Hello JOJO,
Here is a link I used to see exact tire dimension's of the current tire, and see the differences in ones I may want. But actually I was surprised I did a Google search "How big of a tire can I fit on a 2015 3500 HD stock not lifted" And that told me on mine I could go as high as a 295/65R18 But the tire I wanted was not available in that size, So I purchased what they did offer which was a 295/70R18 And it was about to the limit. The tire I actually wanted was the Bridgestone RIVA I THINK? Butit did not come in my size either. Alot depends on what "YOU" want and your use. For me I just found the OEM tires slick... As in I got on a small hill here in Tennessee and slipped in 4 WD because they were standard street tires. I really like The Firestone Destination XT and the Bridgestone Riva.. For a good meaty balance. And no Firestone told me my OEM spare would be fine to use as a spare to get to a tire shop so I just did the 4.... Personally I would go to this link TireSize.com - Compare Tire Sizes, Specs, Prices & more AND zero in on my size then call a Firestone store and ask to speak to the owner/manager and he or she will be happy to help you. I only deasl with managers in things like this as it tends to cut down on BS and false promisers. My manager was even happy to order 3 different sets that he and I though might fit my needs and tried each one on the front right to let me look at it. I thought that was awesome and sold me on him really caring about his customers. I guess you could go the cheap rout and get them direct from a wholesaler but this IS important enough I want someone I could trust because in your case your looking at close to 2500-3k I'd guess. But then I got the road hazard warranty which I NEVER DO because it was a kinda big investment. Hope this helps my friend!
DS