Tips and Advice: The Case for OEM Replacement Tires

The first thing that many people do when their new car is ready for its first set of replacement tires is to ditch them for something else. Whether that’s a tire with a high tread mileage guarantee, an ultra-grippy tire, or the cheapest thing they can find, few people actively seek out the exact same tires that their vehicle came with from the factory.

If OEMs could have just selected the best-rated tire from the Tire Rack website, or asked people on a forum what they preferred, and called it a day, wouldn’t they have done that? If you think you’re smarter than an entire team of OEM engineers, then go ahead and slap on that 80,0000-mile-guaranteed set of tires. But, you will want to understand what you might be giving up in the process.

The first thing that we all need to understand is that tire design is largely a balance of trade-offs. You don’t get low rolling resistance without giving up grip. Likewise, you may have to give up tread wear to gain more snow traction. If you’ve seen a spider chart, these are a good way to compare tire attributes for their tradeoffs, but its rare (or expensive) that a tire enlarges the overall area of the chart. That is exactly what the OEMs are pushing the tire manufacturers to achieve, though.

The second thing to understand is that tires are the largest tuning element on the chassis for ride, steering, handling, acceleration, and braking. Literally everything that the vehicle does in regard to its motion is filtered through the tires. Therefore, it is critical that tires are tuned to the vehicle and the vehicle is tuned to the tires.

Changing out the OEM tires is undoing an extensive amount of tuning and development work that cost the OEMs hundreds of thousands — if not millions — of dollars to complete. The process started with vehicle program leaders determining functional attribute targets for the vehicle. From there, engineers worked with tire manufacturers to develop the tires. While “off-the-shelf” tires may sometimes be selected, in most cases the tires are tuned specifically for the vehicle.

With targets in hand, the tire manufacturers would produce several versions of what look like identical tires and submit them to the OEM, along with their internal test data. These would vary in certain ways, sometimes emphasizing one attribute at the expense of another. Upon testing on the vehicle, the OEMs would either select a submission to proceed forward with or request additional revisions. The final tire tuning must be done quite early in the development process, as the rest of the vehicle tuning will be built up around it.

There are often several tire developments for a single vehicle platform for purposes of offering multiple wheel diameters, differing performance attributes for various trim levels, or just to avoid single-sourcing. If there is only one tire for a car and that production plant has a problem, it could shut down an entire vehicle assembly line.

With the tires selected, the OEMs can proceed with development and tuning of the rest of the vehicle around them. While the vehicle dynamics areas of ride, steering, and handling might be most obviously impacted, there are many other functional areas that are tuned around the tires. Braking performance, fuel economy calculations, and powertrain tuning are all impacted. The “active chassis” group will be developing the stability control, ABS, and traction control around the tires, as will the ADAS (advanced driver-assistance systems) group. The NVH (noise, vibration, and harshness) and durability groups will also be conducting their work around the tires.

As a former vehicle dynamics performance engineer for FIAT Chrysler Automobiles, I want to give you a glimpse into how specific vehicles are tuned around the tires. One of the platforms that I had responsibility for was the Pacifica, which had eight unique tires across 17, 18, and 20-inch wheels. Between gas, plug-in hybrid, Waymo, and the new Voyager, there were nine unique damper (shock) tunings to accommodate the ride characteristics of each car riding on each tire for their intended audience. Consider for a moment that, from a ride perspective, a tire is essentially an undamped spring. You can then see how varying that component would affect things such as impact harshness and shake and how that would need to be accounted for in the damper tuning.

In regard to steering, almost every tire had a unique electronic power steering calibration for its application in each model variant. I count 13 by memory and that is without variable steering modes. Each tire possesses unique traits as to how they respond to an input and build force to move the car. In general, it was desired to make all the packages feel roughly the same in terms of steering. However, certain packages varied slightly due to purpose or market.

Based on how the tires differed, changes to EPS tuning were usually required in a few areas of the calibration. The “on-center” feel, or how “positively” the car would steer straight-ahead, was critical to driving comfort, especially over long distances. The response characteristics of the tire would alter the linearity of the effort build in the wheel as the driver steered into a corner. In other terms, as you steer, you want the wheel to communicate with a continuous build in effort as you turn harder. Not having the effort build well-matched to the tire and vehicle response could result in a car feeling anywhere from nervous and hyper-active to sluggish and unwilling to turn.

So, if you value the performance and feel of your car as it came new from the factory, it would behoove you to replace your tires with the original ones when they’re due for it. And, not just the same brand and model, but the exact OEM tire that was tuned for your car. The aftermarket replacements look the same, but if you’re not buying the OEM part from a dealership, it is not likely to be the same OEM tuning.

Even if you’re not a particularly perceptive driver and you wouldn’t notice a relatively small change in something like steering feel, think about how a tiny bit of extra driving fatigue might accumulate over the course of the life of those tires. Or, how that extra 10 feet of stopping distance might not be a big deal until the moment that you need it.

Now, here’s the big caveat for everyone screaming at me right now. The exception is for those who desire a big improvement of a particular attribute of the vehicle’s performance and are willing to trade off many known and unknown factors in order to achieve it. For example, for maximum traction, one might buy a dedicated snow tire or a track-focused tire with the understanding that tread wear, NVH, fuel economy, etc will be significantly impacted. Others may still go ahead and buy the cheapest or longest-lasting tire that they can, but I caution against that. It isn’t worth undoing all the work that made your vehicle what it is and degrading something that cost tens of thousands of dollars in order to save a couple hundred.

Changing out the tires is almost akin to changing the engine in the car. You can do it, but you’d better know exactly what you’re doing, because the whole car was designed around it. If you’re satisfied with the performance of your vehicle, stick with the OEM tires when it comes time to replace.

[Images: FCA, Michelin]