I recently switched from KM2 to M55. Although I only have a few hundred miles on the M55, I think I can make some useful comparisons between the two tires because my KM2 were not worn out before switching them. The KM2 had about 13,000 miles on them, and I swapped them out primarily because I was concerned about accumulating sidewall damage. They had some scary-looking cuts in them, although they did not ever actually fail. I do a lot of backcountry travel solo and it is not a practical risk to drive on four damaged tires. I made some comparison photographs using my spare tire which had zero miles on it.
I chose the M55 primarily because I chose the 255/85R16 size and the choices in this size are limited. While there are a handful of tires in this size, there is not really anything better for offroad driving than the KM2 except possibly the Toyo M55 and Open Country M/T. The Cooper and Maxxis are not tough enough. Interco's LTB in 34x10.50 is close in size, but comes with significant tradeoffs. Although I was satisfied with the KM2 (having gone through two sets), I wanted to try something other than a mud tire. I was tempted to try the Duratrac in 285/75, but decided that for my driving, the size was more critical than the tread design. Although the wider tires may offer better lateral stability in handling, I don't expect my vehicle to handle especially well, and I believe the taller, narrow tires offer more traction. I also know the 255/85 fit my fender wells better. As it is, I only have a few millimeters of clearance between the inner sidewall and fender well when fully flexed. I already have wheel spacers installed. Going to 285's would result in less suspension flex and less ground clearance. Going to larger tires would involve cutting doors and re-working the plate-steel rear bumper. The 255/85 is the best size for my vehicle and slight variations in tire design are not going to make up the difference when going to a less compatible size.
I live and drive primarily in northern Nevada and the Eastern Sierra Nevada mountains of California. We don't have much mud or for very long. This is hard rock country. We have snow, but it does not remain on the roads for long like it does in the north and east. Off road, the snow is very deep. With the little 33's, I am more limited by ground clearance than by tire tread design. Nevertheless, I was hoping to gain three minor advantages switching from KM2 to M55: reduced road noise, improved snow performance, and improved ice performance. The tradeoff would be reduced mud performance. I was concerned about the sidewalls being stiff and loosing some traction at low pressures since my application is not typical for the M55. My vehicle is a relatively light-weight Land Rover at about 5400-6000 lbs. It is not the typical 8000-9000 one ton commercial fleet pickup.
I drive this vehicle offroad or on the highway to the trail. I don't commute to work, and I don't really go anywhere else. I have another car if I need to drive to the airport in Reno or something. For most of my driving, tire noise is not a concern. I don't go fast enough that you can even hear the tires. On the highway, I don't need mud tires interrupting conversation, but the KM2 are not a loud tire at all. What I found with the M55 is they are not quieter. Although I have more noise from gears than I do from the tires, the M55 are not quieter than the KM2. They're not noisier either, but about the same. So the only advantage I can hope for now is better snow and ice performance. Although the tread life should be considerably longer as well, I don't expect to wear out the treads before the sidewalls are gone. The E-rated M55 are reported to have tough sidewalls, but I don't expect them to last much longer than KM2.
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Some people reported the M55 being heavy. They may be in the larger sizes, but for 255/85R16, the specification is only a few pounds heavier. I weighed them mounted on my wheels. The wheel and zero-mile KM2 was 80 pounds. The wheel and new M55 was 83 pounds -- three pounds difference. And the KM2's wheel had fewer balancing weights on it. I have the M-speed rated M-55's which are a couple pounds lighter than the Q-rated tires. This is not an issue on the Land Rover which cannot go 100mph the way I have it configured.
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The first place I drove the M55 was the Rubicon trail. It's just around the other side of the lake and I had a three-day trip planned with a site at Dirty Dozen reserved. Unfortunately, it was cut short by the King Fire that resulted in the trail being evacuated. I still managed to try the M55 on a few obstacles. After we had to evacuate, I headed down to the hills between Deer Valley and Hermit Valley in Stanislaus County and had more time on trails and some obstacles. I also had more highway time coming over Ebbett's pass and through Alpine county back into Nevada.
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I typically ran the KM2 around 12-14 psi off road. I lost a bead once at 12 psi on a rock at full lock , so I tried the M55 at 14 psi to be safe. The Rover was loaded with supplies and 4 people and a large dog for 3 days so it was probably about 6000 pounds. I didn't have a problem getting good deformation and traction at all. I can see that maybe they're a little stiffer, but they could still be dropped another couple psi without an issue. You can see them working in this video at 14 psi.
[video=vimeo;106735091]https://vimeo.com/106735091[/video]
I see the M55 as one of the best choices in 255/85R16. In other sizes, there's more choices. The KM2 would have been a better bargain. They are $100 less expensive per tire. For a highway user, I am sure they will get more miles from the M55, but for my use, the snow and ice performance are the only clear advantage I can hope for. Nevertheless, off road they are not disappointing at all. I don't see a disadvantage compared to the KM2 except possibly in mud -- and the KM2 was not an awesome mud tire anyway. I'm happy with the M55 and I expect to get even happier as fall and winter set in over the next couple months. I don't expect any of my trail companions to swap to the M55. They'd have to give up their 37's and 40's. For a narrow 33, I don't see what more one could expect.