 - the other cars have compact spares, so they don't even apply.

ex. My current set of Pirellis on the Matrix - they recommend against rotating them. This makes set #2 on that car, original ones were BFGoodrich g-Force T/A KDWS tires - pretty decent, but never lasted more than 14K-17K miles before wear down, regardless of how they were rotated (one set was not rotated as a test) - tire pressures were maintained well, alignment was spot on. After the 3rd set of tires by 40K miles, switched to Pirellis. Now on the second set - rotated the first set against the manufacturer's recommendation - got about 35K miles out of them. Second set, followed the manufacturer's recommendation - closing in 25K miles, tires shown less wear at this point compared to the previous set.

Correct. Tread design can handle cross-rotations. Just note that tire wear/tire noise will be slightly higher immediately after cross-rotating them due to how the existing nap of the tire is working with the new direction of rotation. That will quickly wear off and the tire will roll true, but before then, the car many drive a bit "off". Cross rotation with a symmetrical tire tends to level wear more effectively, especially since most cars are driven with only the driver. This all assumes that the alignment is true and suspension is sound to start with.

That will work as well, it just won't level all wear (each corner) - just levels the wear front and back (axle pairs). Your call, all the rotation patterns are valid for symmetrical tires.