Biggest Tire on Stock 3500 DURAMAX [W/Without A Lift Kit]

What are the biggest tires I can put on stock 3500 CHEV or GMC? That is one of the most commonly asked questions amongst truck owners and the 4WD community. Remember, when fitting aftermarket rims with bigger tires, your main concern should be the wheel offset and the tire width, since diameter doesn’t affect the fitment. So what is the max tire size you can fit on a 3500 CHEV or GMC?

The biggest tire you can fit on a stock CHEV or GMC 3500 is a 295/65R18 on the factory stock wheels at stock ride height.

It’s interesting to note that GM has designed the front IFS suspension on the 3500 to be slightly adjustable by manipulating the torsion bars. This will allow you to fit a larger tire without much scrubbing at full turn and at full compression. This is good news, however, be warned that when you start fiddling with the torsion bars, you start altering the geometry of the IFS suspension, which includes changing the angles of the CV joints, upper & lower ball joints, and the tie rods.

With that being said, let’s see what the max tire size you can fit on a stock 3500.

Biggest Tire on Stock 3500 Duramax [Without a Lift or Levelling Kit]

When you adjust the torsion bars to accommodate a larger tire, you will lose suspension travel. For example, a 2-inch adjustment will mean the Upper Control Arm will be significantly close to the front bump stops, which limits downward travel considerably. Sure, you’ll be able to squeeze in a 35-inch tire, however, your ride will be firm and somewhat bouncy. You also might need to trim away some fender plastic to avoid scrubbing at full lock.        

18 Inch Tires on a CHEV & GMC 3500

Many 3500 owners have reported that they have successfully fitted 295/65R18 on the stock wheels and stock suspension height without any hassles. 295/65R18 equates to a 33.1” tire and should still be within the acceptable factory specification.

Personally, I prefer the look of a 34-inch with a higher aspect ratio i.e. sidewall. It looks proportionally correct in my opinion. With a 2-inch suspension kit or front leveling kit, there’s no need for a diff drop since the front CV angles should still be within acceptable geometry. It will look and ride better too.

Alternatively, you can opt for a wider 305/55/20 which equates to a 33.2” combo. You’ll need to watch that offset so you will need a 20×9, with a +12 offset to clear those UCA’s nicely.

20 Inch Tires on a CHEV & GMC 3500

Certain 2020 model trucks are sold factory stock with 34-inch tires, which equates to 275/65R20 to be specific or 34.1 inches to be exact. You can easily go one jump up and fit a 285/55/20 or even 285/60/20 on factory stock suspension, however, the latter will look much better and not like the stock ’20s.

If you really want to see a significant difference then consider a 305/55/20 (33.2”), or 275/65/20 (34.1”), or 295/60/20 (33.9”.

If your upgrade is purely for aesthetics, then sure ’20s are the way, however, If you intend to still have the option of venturing off-road and maintaining full practicality and versatility then a 20” might not be the best option. 20-inch rims look good, but you lose a significant aspect ratio i.e. sidewall, which means you cannot deflate as much and this means less sidewall flex. You also won’t get a much longer footprint when deflating for sand driving. Lastly, a 20” delivers a stiffer ride, and tires prices are generally mode expensive.

However, if you are dead-set on 20” wheels then you’ll need to do the following to make them fit properly:

1. Fit a +20 offset rim
2. Adjust front and rear psi (60F and 80R)
3. 6 Full turns on the OEM torsion bar keys.
4. Cut a small 3″x3″ piece of the inner fender on passenger side front to prevent rubbing over speed bumps, and potholes.

Biggest Tire on Stock 3500 Duramax [With a Lift Or Levelling Kit]

With a level, 305/70/18 is defiantly possible, with some trimming. That equates to 34.8” in metric values. Unfortunately, it won’t be a straight fit and finish, since you’ll need to still do some minor trimming on the inside fender liners and guards where rubbing occurs.                  

Many DURAMAX owners have reported running 295/70/18 (34.3”), 285/75/18 (34.8”), and 305/65/18 (33.6”) without any hassles on the DURAMAX with a small 2-3” level.  There are also guys running 35×12.50×18 BFG’s with leveling kits and no problems.

Big Tires on A 3500 DURAMAX and Fuel Economy

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, they form part of the unsprung weight of the truck. The quickest way to increase fuel consumption is by increasing unsprung weight. The heavier rolling mass of the tires has a direct effect on your fuel consumption, especially when pulling off.

So before you do the big tire upgrade make sure you understand the impact it could have on your fuel economy. Your truck might look more aesthetically pleasing and have a more aggressive stance, but it will be thirstier, so be prepared to visit the pumps more regularly.

Also, depending on how big you go, you’ll feel a drop in low down torque which affects the pull-off performance as well. The weight difference in the tires might only be a few kg heavier per tire, however, this makes is a big difference in torque transfer on the wheels, and if you decide to go bigger than 3” then re-gearing of diffs should be a consideration since there will be more strain on diffs and other drivetrain components.

Difference between a 33 and 35” Tire

Below you can see the physical differences between a 33 and 35-inch tire.

I find this tool very useful when determining tire size differences and what effect it has on your truck: https://tiresize.com/comparison/

Resources

https://www.duramaxforum.com/threads/biggest-tire-size-on-stock-3500.1009952/

Conclusion

So the takeaway here is, without a 2” or 3” suspension upgrade or leveling kit you’re limited to bigger-sized 33” tires.  You will need to do cutting and trimming of wheel wells if you decide to go bigger 34” on stock suspension on the stock rim. A leveling kit will help with wheel clearance and some suspension travel, however, the best option for your 3500 is to opt for a 2-3” suspension kit if you want to run wide 34-35” comfortably. If you can include +12 offset aftermarket wheels, it would improve UCA clearance and backspacing with no need to do any major trimming.