You may run into 1 or 2 maybe 3 non legit favorable reviews, however the flip side is true too, you may run into 1 or 2 maybe 3 non legit negative reviews. If the average review is quite high then chances are the tire, or whatever you're wanting to buy, is good, just don't read reviews the manufacture website posts those they cherry pick.
I've been reading reviews for years, if the subject I'm reading on only has 1 to 3 reviews I may wait or if I really need it just buy it. For example I had to buy all new kitchen appliances for a new house we moved into and I read all the reviews I could find then selected the brand and model based on that. I'm I guaranteed that I will never have an issue with a product that received high reviews? No of course not, but the odds are in my favor that the product will be good and so far I haven't been disappointed, I would rather read reviews than to just point to a product and say I'll take that one because some sales rep said it was good.