Where are you?

If your car is a “normal” passenger variant, 215 75 15 on the standard rim.
If your car is a commercial variant placarded for 16” rims these ran a taller tyre, so you have more options up to just over ~30” tall.

If you are concerned about staying legal pay careful attention to maximum permitted rim width. Sierras run 16x 4.5” or 15x5.5” rims.

Most people do not seek legal approval for swapping in WT axles as it’s basically an undetectable modification but it does require fabrication and theoretically engineering/mod plate approval. All the spring perches on the WT axles need to be moved inboards to match the narrower spring spacing of the NT chassis.

It will increase track width around 80mm.

Suspension lift has nothing to do with tyre clearance. What size is 30x7.5? Is that a conversion of a P metric size?

This is off topic, but how relevent is the tyre size the car came with if the axles have been swapped? I'm sure there's more to it, but i was under the impression engineerable tyre size has a lot to do with what axles are under the car.

As far as i know, no wide track sierra came out with 16" rims(?) So i thought, technically, you would be limited to 215/75/15 if you swapped over to wide track axles.

Obviously your car passed brenno, was this ever mentioned? Or can you give any more insight? I'm aware narrow track and wide track axles are very similiar apart from width, so an engineer may be happy to pass either based on 16"rim cars, but i'm a bit curious.

Engineer was happy to do it on WT axles as they are the same strength and brake wise and all that mattered what the tyre placard which I got from a dash swap by chance. Wasn't looked at in any more depth than needed.

30x9.5x15 will be under the legal limit.

This is off topic, but how relevent is the tyre size the car came with if the axles have been swapped? I'm sure there's more to it, but i was under the impression engineerable tyre size has a lot to do with what axles are under the car.

As far as i know, no wide track sierra came out with 16" rims(?) So i thought, technically, you would be limited to 215/75/15 if you swapped over to wide track axles.

Obviously your car passed brenno, was this ever mentioned? Or can you give any more insight? I'm aware narrow track and wide track axles are very similiar apart from width, so an engineer may be happy to pass either based on 16"rim cars, but i'm a bit curious.

Sorry Beery, I missed this.

VSB14 is specific that the rim and tyre options permitted are linked to the axles from the donor vehicle, so, for example, if you put landcruiser axles under your Sierra you could run a 34" tyre legally (assuming other parameters can be met like overall height, tyre clearance etc)

You're correct that there was never a 16" placarded WT and that, strictly speaking, means you're back down to 215's if you swap to WT axles, however, there is a good argument that the 16" placard can be carried out on the grounds that the brakes, wheel bearings, GVM etc are all identical. I believe 15" placarded sierras have been engineered with 31's on that basis, but I don't know if it's 100% compliant with VSB14.

Sorry Beery, I missed this.

VSB14 is specific that the rim and tyre options permitted are linked to the axles from the donor vehicle, so, for example, if you put landcruiser axles under your Sierra you could run a 34" tyre legally (assuming other parameters can be met like overall height, tyre clearance etc)

You're correct that there was never a 16" placarded WT and that, strictly speaking, means you're back down to 215's if you swap to WT axles, however, there is a good argument that the 16" placard can be carried out on the grounds that the brakes, wheel bearings, GVM etc are all identical. I believe 15" placarded sierras have been engineered with 31's on that basis, but I don't know if it's 100% compliant with VSB14.