Are you keen to know what the biggest wheel and tire combo are you can fit on your AT4 without any suspension mods or aftermarket rims? Perhaps your truck is still within the warranty period and you don’t want to void any future claims. Or maybe you just want a more aesthetically pleasing look on your truck and qualify for your “man card” whatever your reason is, this article will cover all the possible wheel upgrade options without any additional modifications.
The biggest tire you can fit on a 2021 GMC AT4 running factory stock suspension and rims are a maximum of 34.8-inches. This equates to a 285/75R18 (34.8”) or 285/65R20 (34.6”) with no trimming or rubbing.
If you are not prepared to do trimming and cutting and you are running stock suspension then you’ll be limited to a maximum of 34.8-inches, depending on the tire brand of course. On the upside, there are many other alternatives you can try in both 18” and 20”.
Let’s now look at exactly what is required to make the biggest tires fit, as well as some of the most popular tire and suspension combinations guys have done with 100% success.
View all GMC AT4 Suspension Upgrade Kits – AMAZONTable of Contents
GMC AT4 35-inch tires and no lift – 18” Options
When looking at the current At4 shapes, the maximum tire size you can fit in 18” is any of the below configurations:
18-Inch Tires on a AT4
| 285/75R18 = | 34.8×11.2R18 |
| 295/70R18= | 34.3X11.6R18 |
| *305/70R18= | 34.8x12R18 |
*UCA clearance might be an issue with the 305/70R18. The wider tire could make contact when you turn at full lock and thus interfere with the control arms. The wider tires could also rub the UCA on a bump so clearance is very minimal. A small wheel spacer or negative offset aftermarket rim would rectify this.
When looking at the current At4 shapes, the maximum size you can fit in 20” is any of the below configurations:
20-Inch Tires on a AT4
| 295/60R20 | = 33.9X11.6R20 |
| 275/65R20 | = 34.1×10.8R20 |
| 285/65R20 | = 34.6×11.2R20 |
| *305/60R20 | = 34.4x12R20 |
*UCA clearance might be an issue with the 305/60R20. The wider tire could make contact when you turn at full lock and thus interfere with the control arms. The wider tires could also rub the UCA on a bump so clearance is very minimal. A small wheel spacer or negative offset aftermarket rim would rectify this.
Many truck owners want to know if it’s actually possible to run bigger than a 34.8” tire on a stock suspension without issues. How many actually end up running bigger than 35’s on stock suspension is another story altogether, since the amount of cutting and trimming without a leveling kit is ridiculous. You also lose a lot of suspension travel and your gas mileage is affected drastically. If you really want to go bigger than 34.8”, it is highly recommended to opt for a small 2-3“ Lift kit to allow for improved clearance inside the wheel arches and increased clearance from the UCA.
On the other hand, a minimum of a 2”-3” leveling kit upgrade (Amazon) will resolve many UCA and body clearance issues associated with fitting bigger tires. Also, tire width and rim offset is actually the most important issue to be concerned about. Once you go bigger than 35” you’ll need a set of aftermarket rims with a negative offset to clear UCA and other vital suspension components.
Big Tires on the GMC AT4 – (Fuel Economy & Performance)
With any upgrade, comes a small tradeoff and a big tire upgrade is no exception. Fitting wider, heavier tires definitely comes at a price and since the tires are positioned below the suspension, that makes part of the unsprung weight of the truck.
Expect to lose 1-2mpg with a heavier and more aggressive 35” AT tire upgrade. The quickest way to increase fuel consumption is by increasing unsprung weight. The few pounds of heavier rolling mass of the bigger tires have a direct impact on your fuel consumption, especially when pulling off. Your truck might look more aesthetically pleasing and have a more aggressive stance, but it all comes at a price.
Also, depending on how big you go, you’ll feel a drop in pull-off performance as well due to the heavier tires needing to be pulled along. It might not even be more than 5 pounds heavier, however, this makes a big difference in torque transfer on the wheels.
Also, if you go bigger than 35” then re-gearing of diffs should be a consideration since there will be more strain on diffs and other drivetrain components. Don’t forget about your speedo which will need recalibration to be accurate again.
What AT4 Owners Say
User Feedback #1
For sizes, I just put 295/60R20 Cooper Discoverer STT Pro tires on my AT4 and they fit with “no” rubbing. I say “no” because I did manage to get them to very, very slightly hit the mud flap bracket on a hard right turn into a steep uphill parking lot, so a mud flap removal may be necessary depending on where you drive.
https://www.gm-trucks.com/forums/topic/223068-biggest-tires-on-at4-18-wheels-with-no-modification-or-wheel-change/page/3/
User Feedback #2
I put 285/75 18’s on my truck like you guys said and they rub. Not bad tho so I guess I’ll just deal with it. Look great tho!
https://www.gm-trucks.com/forums/topic/223068-biggest-tires-on-at4-18-wheels-with-no-modification-or-wheel-change/page/4/
User Feedback #3
Finally got my AT4. This is totally stock with the stock 18″ rims. 35×12.5 Duratracs *just* fit. Some rubbing of wheel well liner at full lock but a tiny bit of trimming will fix that. Last pic is of the control arm on the front, which was my biggest concern. It’s close but no rubbing or issues.
https://www.gm-trucks.com/forums/topic/221897-at4-with-duratracs/page/3/?tab=comments#comment-2241208
Resources
View all GMC AT4 Suspension Upgrade Kits – AMAZONGMC AT4 2″ Leveling Kit (Amazon)
2019-20 AT4 & Trailboss compatible 2″ Lift Kit (Amazon)
07-2020 2″ Front leveling kit (Amazon)
https://tiresize.com/comparison/
Conclusion
So the takeaway here is, without a 2” or 3” suspension upgrade or leveling kit and you really limited to a 34.8” tire. Anything bigger and you’ll need to do cutting and trimming of wheel wells. You’ll be limited to the biggest 34-inch. A leveling kit will help with wheel clearance, however, the best option for your AT4 is to opt for a 2-3” suspension kit if you want to run wide 35’ comfortably. If you can add a slightly negative offset aftermarket rim, it would improve UCA clearance and backspace with no need to do any additional trimming.