While it's true car dealers/manufacturers do buy tires in volume, they are purchased on a 'Just in Time' inventory plan and thus usually are not more than 18 mos old when sold on the new vehicle, so age is rarely a problem. Occasionally they might sit a little longer than that, but not the norm. And they do typically wear at a faster rate than tires one would buy aftermarket, but that has more to do with the tire/rubber compound than any quality issues. Just compare the mileage rating one to the other (OEM vs aftermarket) and you'll see.
Vehicle dealers/manufacturers will typically provide a softer and more stable tire for new cars/truck deliveries to give a much improved ride/comfort level and a more confident road feel for the customer than a harder compound. These tires will also usually be run at a little less than optimum pressures, and both these reasons will promote what seems to be a faster than normal tire wear after the sale of a new vehicle.
Another issue that sometimes/often takes part in tire wear, especially on trucks, is that when we replace the dealer's tires with new aftermarket ones, we sometimes go up a size or two for looks and/or utility while not realizing how much just a little different size will affect longevity. For example, if you go from a 34" dia tire to a 35" dia tire (just 1" in dia difference or 1/2" taller to the axle), you will improve the wear/mileage of that tire over the smaller one by about 3%. So, if you're getting 35K miles from the bigger tire, that's an increase of approximately 1050 miles over the mileage of smaller one just from the size difference with every other thing being equal. But know this .... the vehicle manufacturer will almost always be 'on the money' with recommended tire size for optimum results. He knows his vehicle's limitations and/or requirements best.
The larger tire will often also call for a few pounds more in tire pressure too which will also provide a longer life, all other things being equal. But the single biggest increase in actual mileage of aftermarket tires over OEM's is the fact they will typically be a harder/firmer compound having a higher mileage (and sometimes load and speed) rating than OEM's, and often run those little higher pressures. At the same time you may/may not affect your actual mileage (good or bad), and you will be adding extra wear and tear to all your truck's drive train (especially gear ratio), suspension system, steering system, and braking systems. With only minor tire size differences it usually won't be enough to notice, but there will be a difference! It's those ever present, sometimes PIA laws of physics.
And lastly, there's no way a new car/truck manufacturer can tell you, the prospective buyer/owner, what the optimum tire pressure will be for 'your' particular needs. They will give a routine, average pressure requirement that will provide the best of mileage, comfort, longevity, and road feel to the most average drivers of that particular kind/type of vehicle. They cannot possibility know how anyone will/might use that vehicle after it's bought.
So, the thing to do is once you've settled on and mounted the aftermarket type and size of tire you want, or to extend the life of the OEM's, go to that tire manufacturer's web site and take a look at their tire use/pressure comparison charts. Find your tire model, use and size (and any other particulars), compare that info to their chart and see what the manufacturer's recommended tire pressure for your specific use/needs is. Rarely will it be the maximum pressure molded into the sidewall, but could be. You will also be able to determine if/and by how much you may be overloading/overworking your new tires. You'll need an actual vehicle weight (not the book weight, but ideally actual individual corner weights) with big, heavy, across the bed tool boxes, typical loads you might carry, or pulling too heavy trailers, etc. Obviously some of this isn't really necessary on most of our personal vehicles, but the bigger/heavier the vehicle is, the more important it is. And the bottom line is this ..... running tires underinflated is
BY FAR......... by far ...... did I say 'by far' the single biggest contributor to premature tire wear/failure of almost any means.
Tires are a really interesting item to understand, especially for their simplicity. Take a look and you'll see.
