40" Cooper Discoverer STT Pro Review

Jun. 20, 2017 By Pete Bach

Let's face it, when it comes to off road tires, 40's are the new 33's. We remember - and it wasn't that long ago - that 33's were big enough for any Jeep and any trail, but not anymore. One of the most talked about and anticipated tires in the off road industry has finally been released in the increasingly popular size of 40x13.50.

Tire size, and the way four wheelers build trucks, Jeeps and SUV's, has certainly changed, and Cooper Tires has realized this with their introduction of the 40" STT PRO Tire.

The Cooper STT PRO is now widely used by many that want the ultimate in traction when off road but don't want to sacrifice on road civility. Fortunately, our Jeep Wrangler JKU is set up to to run a 40" tall tire and we were able to experience the new tire that replaced the already popular Cooper Discoverer STT Mud Terrain. In our testing, we were able to put our STT PRO tires into every situation mother Earth could throw at them. Mud, snow, ice, dirt, rocks and all man made surfaces were present to do their best at discrediting the big Cooper Tire.  Here's what we found...

Mounting And Balancing

Many of us that have been four wheeling and playing with large tires for quite some time now realize that trying to balance a tire that is larger than 37" is typically futile. Large mud terrain tires have big sidewall lugs, three ply sidewalls and larger lugs and voids, making them good candidates for pyramids of lead on the insides of their wheels.

Our Cooper STT PRO's are mounted on aluminum ProComp wheels and it was no problem stretching the sidewalls over the seventeen inch diameter wheels. The beads popped right into place with very little force and sealed up nicely. We noticed the flexibility of the tall sidewalls but also realized the toughness of the bead while under the pressure of the tire machine.

We then spun the tires on the balancer using both methods; dynamic and static. With lower weight numbers being asked for by the balancer, we ended up using the static process and placed tape weights on the insides of the wheels.  Surprisingly, each tire balanced to under seven ounces!  This is always a good sign as to how well the tire is made in the factory.  We see some of the big off road tires out there come from the factory with balancing patches on the inside of the tires.  Cooper does not need these. The 40" STT PRO's are very true right out of the gate. 

Grade:  A-

Actual Size and Weight

All tires on the market, whether Flotation or Metric, have their "actual size" or aspect ratio molded into the sidewall. We've found that most tires are typically an inch shorter than what is "advertised". We are happy to report that the Cooper STT PRO 40x13.50 17 is exactly 40 inches tall, with a section width of 13 inches, mounted on a 9-inch wide wheel.  Each monster of a tire weighed in at 78 lbs, which is less than many other large tires that we've tested in the past. This could be a good thing when considering rotating mass on a JK without eight cylinders. As a note, we plan on mounting these tires onto a slightly wider bead lock wheel that will increase section width and contact patch.


Cooper designed the STT Pro as a very aggressive mud and rock tire, however, still calls the tread a hybrid. By using a three to two lateral tread pattern, better traction is achieved and less noise is produced, making it perform like a mud terrain and sound more like an aggressive all terrain.

Cooper used their very own rubber compound, ArmorTek3, that consists of black carbon and silica. According to Cooper, this combination in a three-ply sidewall, offers 50 percent more puncture resistance and less bruising. The STT PRO also incorporates a wheel protector around the bead area of the tire that should fend off rocks that harm vulnerable wheels. 

Our favorite part of the STT Pro Design, is the Flex Groove. The Flex Groove is strategically placed where the shoulder lugs end, and the sidewall begins.  This allows the tire to flex and contour over and around objects by not pushing and pulling on the large shoulder lugs.  It's almost like having a two piece tire...the tread can flex one way and the sidewall can go another.  This allows for a smoother ride on all surfaces, especially when aired down, and pliability is exceptional when in the rocks. There are so many technological advances and details incorporated in the STT Pro, it's a little hard to get them all into this review. The outer shoulder lugs include dimples that are said to aid in the release of mud and snow that can become packed into the large voids of the tread. In addition, scallops and scoops were added to evacuate dirt, mud and snow before they can load up in the tread. 

The tread design in itself is pure genius. By using lateral type tread blocks, it increases the tire's ability to climb and claw up vertical ledges and surfaces.  However, by alternating and staggering the center lugs, it gives better lateral traction, reducing side slipping and increases side hill abilities. Cooper has incorporated everything they've learned into this one tire...and it works.

Grade:  A+

Street Manners

Street manners of 40-inch tires has become more important over the last decade. In the past, most wheelers that ran a tire as big as this typically used them off road only.  With the advances in tire development over the past ten years, more and more of us are using 40"(and larger) tires on a daily driven vehicle. Our first impression of the Cooper STT PRO on the road was that they could use some dialing in before testing. Our air pressure was something we had to experiment with. Too low and our handling suffered due to the tall flexible sidewalls, but go too high and the tire wanted to grab everything on the pavement and concrete with a small contact patch. 

As mentioned, we plan on running a wider wheel, as our STT Pro's crowned a bit on our 9-inch wheel, which is likely why we experienced wandering. Once we found our sweet spot (about 36lbs.), the Cooper STT PRO became very impressive on the street. 

Remember, this is all relative. No, the Cooper STT PRO doesn't offer precise handling. It's a huge, off road beast made for dirt, mud and rocks. However, it handles as well as most 35-inch tires we've run in the past, even with the taller sidewall.

Noise can be a problem with a tire this aggressive and the STT PRO's certainly make noise. There's no question about it. With that said, they are no louder than any other aggressive mud terrain we've tested. All mud terrain tires make some noise so it's fair to say that you can expect the typical buzz or hum down the road with the STT Pro.

Grade: B

Snow and Ice

We were fortunate enough to test our STT Pro's in snow levels ranging from one inch to 24 inches. We can report that in all levels of snow, the big Cooper tires performed incredibly well. The large lugs, deep tread and huge voids tore through drifts, wet snow, light powder...anything the cold could toss at them. After the snow was packed, however, they didn't shine as well.  Snow packed and icy roads were the STT Pro's kryptonite. Mud Terrain tires do not perform well on ice in general, and while this terrain was not ideal for the Cooper, it still faired better than several other MT's we currently run on other Jeeps. 

The three to two tread pattern that has been strategically staggered, helped bite into the ice more like an all terrain. The STT Pro has some siping, however, we feel that a few more small sipes could do wonders for the big MT. 

Grade: B-


We don't know whether to say that the Cooper STT Pro's love rocks or hates the rocks. The tires grip well and seem at home in all types of rock, including river rock, granite, slick rock, etc.

On the other hand, the STT Pro's seem to almost punish the rocks and simply laughed them off as they tried to offend them. We also ran our STT Pro's through many of the tough trails in Moab with 30lbs of air. They never skipped a beat. 

When aired down to 12lbs, they were even better!  Colorado offers up rocks that are typically covered in loose sand and dirt. Our Coopers clawed over anything Colorado could muster. The huge sidewall lugs shrugged off sharp rocks of all types and wrap around the sidewall enough to protect the tire very well.

The sidewall flexes excellently, as the Flex Groove allows the tires carcass to form around anything in their path. We've owned many sets of tires in the past that require a slight "break in" period, as to get the thick sidewalls to soften up, but the Coopers were soft and flexible right from the get go. The STT Pro rubber compound seems to grip just about any type of rock and is soft enough to allow graining of the lugs edges and best of all, we have not experienced any major chunking yet. 

One noticeable attribute of the Cooper tires is that they display excellent lateral stability. In off camber sections of trails, there is very little side slip and the tire's sidewall doesn't want to fold over, or over flex, from the angle and vehicle's weight. We suspect that the staggered lug pattern had a lot to do with tires stability in the rocks.

Size of the tire for rock work?  Perfect.  We love the new 40x13.50 17 tires from Cooper and they are an excellent choice for any rock crawler. We have caught rumors that even larger sizes might be on the way for us rock crawlers, racers and bouncers.  40x15.50 perhaps?  And if we could beg for taller STT PRO's?  42" please! 

Grade: A

Dirt and mud

Everyone knows that to truly test out the effectiveness of a good off road tire, one must test them in two-wheel drive  We drove our Jeep as if we were Walker Evans in Baja to see if we could get these tires to break free in the dirt. As expected, they hooked up well. Very well.  Forward momentum under extreme throttle was not slowed down due to the lack of traction. In hard packed dirt, soft sandy dirt and mud, the STT PRO's grabbed and clawed for excellent traction. The large voids cleared easily of sticky mud thanks to the small divots, scoops and scallops that keep mud out of the tread. Our slightly narrower tires (on 9" wide wheels), with a smaller contact patch really shined on dirt roads at higher speeds.  Cornering was crisp and predictable and was actually hard to get the rear end of the Jeep to break free.

Our only mishap in the mud was the tire's encounter with an object buried several inches below. While the tire was churning through, it caught something sharp and put a large gouge into the sidewall. It appeared to have punctured the tire, but once the mud was cleaned up, we saw that no permanent damage was caused and we've had no problems with the tire to date.

Grade: A-

Overall Grade: A-

The Cooper STT PRO is a great all around, aggressive off-road tire. We applaud Cooper for listening to their customers and giving us not only a competent off road tire, but for giving us the size that many of us now need and want. If you are looking for a serious, do anything, off road tire, take a look at the Cooper STT PRO. We did, and in fact, we recently went out and bought another set of 40x13.50 17 Cooper STT PRO's for another one of our project Jeeps that we are currently building. 

If that doesn't speak volumes, we don't know what does! 

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