When we found ourselves looking to upgrade the tires on a stock Wrangler TJ, we didn't want to compromise a smooth, quiet highway ride for good off-road performance. We found several computer-designed, high-tech tires that promised both great off-road traction with a civilized on-street ride. However, while there are a number of manufacturers that offer the performance we were looking for, relatively few offered the size we wanted. That's right, we actually had a tough time finding a newly designed tire in a 31x10.50-15 size.
Thankfully, the Cooper Discoverer STT is offered in a range of P-metric and flotation sizes up to the big 35x12.50 for 15-, 17-, and 18-inch rims. These are the sizes most of the real-world Jeeps of today run on the street.
For starters, the Cooper Discoverer STT tires have an aggressive-looking tread design that extends down to about the middle of the sidewall. The lugs are deeper than most mud-terrain tires of similar heights we've tested and are siped through the center of each tread block. In our opinion, it's a bitchin' looking tire that is right at home on a Wrangler or early Jeep.
On the Street
Looks aside, the Discoverer STTs made us a little nervous after our initial test drive home from the tire shop. For the first 100 miles we noticed a lot of road noise and a bad wander in the steering that wasn't there with our smaller all-terrain tires we had been running. However, after about 250 miles of street driving we found the tires really quieted down and the wandering issue abated as the tires wore into the Jeep's track. On the highway you can barely hear any tread hum above the sound of the soft top. We'd rate the on-road noise at the low end of the mud-tire spectrum, which is to say it's about on par with a BFG MT. Because of the extremely deep tread design and ample siping, wet weather traction is very good. There's no hint of hydroplaning in deep puddles and on- and off-ramp grip is tight. However, we feel the tall, supple treads squirm a bit when pushed relatively hard in corners on the road, resulting in a bit of understeer.
In the Dirt
Off-road we ran the STTs through the standard gamut of terrain - mud, rock, loose dirt, and sand. We kept our tire pressures at 12 psi at first, only airing down to about 6 psi after the higher pressure proved less than ideal. Even at the relatively high 12 psi pressure, the tires exhibited a nice bulge. The sidewalls are supple, yet when ground into rocks and obstacles proved to be quite strong.
What little mud we were able to put the tires through was cleared rather easily with little tire spin. Copious throttle application in two-wheel drive resulted in more forward momentum than we were expecting. In the sand, the tires tended to dig a little too readily for our tastes. Airing them down to about 6 psi increased the flotation and helped prevent this, but only at the risk of throwing a bead off the rim. On hard rocks the tires gave very satisfying performance. We were able to crawl ledges and obstacles at extremely low speeds with nary any slippage. However, once the tires slipped we found that increasing the wheelspeed served only to take rubber off the tread. The tires just spun without any increase in grip. Once again, the lower air pressure proved beneficial. Where these tires really shined is in hard- and loose-packed soil. In short, they love the dirt. In hard pack, the tires bite without breaking up the surface and digging holes. In looser terrain, the tires claw and tear their way forward without burying themselves.
Bottom Line
The Cooper Discoverer STTs are aggressive-looking tires that offer a smooth and relatively quiet ride on-road and decent off-road performance. We think that a source of onboard air is required to get the most out of these tires off-road because they work much better at lower pressures.