I would doubt your lexus has a center diff. It'll be running in 2wd until the fronts slip or something like that. Its a non-issue. I'd have an ounce of concern on a subaru or a discovery or something legitimately running in awd on dry pavement all day. But not on a lexus.

Put two new ones on the front and forget it.

As for where to put the new tires, where do you want the grip? The wheels doing all the steering and stopping in an emergency, or the wheels that might slide sideways in an extreme maneuver that was necessary because you couldn't stop?

harddrive - Don't know if your wife's Lexus is 2WD, 4WD or AWD (you didn't state in your OP) - that is why I came in with what I know about AWD vehicles.

Do what you think is best - with the information given, I haven't ever heard of putting different diameter tires on the vehicle hurting the stability control system, but I have heard (and seen) the damage that it can do with an AWD drive vehicle and the center diff.

Sorry guys - I very rarely chime in on technical questions, now I remember why.

So with the logic of tossing the 50% worn tires out.

My 996 TT goes through 3 times the amount of rears than front. At the end of the life of the rears I should be blowing diffs?

With the worn tires on the rear you will have too much overstear. So if all 4 were worn to 50% the car would just slide off the road on the first corner?

Sorry for the sarcasm, but ......

I don't think you were sarcastic enough.

This.

Changed out a single rear tire on the junk 95 legacy from a nail just outside the repairable band. The tires only had 20-30% wear. The shop had me sign a waiver but at the time I thought they were just trying to up sell. I drove it home at lower speeds and the car pulled a lot.
Went back in and changed all four. Much better now.

On a 2004 Prius (with traction control), if one of the tires was slightly low the mileage would drop 5-10mpg.

There is something to what you say.

Your legacy has true full time awd with a centre diff. Unlike the op and his Lexus.

Boy there is some serious bollocks going on in this thread. Best tyres to the front, theyre doing all the steering and braking. Changing a pair of tyres on the same axle is fine, changing one corner on a road car is a decision about the state of the remaining tyre. ABS and stability systems will compensate for the level of grip available though any given tyre, what drivers used to do. AWD Subie and EVO pocket rocket tarmac and gravel rally cars get one corner changed all the time, and the R tyres and nature of use are putting loads through the driveline that road cars rarely experience.

Are you serious? By the book with measuring instruments, maybe? During my early college, running wild with my ratty car and no money, so used tires were all I can afford. Better tires were us front all the time. Same diameter, we hoped but I am sure if I measured it, they weren't. I still raised hell flying, sliding on the canyon roads. didn't notice much difference but the steering was a bit better. I slammed on what ever I can find at the local tire shop. Maybe today's super computers in cars pick up the difference in diameter so it blows up the diffs on purpose. That was crazy talk, bud.

All things being equal, a tire that is worn to half tread depth will have better grip than a new tire of the same construction and tire pattern, because the tread blocks will have less squirm.
This doesn't take into account the heat cycles that have gone into the older tire, which will cause the rubber compound to be a little harder, but the tire still has plenty of grip in the dry.

If it's wet, then tire performance will be compromised by any number of mitigating factors. It's up to the driver to determine what the tire wants to do when the weather turns, but that is always the case, no matter how old or new the tires are.

To suggest that half-worn tires will cause massive and dangerous oversteer or understeer is a ridiculous assertion.

AWD has the ability to slip, front to rear, or it wouldn't be AWD, it would be 4wd. You don't use 4WD on dry pavement, only off road or in slippery conditions. With AWD even if your tires are the same you would ruin your center differential every time you go around a corner if FOXPAWS statement were true. Yes, use tires of the same size but don't worry about the tread wear. Sorry PAWS you are wrong, the people you work with are wrong. It happens from time to time, get use to it. AWD, true AWD has to have the ability to slip from front to rear or your AWD will hop around the corners. Any shop would rather sell you 4 tires instead of 2.

Quoted from Tire rack:

"Here are recommendations from some of the manufactures that Tire Rack currently serves for matching the tires used on their four-wheel drive and all-wheel drive vehicles. Additional recommendations from other Original Equipment Vehicle Manufacturers is pending.

Audi As published in their vehicle owner's manual, "rolling radius of all 4 tires must remain the same" or within 4/32-inch of each other in remaining tread depth.

Porsche Cayenne within 30% of the other tire on the same axle's remaining treadwear.

Subaru Within 1/4-inch of tire circumference or about 2/32-inch of each other in remaining tread depth.

Change the two tires already... Most of what we worry about will never happen.

This. Changing one pair is super common on modern Porsches where they are staggered sizes and you can't rotate. Modern tires are just as effective at 50% as they were at 100%.

BTW, with new tires at 10/32 and half used tires at 5/32, you are talking about a very insignificant different in diameter. Change the pair and move on with life.

Was just changing the Camry snow tires/rims to summer tires and came across this very subject in the owners manual. Hdrive would suggest you check the owners manual and see what is recommended.

I'll throw in my stupid comprehension of my volvo xc70 haldex drive so someone can correct me.

At least on my volvo, the car is usually front wheel drive. If the computer senses the front wheels are spinning faster than the back, it starts to engage power to the rear wheels through the haldex clutch, so if that is correct, as long as the bigger tires are on the front, and they spin slower, everything is okay. Volvo owners manual states to put chains only on front. Maybe that keeps front spinning slower, or maybe because entire system is such POS it can't handle chains on all four corners. Right/Left bias is handled by ABS. I'm sure other cars are different, but this is my perception of the vulvo...
Correct?

Years ago I read that you have to be very careful about the tires you put on a 964 Carrera 4.
Apparently the front tires must be a slightly larger diameter than the rears. (or is it the other way around?)

But this was a long time ago and I'd imagine 4 wheel drive systems like everything else have progressed.

I really can't imagine such a small difference having and real effect regardless of what the owners manual may say.

I owned one, and used to change one set of rims for another identically sized set but using with a slightly shorter profile R tyres, and track it regularly. The rolling diameter shortened the gearing, affected the speedo, no other problem.

That driveline in that car is from the 959. You could take Poland with it.

If you are worried about the center dif, replace two, then rotate one side. The OD will average out to the same :)

Buy new front tires and have them shaved to 50%. Everyone is happy. J/K.