GM Tire Performance Criteria (TPC) "Survey"

e_net_ridere_net_rider Member Posts: 1,380
edited June 2014 in Chevrolet
The TPC Spec number you will find on the sidewall of your GM OE tires. They are proud of having developed this and do a long list of tests to find the best tire for GM models. That number is all you are supposed to need when you go to the dealer to have your tires replaced.

I would appreciate very much if those who have OE tires would post the specifics of your vehicle along with the TPC number found on the tire. It may have a couple of letters behind the number such as MS to indicate all-season.
Reply in this manner.
09 Malibu LT2 I4 AT6 (OE tire size) (TPC number) (make & model of tire) ( did you pay extra for wheels, ie price listed separately on sticker such as maybe a chrome upgrade. Simply answer, yes or no.) (same for tires. Answer yes or no.)

When sufficient data starts coming in, I'll analyze and post back.


  • e_net_ridere_net_rider Member Posts: 1,380
    The reason for the survey is that so far I have not been able to find GM's chart of TPC numbers that will point to specific vehicle and tire match.
    I do have a converter for specific TPC numbers, found at a GM site, and that is why I need the info.
    You can find a short explanation of TPC criteria in your owners manual and at GM tire replacement web sites.
  • capriracercapriracer Member Posts: 901

    This is indeed your lucky day. I just happened to be surfing the 'net and ran across your post. It might be good for me to explain who I am:

    I am a tire engineer for a major tire manufacturer - and I have been assigned to service the GM account. Among the many things I do is dealing with consumer issues after the tire is in production.

    I happen to have a list of 2010 GM tire fitments. This would be cataloged by vehicle for the various tires supplied to that vehicle - size, manufacturer, and trade name (I would have called it "tire model"), plus the TPC number.

    And just so we are on the same page: The way the TPC system works is that GM assigns a number to a particular qualification program, and all tires that get approved would get this number. Each time a new tire qualification program is released, a new number is assigned. Tire manufacturers are under no obligation to supply to the general public the tire with this assigned number - EXCEPT the tire manufacturer who is given the supply contract. He is not only obligated to supply all the GM dealers with this tire, but he has to stock a 3 year supply after the tire is no longer delivered to the assembly plant.

    The net effect is that there is usually one - and only one - tire that has a particular TPC number. There are occasional exceptions to this.

    But you may find a tire with a particular TPC number that appears on different vehicles - indicating these vehicles are very similar (at least from the tire's perspective).

    So why are you curious about the list you are proposing to compile?
  • e_net_ridere_net_rider Member Posts: 1,380
    I would like to see that list, for sure!
    As to compiling the list, I am curious if GM used more than one TPC for this car. They certainly used several different tire manufacturers, 16, 17, & 18" wheels, and varying widths/profiles.
    I find it hard to believe that all these fitments would perform to the same TPC.
    If I plug the TPC into the GM tire finder it finds only one tire, the ones I have. Yet the TPC is supposed to be the best tire or tires for the vehicle. It just does not add up.
    Reading this,
    link title
    and other info about the TPC, it seems to indicate that they strive to find the single best tire for a vehicle and thus a vehicle would have only one TPC. or several tires that meet the same TPC for that vehicle???
  • capriracercapriracer Member Posts: 901
    First, the list is 10 pages long, and I'm not even sure I am allowed to share it.

    Second, each TPC number is for a specific set of performance characteristics. Change those characteristics, and the TPC number would change. Needless to say, changing tire sizes, changes a whole host of performance characteristics - including those the average motorist would not have a clue about.

    Like Force & Moment Characterisitcs: These are of value in the vehicle design process when modeling a vehicle's handling characteristics. They are almost meaningless except in this context. Nevertheless, GM specifies some of these characteristics as part of their TPC number.

    So, YES!, the TPC numbers for 16" tires would be different than for the 18" tires. But there is more to it than that.

    Clearly an 18" tire would perform differently than a 16" tire, and the intent of specifying a different tire size would be to take advantage of those differences. It just might be that the 16" tire is designed for the mom and pop, garden variety car, where the 18" is designed for the sporty version of the same vehicle. Clearly these tires would be purposely designed to be different.

    So let me put it this way: EVERY car comes from the factory with tires designed SPECIFICALLY for that vehicle. So putting the same exact tire on is what the vehicle manufacturer thinks is the best tire for that vehicle.

    Will other tires work exactly the same? No, but they might work good enough for you! Plus, what you might find desirable might be quite different. Tires are a compromise and it is not unusual for consumers to have different priorities than car manufacturers.

    GM cars are no different, but GM feels they need to identify these tires in some way - hence the TPC numbering system.

    So a given vehicle may have several different tires on it - depending on the version - with sporty versions being different than the fuel economy versions. Not only would the tires be different, but the springs, shocks, and sway bars, too! - and probably a whole lot of other things! And, of course, the tires for these different versions would have different TPC numbers.
  • e_net_ridere_net_rider Member Posts: 1,380
    Sorry that it took awhile to respond, but I wanted to research a couple of things, but still a bit vague of some items.
    Lets look at 09-10 Malibu since there seems to be no changes as to tires, suspension, or powertrain for those years.
    The base LS is 2.4L w/AT4 & 215/55-17 tires.
    The LT is 2.4L w/AT6 & 215/55-17 or 225/50R17 (*maybe)
    option LT is 3.6L w/AT6 & 225/50-18.
    The LTZ is 2.4L w/AT6 & 225/50-18
    option LTZ is 3.6L w/AT6 & 225/50-18
    This assumes that GM's tire finder lists all of the OE sizes used.
    09-10 dropped 16" and added to 17" size P215/55R17 to 225/50R17. 18" tires remained the same size.
    Using some GM parts lookups, it appears that maybe different springs and struts may have been used by code, but I don't have a way to match code to model.
    It appears different struts by engine, 2.4 or 3.6.
    Possibly three different springs for model, only identifiable by code, not trim or engine, or least I don't have a cross.
    Yet there is only one part number for the bar.????
    The LS should be the lightest, being stripped and having 4 speed.
    The LT somewhat heavier because of options and the 6 speed which is built heavy enough that it can accomodate larger engines simply by adding more clutch plates and changing the computer programing. Yet these two get same tire.
    The 3.6 option would be heavier, mostly because of engine and maybe slightly because of extra clutch plates in tranny. All 3.6 get one size of 18" tire.
    The LTZ, presumably the heaviest because it is fully optioned, all get 18" tire, even with the 2.4L.
    There are two 18" tires as OE, only difference in standard specs is one is T speed and other is W. Hard to guess which went on what vehicle.
    The 17" drop to S and load drops from 94 to 93.
    (*maybe) The 225/50R17 is listed as applying to 09Malibu and since 17" are only on the LS & LT models with 2.4L it can be assumed that it would apply to either LS or LT or both.
    The one tire used in that size is listed as a grand touring. The other 17" used as OE is a basic passenger tire.
    Yet my window sticker clearly states "Touring Tires".
    And with the GM parts locators I used, there is only one strut for 2.4 L, different one for 3.6. So all trim levels including the LTZ get that strut unless it has the 3.6L option.
    With three springs available, and assuming that the LTZ would be better tuned than baser models, that uses two of the springs available. One for the 2.4 and another for the 3.6, which only leaves one spring for all others.
    But it would be more logical to maybe match by engine/tranny first. One for 2.4 and 4 speed, another for 2.4 and 6 speed, and the third for the 3.6 and 6 speed.
    Other combinations are possible but with those assumptions there is only one spring/strut combo that would be on my LT2 2.4L AT6 with 17" wheels.

    So how do two totally different tires with different TPC's match up as the best tire for my vehicle?
  • capriracercapriracer Member Posts: 901
    If you'll forgive my not posting promptly, I'll forgive yours. I was out of the country and without access to a computer.

    Recognizing that you've just pointed out at least 5 additional opportunities to have different tires, I think you can see why this is such a complex subject.

    If we assume that the goal is NOT to have many different tires - that the goal is to MINIMIZE the number of tires needed, but at the same time have some flexibility to tailor the car more precisely to the customer's desires - then we have to expect there to be compromises. But even within those compromises, efforts could be made to maximize the situation with careful tuning.

    But let's approach the problem from the other direction: 99% of the car owners replace their tires with exactly the same tire size that came originally. For GM cars, GM has a TPC numbered tire that is the exact tire placed on the vehicle from the factory - and if the consumer orders that tire by TPC number, he can not make a mistake and order the wrong tire.

    However, some folks want to change tire sizes, and in that case the TPC number is of no help. Part of the TPC specification is the tire size - and the TPC numbering system does not address the issue of which of the 2 tires sizes is the best. Once your vehicle has a particular tire size on it, the TPC number will guide you towards the same tire that came on the vehicle originally - which GM feels is the best compromise.
  • e_net_ridere_net_rider Member Posts: 1,380
    You are obviously right in the last paragraph, but GM claims that the TPC is supposed to address the best tire for a vehicle and from my earlier post about which tire size for what vehicle that seems to be an untrue statement. That is by looking at a couple of different tire manufacturers sites, they show either tire size, 215/55r17 or 225/50r17 were possible OE sizes for my model and trim level.
    I did not realize the percentage of same size users was quite that high, but certainly there are a lot of choices within a size. Load, speed, temp, etc. which can all effect the performance of a particular tire on a vehicle.
    The last tires I bought were for an Aurora. I used reviews of other owners to guide me. Those who had replaced the OE tires, Eagles or a Michelin, were much happier with other tires, especially over eagle, a high priced tire at that time. Reasons were improved ride, quieter, stickier, etc. and the hands down winner was half the price. I was so impressed I considered putting them on other two vehicles. One vehicle, customers loved them on it. The other it was rated poorly. So different tires can make the vehicle, the challenge is getting the right one. And from what I've seen of the TPC system so far, I'm not impressed.
  • e_net_ridere_net_rider Member Posts: 1,380
    Being a tire expert, I'd appreciate you insigt to the wear I'm seeing.
    I'm only able to get a good look at the outer rib. I see a feathering of each block in that rib. On one side of the vehicle, there is more wear at the trailing edge of the block. It is the opposite on the other side of the vehicle. Both front and back are same on each side.
    You might say rotate, but think about it. If I took wheel from LS and stood it next to wheel on RS, the wear pattern would then match.
    Vehicle was aligned at 600 miles and rechecked at different dealer a couple of weeks later. Now almost 6K.
    Do you have any ideas as to what might cause such wear?
    BTW, vehicle handles poorly, like play in steering and different road surfaces effect it drastically. Sometimes it feels like I'm driving on a layer of ball bearings.
  • capriracercapriracer Member Posts: 901
    First, as I said in my very first posting, what YOU consider to be the things you would want in a tire, are not always the same as what GM thinks you want in a tire. The thing that is certain is that the tire with the TPC number was optimized according to GM's priority list for YOUR vehicle. If you have a different priority list, then you will come up with a different answer.

    The wear you described has many names - feather edge wear, heel and toe wear - and it is caused by the natural rotation of the tire.

    As a tread element transitions into the footprint, the element is bent out of the way and the leading edge is rounded. As the same element exits the footpint, it stays attached until it almost leaves the footprint, then suddenly releases causing an extra bit of wear to occur on the trailing edge.

    So between each tread element, there will be "step". This is normal. However, the more toe the vehicle has, the worse this wear becomes. To keep this under control, regular rotation is perscribed. This is condition is also aggravated by low inflation pressure.

    And I think all the other symptoms you've described might simply be due to underinflation.

    But you should be aware that in my long career, I've determined that the alignment tolerances published by the car manufacturers are usually too wide - especially the toe spec. It is not uncommon for folks, especially those you don't rotate regularly, to complain about tire wear for vehicles that are barely "in spec". The key here is "barely". It's a matter of degree - and I think the vehicle manufacturers need to update their specs to reflect advances in technology. They've been using the same tolerance for over 50 years!
  • e_net_ridere_net_rider Member Posts: 1,380
    As a tread element transitions into the footprint, the element is bent out of the way and the leading edge is rounded. As the same element exits the footpint, it stays attached until it almost leaves the footprint, then suddenly releases causing an extra bit of wear to occur on the trailing edge.
    That makes sense, but is not exactly my case. On one side of vehicle it is trailing edge and the other it is the leading edge. So rotation is of no change to wear pattern whether rotating front to rear or cross.
    Alignment might be something to look at although the two printouts I have look pretty good. It is not likely that the machines require the degree of calibration such as in aircraft assembly where sometimes it requires 8 feet of concrete to ensure nothing gets displaced.
    I would think the best alignment would be when the vehicle is on the assembly line unless something happens to the vehicle. If that is not so, then it makes me question just how good the manufacturer spec is in the first place. Of course one can do some tuning to change handling, like buying different tires.
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