MT ET streets measure up differently. the 11.5 is not tread width. According to their site, the tread width of those tires is 9.8" and they recommend +/- 1" rim width (so 8.8" to 10.8")
I have the 26x11.5x17 MT ET streets mounted on stock 17x8" wheels and they seem fine. (tread width is 9.5" for that tire)