Okay first of all feel free to just ignore this inane tale of woe.
It is both highly theoretical and really just therapy for OCD in the middle of the night because of a dumb thing I did yesterday.

I can't believe my stupidity. I've been seasonally changing tires on two vehicles for 35 years of my life. I can do it, and possibly was doing it, in my sleep. I check the lug nut torque fastidiously. At least twice while changing them and then within 50 to 100 km afterwards. Plus every month or so thereafter.. yes I'm paranoid and OCD on lug nut torque.

Anyhow put the KO2s on for the summer and thought all was good. Drove the day, maybe 50km around the city. In the evening I grabbed the torque wrench and went around. To my horror two of the lug nuts, opposing ones, on the left rear wheel were pretty loose. In hindsight I wish I paid attention to their exact torque and rotation to tight, rather than tightening them up in panic, as if doing it fast helped LOL. I don't think they were finger loose but they certainly needed to be torqued up a noticeable fraction of arc.

🤬 I think know what may have happened. Sometimes the initial impact snugging is very slightly over torqued and I back them off a fraction and retorque them. I must have got cross-eyed and loosened a diagonally opposed pair and tightened the adjacent ones. 🤦 Maybe 🤷. Then somehow maybe compounded my error and missed my double check torque on that tire at the end of the evening. Or by some weird fluke only those two lug nuts loosened a bit over the day. 🤔 Unlikely...

Anyhow about the only thing I'm trying to soothe myself with is it was only lightly driven for one day and if you're going to have two lug nuts looseish, then having them opposed is probably the best scenario for force symmetry.

The last thing rattling around in my brain is what's the safety factor anyway? This was obviously missing one third of the designed clamping force. I doubt it's a full 50% margin although it's not like I was towing or off-roading that day.

Sigh. I'm Just going to have to settle with the discomfort of uncertain ( and likely very minor) overstress on my studs and wheel unless I want to proactively, and likely pointlessly, replace all six studs on my left rear hub.

Grrrr. Going to try to get to sleep now.

You're good, this is not a big deal since you caught it the same day, don't lose any sleep over this!

No need to replace studs if 4 were tight. Those 4 tight studs keep the wheel bore clamped against the hub, so no real stress on the studs.

Lost my front right wheel on a twisty back road at 50mph a couple years ago.

You should not stress over (2) nuts being loose. The conical shape of your lug nuts mean as long as nuts are in position and in contact with the female surface of the wheel then all studs would be carrying the weight of the vehicle. Conical surface of your lug nuts not only self centers the stud in the wheel hole but increases surface area to reduce stress here.

In addition to carrying the weight of the vehicle there is some moment created by your wheel / tire based on how offset the center of your tire / wheel is to the hub surface and also this also changes while driving, turning, wheeling. This is a pretty small moment but it cycles every time your wheel spins. This moment is also why wheel bearings wear out, the larger the moment the faster they wear. Assuming you don't have a massively negative wheel offset this should be a fairly small moment and with the conical nuts in contact with the wheel surface this would still put some tension on the loose stud / nut as well as the adjacent tight studs / nuts. At the distance you drove this would only cycle about ~20,000 times...

The issue with the loose bolts is they can fatigue, Torque (rotational force) is required on hardware to exert pre-tension on the bolt (linear force). A bolt with correct pre-tension never feels additional tension as long as the new tension does not exceed the pre-tension. This is why we pre-tension hardware. Bolts / studs last many times longer as they dont fatigue from tension cycles that a "hand tight" bolt would feel. I dont think you caused any damage over the distance you drove but I would argue you would not have overstressed the tight studs but you may have fatigued the loose studs.

Note that measuring torque is actually a terrible way to get pre-tension because differences in friction change how much torque is actually converted to pretension. IE imagine a rusty nut vs a waxed /lubed nut. You apply 100 ft lbs to each nut. The waxed nut has significantly less friction, spins further under the same torque and now has more pre-tension now than the rusty nut. Torque is easier to measure than pre-tension but this is a good reminder to keep your hardware clean and replace if rusty.

That's an excellent review! Also thanks for highlighting the tension cycle fatigue. I had that in the back my mind a bit too. Sort of why if I were truly crazy enough to do anything I'd replace all 6.

It is indeed the moment forces I worry most about. As you note the load is taken up by a combination of the friction, the hub centric wheel rims, and conical lug nuts. In my paranoia I can almost feel the slight over stress on the 4 torqued studs as the truck was cornering that day. It's this visceral mechanical empathy and self recrimination that's the real torture when I make these mistakes. Oh and the fact that now I'm going to check at least 3 times and probably take a picture of the torque meter on each lug in the future. Most aircraft proceedures have half the checks LOL - and Boeing probalby has a quarter.

Anyhow that was more of a technical response than I probalby deserved. I am though still curious as to what a typical design safety factor is in this situation. I'd wag more like 20% or one stud/nut, but I might be wrong.

I’m a bit OCD too and what I do is after torqueing all 6 to spec in pattern I follow up with checking them in a circle. I find this helps prevent me missing one. Above 5 lugs starts to mess with your head. My other truck is 8 lugs!

The built-in washer on our lugs often results in false torquing when you go back over the lugs multiple times, often find one that "screeches" and moves significantly. Would you recommend lubing the washers lightly between the washer/lug face and washer wheel face?

I think that would be a great question for a toyota engineer.

I would not / have never lubed the lugs or surfaces personally. I do think it would be safer to err on the side of slightly more pre-tension than less to prevent fatigue. That screech is some vibration between surfaces and if you suspect the friction is high or its popping you could lube or add more torque at that location.

Some more related bolting thoughts, In structural bolting we have a few different tension conditions:
-"Snug Tight" - as tight as an able person can tension a bolt using a standard wrench. Good for shear / bearing joints and usually require a locking device.
-Full Pretension - Tight enough to develop a high percentage of the bolts ultimate strength, usually 70%. The tensions here vary by bolt grade and size.

Measuring torque to develop full pre-tension is rarely used again due to varying friction and carrying a calibrated torque wrench is uncommon. Instead most pre-tension construction happens by "turn of the nut" method. This involves getting the bolt to the snug tight condition and then chalk marking a line down the side of the nut and onto the surface it bears on. The nut is then tightened until it reaches 1/3, 1/2 or 2/3 of a turn when measuring the rotation between the chalk mark placed on the nut and surface. This yields more consistent tension regardless of the friction conditions. The rotations needed depend on bolt size and bolt length.

For turn of the nut it takes less torque to achieve the correct rotation using a lubricated nut. Generally nuts are coated in a colored wax and different colors are used per size to help identify them easily. Note that there is significant friction between the threads of the bolt / nut and also the bearing surfaces so wax coating inside and outside of the nut is needed. Material finish also plays a role, galvanized finish can really roughen up the surfaces of the threads.

There are other pre-tension methods used in many industries. One more reliable one is the use of a direct tension indicator washer. This uses a one time use washer that under sufficient tension crushes small steel bumps and squirts out a rubber like substance indicating its tensioned. Really cool and nerdy stuff I know. Great video below showing how much friction effect tension.

https://www.youtube.com/watch?v=O96AUlIBkU8