GMC AT4 Oversized Tire Trimming Guide

Looking to Run 34″ or 35″ Tires on a GMC AT4?

In this guide we’ll show you everything required to run over sized tires and aggressive offsets on the GMC AT4 Wheels. The primary issue with the GMC AT4 and Sierras in general is the design of the fenders are more squarish than that of it’s sibling the Silverado which as of 2019 went to a more rounded shape.

The square design on the fenders make it very difficult to run over sized tires and aggressive offsets as you will incur rubbing in several places which we’ll address in this post.

Here are the two most common questions we address related to the GMC AT4:

Do 35″ Tires fit on a GMC AT4?

Yes but not without some modifications and very specific offsets. Running 35s on the GMC AT4 requires a very specific wheel offset in addition to the below trimming. To be on the safe side you want to maintain a 8.5″ max width and at minimum a +18 offset (higher will be better).

If you’re running the stock wheels you’ll be in good position however if you’re running the ReadLift you will come across issues with the Upper Control Arms. If you’re looking to achieve the stance look with a more wider fitment and more aggressive offset, you’ll likely end up modifying a little more aggressively than below. It’s likely we’ll attempt that in the future and document that on a separate post.

Do 295/70 or 34″ Tires fit on a GMC AT4?

Yes, with minor modification. Running 295/70s or 34 inch tires on the GMC AT4 requires a very specific wheel offset in addition to the trimming below. To be on the safe side you want to maintain a 9″ max width and at minimum a +0 offset (higher will be better). We offer these fitments in our database and the fitments we offer will have sufficient clearance for the ReadyLift Upper Control Arms.

If you’re looking to run 34 inch tires and ok with the below trimming you can search our database enter the year of your GMC AT4 and select ‘ReadyLift Moderate’ option as your lift. Our ReadyLift Moderate offerings will clear the ReadyLift Upper Control Arms and will be the max diameter tires we recommend with the below trimming.

If you rather not trim you can select the ‘ReadyLift Conservative’ option when conducting your search and we’ll show you the most aggressive options you can run and note the ones that won’t require trimming as well as the ones that do. In short the + option is the more aggressive option and the one we ran on the sample below.

Where do over sized tires rub on the GMC AT4?

When running anything bigger than a 33″ you will encounter rubbing in four places.

In the front you’ll be rubbing in the liner and on the tab where the front lip is mounted.

In the rear you’ll be rubbing on the rear mud flap and the liner once the mud flap is removed.

The good news is that the fix is fairly easy which we’ll share with you below in detail. Please note that this fix has only been tested running a 34″ tire on a 8.5″ wheel in a Zero offset, anything over 34″ and wider than 9″ may require additional modifications but at minimum it will require the below.

GMC AT4 Oversized Tire Trimming Guide

To start you want to turn your tire outward so you have the room required to work in the front section with the most room possible.

Start by removing the bolt holding the front lip spoiler (7mm socket) and also remove the retainer the bolt screws into (may require some prying with a flat head).

Trim the section off that’s making contact with the tire. See highlighted Yellow area below. You can use a dremel for this as pictured in the background.

Drill a small hole at the top that will allow for a tie wrap to be passed thru. Use a drill bit that’s suitable for whatever tie wraps you have available. NOTE: When drilling this hole be careful to not drill thru the bottom lip spoiler itself. The plastic is really soft and drills easily so use caution.

Once that tab has been trimmed and the hole has been made zip tie the front lip spoiler to the upper plastic portion it was previously bolted on to. The end product will look like the below (notice the zip tie).

Now we need to address where the tire was making very minimal contact with the front liner (see blue tap above for contact point).

Remove the three screws holding the felt with a T15 star bit.

That felt goes screwed on to a plastic plate. That plastic plate flexes so it can be tie wrapped back to a bumper holder that’s right behind it. You’ll have to drill a hole on that small plastic plate for a tie wrap to pass thru. Don’t push it back too much as the holes for the liner will later not align, I probably pushed it back about a quarter of an inch. If you push it back too much you’ll have to cut the tie wrap and do it again with just a little less tension. Our 295/70/18 was just barely making contact here so we didn’t force it too much.

Once this step is done screw back on the liner and the front is complete. Below is a picture of the final front lip and the clearance we now have with the liner.

The below is a view from the bottom up at full lock you can’t see the clearance too much but I can easily fit my hand. Before tie wrapping the liner back the tire was touching the liner.

 

Rear Mud Front Modification

Start by removing the mud flap and the bracket that holds the mud flap. I did not take pics of the mud flap removal but it’s really straight forward.

The mud flap is help in place by the same T15 screws that held the front liner and also a 10mm bolt. The mud flap bracket is help on with (4) 13mm bolts.

You’ll now have to trim the mud flaps as shown below. The most important part of this step is to note that the mud flap will be re installed by use of three T15 screws so make sure before you cut that you’re not cutting out more than those three screws. Below I marked with tap the exact area that I cut for reference.

Here is the mud flap cut with the piece that was cut off still present so that you get the idea.

And here is the final product

Before installing you’ll want to move back the fender liner. You’ll notice that the fender liner has a hole at the bottom where it once bolted on to the mud flap bracket. Use that same hole to run a tie wrap thru as the below example.

Once that’s done you can re install your mud flaps without the brackets (they’ll be held on with the three T15 screws).

You’ll now see that the portion of the mud flap that once made contact is no longer in sight and the liner is far off.

 

This is what the finished product looks like

2020 GMC AT4 w/ 1.75″ Readylift 

Wheels: 18x.8.5 Offset: 0 (Black Rhino Chase)

Tires: 295/70/18 (Falken Wildpeak AT3)

 

Looking for wheels for your GMC AT4 or Silverado Trail Boss?

Let us know the year, make and model of your truck in addition to the lift (if any) and we’ll show you exactly what fits. Use our search tool below to get started.