I had a tire go down last week (295 70 17 duratrac on 2020 f150 fx4). The tire was more than halfway worn and I really can't afford a new set of tires right now.($1,500).

I ordered a new tire from tirerack.com and had them shave it down to 8/32. I gave them this measurement based off the depth of the center of the tread. I did not notice or take into account that the center of the tire tread was more worn than the outsides of the tread on the old tire.

When I got the new/shaved tire I immediately noticed that the tread looked smaller. The inside tread depth of the old tire measures 7 to 8/32s while the outside measures 11 to 13/32s. The new/shaved tire measures 8/32 evenly across and I can tell a difference in the outer treads.

I was planning on putting the new tire on the rear but am now concerned about the difference in size and don't want to do major damage to my drive train.

What is the allowable difference in tread depth between tires on the rear axle?

 

You should be fine to have a tire that is just a couple thirty-seconds smaller. The rear is probably best as you have an open differential.
The front, if you have 4a option, may cause an issue. I think the difference is too minute though.....and there are cases when a mismatched tire can contribute to 4x4 shift issues when on the front of The vehicle. the hubs could spin at just enough difference in rate to trick the IWE engagement. That small amount probably okay though.
I've seen one example (internet) where the same identified size tire was different enough between manufacturers that it contributed to the shift concern. For example two tires, different manufacturer, of 265/75-17 was just enough to be a problem.

 

You can not do damage to a stock drive train since all F150s have open diffs. Even using the Elocker on the rear with such little difference in circumference would not be noticeable.

 

I have one mis matched tire, have been running it for over 40K miles and no issues. The mis matched tire is about worn out now, the other 3 have probably 8/32 tread.

 

A couple years ago on my prior F150 I bought two tires from WalMart.com, had them delivered to local store and went into have them mounted and installed. They were exact same tire as was currently on the truck. They wouldn't do it because the other two tires had less than 50% tread left. Said they couldn't do it on a four wheel drive vehicle. Took them to the local tire store and they mounted and installed with no problem. Had never heard of that before.

 

did some math A 33" diameter tire travels 103.67" per revolution
A 33-10/32" tire travels 104.65" per revolution.

With open differentials the truck will never notice.