Why doesn't the TPMS system include the spare tire?
Posted by IKnewThisYearsAgo@reddit | askcarguys | View on Reddit | 69 comments
Every spare I've ever checked needed air, sometimes a lot. Seems like a no brainer to put a sensor on it.
UmatterWHENiMATTER@reddit
Some do.
LongOrganization7838@reddit
Some will have it in the spare but in general the wheel needs to spin in order for the tpms to wake up which is also why you need to drive for it to get the updated numbers
cvad1@reddit
I had an 04 jeep grand Cherokee that had tpms in all 5 wheels and would display the spare pressure when you flipped through the menus
Signal-Confusion-976@reddit
Some cars do include the spare tire.
ApollymiKatistrafia@reddit
Toyota had them on their full size spares. They dont bother with donuts, and now tons of cars dont have spares at all.
BassWingerC-137@reddit
For some cars and some sensors the wheel must spin to wake the sensor up and communicate to the car where the wheel is located.
ion_driver@reddit
When I swap my tire with my spare, the TPMS loses the signal from that tire. I've always wondered why. Like, the tire is right there under the truck.
CommandoLamb@reddit
When I had a BMW I had a full size spare and it had a tpms. If I put it in the trunk the wrong direction I would get a tire pressure light.
The valve stem had to be facing the rear right for it to be in range of the sensor. It was stated in the manual.
00s4boy@reddit
Specifically it's an inertia sensor to prolong battery life since the batteries are not replaceable.
Drunken_Sailor_70@reddit
My tundra does and so does my wife's 4runner. Sometimes its a PITA though, especially when yhe temps dip and the spare pressure goes a psi or two below the setpoint.
DismalAd6639@reddit
It wastes money, it’s that simple. The sensors aren’t cheap
SeriousPlankton2000@reddit
You can get a four tire sensor kit (screw on valve sensors) for 13 €, complete with receiver.
DismalAd6639@reddit
The temu tpms kits aren’t comparable to the oems
wpmason@reddit
Sir, cars are increasingly shipping without spare tires in order to save weight and cost.
That’s the future, not adding more cost to the past.
KOhReally@reddit
I believe the good expensive ones with sensors in the tire do in fact work for the spare too. But a lot of the cheaper modern ones use more calculations so there’s no way to test the spare (no weight on it, no displacement).
PetriDishCocktail@reddit
I love the TPMS systems that use the ABS / wheel speed sensor. There's nothing to go wrong. No battery to change. No expensive sensor for the tire guys to mess up....
Skensis@reddit
The simplicity is nice, but I much prefer getting the data I get from an active TPMS. Getting both PSI and temp in real time is useful for me.
ReversEclipse1018@reddit
If the tire guys break the sensor, there’s shop replaces it at no cost. If they don’t, find another shop, cause that one is shit. Overcharging (more than a standard profit up charge) and making you pay for their mistakes is just bad business.
KOhReally@reddit
Agreed! Much more elegant solution, even if there are some cons.
kaack455@reddit
Huge con is where you live, when gm used that system every time you went down a gravel road with any washboards the low tire light came on
ReversEclipse1018@reddit
Hondas are the only modern vehicles that don’t have sensors. Literally every other brand has sensors. But, no. OP isn’t asking if sensors would work for the spare, they’re asking why spare don’t have sensors at all. Toyota SUV’s do, but that’s the only ones I’ve encountered with sensors in the spare. The reason the most spares don’t have sensors is because generally spares are donuts, and therefore require a much higher pressure to be correct. The vehicles TPMS system has a specific, small, range that it checks for correct pressure. The regular 4 tires being between 30-40psi are the range that the system checks for. The donut at 80psi would have the light on constantly. And if the range went high enough to include the spare at 80, you could way overfill your tires and it wouldn’t tell you. That would then lead to a blowout at high speed with high pressure, which is not a good combination…
KOhReally@reddit
Well this just isn’t true. My modern VW is a calculation based system, no sensors in the tires.
There’s no reason you can’t have different sensors, different alarms, etc. for the spare vs the other tires. It’s just wires and programming.
ReversEclipse1018@reddit
Since you deleted your other comments, I’ll reply to this one with my answer to the deleted one.
No one has claimed there is any resemblance of friendship between us. And no, a pressure sensor based system would not work for the spare, which is what I’ve been telling you. You’re stuck on wheel speed sensors which in theory could work for spares, but manufacturers don’t want to employ the extra people to come up with separate calculations that also automatically sense the spare being used.
I don’t believe you’ve ever programmed a TPMS system. What you’re saying implies that each sensor communicates with a specific wire in the receiver module, which is just false. You can change which sensor communicates to the car where it is, in fact when you rotate your tires, you’re supposed to reprogram the TPMS.
1) All of the brands that that use indirect TPMS (Honda, Tesla, Audi, VW, Mazda, Toyota, Jeep, and Ford) don’t use them on every model. Which reminds me, you never answered me on what model or year VW you have. 2) No, they don’t cover the spare, which is the entire reason for this post, you absolute dunce. OP was asking why TPMS doesn’t cover the spare. And the answer is because it just costs more money to do so. Whether it be an older system that uses at best, a $50 sensor, or more programming that requires paying more employees. And btw a RAV4 is in no way a “higher class car.” It’s an expensive vehicle, but Toyota SUV’s are not high class. Nor are they necessarily off-road focused. You ever heard of the term “Pavement Princess” in reference to Jeeps that never so much as see dirt, much less touch it? That term encompasses every supposedly off-road vehicle that is not used in any off-roading capacity. In modern manufacturing, they’re mainly designed as a comfortable ride, rather than a capable one.
So, like I said, when you can come up with that calculation based system for spares that you keep taking about, let me know.
KOhReally@reddit
When did I say there was a calculation based system for spares? My original post clearly states: "But a lot of the cheaper modern ones use more calculations so there’s no way to test the spare (no weight on it, no displacement)."
You call me a dunce but you can't even read, friend.
To your point: "And no, a pressure sensor based system would not work for the spare, which is what I’ve been telling you."
This is just wrong. Even you keep talking about Toyotas with pressure sensors in the spare. Everyone knows there are spares with pressure sensors.
This is why I deleted my posts, you are aggravating and non-sensical. Impossible to argue with.
ReversEclipse1018@reddit
No, you never said that there are calculation systems for spares. I said a calculation system could work for a spare, and you were saying that it would just require different programming. I did read, dunce.
And yes, I keep talking about Toyotas. They have full size spares, which run the same fucking pressure as the other 4. Donuts run 80. You’re the one not making sense, and it seems more likely you deleted your posts because of my correct statements.
ReversEclipse1018@reddit
It’s also a lot more money that the manufacturer doesn’t want to spend when the customer can pay for a pressure gauge. They’re taking power liftgates out of vehicles and only offering it for higher trims. My parents just bought a 2026 CR-V and it has a manual liftgate for their base model. Their 2015 and 2019 base models both had power liftgates. The manufacturers don’t want to spend more money on something that doesn’t get them paid more.
And what year is that VW? What model? Have you taken the tires off and checked? I just put 4 tires on a 2026 Volkswagen Tiguan today and they all had sensors.
KOhReally@reddit
I don’t understand your logic here, it is less money to use calculations than sensors in the tires.
It’s also the exact same amount of money to have a different alarm on
ReversEclipse1018@reddit
Well this just isn’t true. Your logic about it not being more money is flawed. Sure the hardware doesn’t cost any more, but paying different employees to come up with the system to notify you of different pressures costs more. The person who’s working on the sensors that activate from the centrifugal force of the rotating wheels is not the same person who would be paid to come up with a sensor that is constantly active while not using the power from the battery in the sensor while the spare is stationary. So, when you can come up with a calculation based system that automatically (no driver input) knows that the spare (a different diameter wheel) is on the vehicle and can calculate the pressure of a much smaller tire in tandem with calculating the other three, come back and tell me off for my opinions. Until then, I don’t want to hear what you’ve got to say to rebuke facts.
KOhReally@reddit
Friend, the calculation based systems can’t work for your spare that’s the whole point in my original post.
You said that spares don’t have sensors because of the higher pressures which makes no sense at all. The sensor in the tire, the wire to communicate with the sensor are the exact same as the other tires, you just change the programming.
The reason most spares don’t have sensors is. 1) many cars now use a calculation based system without sensors in the tires. 2) TPMS systems don’t need to cover the spare, it is purely an extra selling feature more for higher class or off road focused vehicles.
I don’t think you understand the systems that are used and their differences.
do-not-freeze@reddit
Weird, we have a higher trim 2015 CR-V with a manual lift gate.
ReversEclipse1018@reddit
The 2015 may not have had a power liftgate then. I thought it did, but the 2019 for sure did. The 2019 was also most definitely a base model, cause a guy t-boned my mom and his insurance paid for the 2019. But the fact that it was standard until apparently this year is crazy. Manufacturers wanted to pull in customers with convenience but now they just want more money.
ChemistAdventurous84@reddit
Lexus puts them in the full size spares. Well, I have a 2009 and a 2018 in which that’s the case.
ReversEclipse1018@reddit
All Toyota SUV’s have them. And they’re required to work in order for the system to reset. Pretty annoying for us techs, but smart for the company.
Any_Web_1784@reddit
Some do
Equana@reddit
Some do have sensors. Most owners would see the light and never think to check the spare or they don't know if they even have a spare. Yes there are people like that.
Theycallmesupa@reddit
My light was on for yeeeeaaarrsss before I realized it was the spare (I found out on reddit)
SeriousPlankton2000@reddit
Nowadays it tells you each pressure individually.
Chemical-Quality-186@reddit
Before I was a tech this happened to my grandfather's truck. I took it to 3 shops local and 1 out of town, most said I needed a new module (not sensor). 1 said they just didnt know what was wrong. Finally after becoming a tech and having access to the proper tools I decided to check myself...turns out no one checked the spare tire.
One-Bodybuilder309@reddit
Mine does
Realistic_Word6285@reddit
This is why I always recommend a portable tire inflator to keep in the car. Rarely anyone ever checks the air pressure in the spare tire.
Far_Chocolate_8534@reddit
Can confirm. 8 years in the tire industry.
CrazyJoe29@reddit
I don’t care why. Just fix it. Tell me the pressure of the spare.
There’s a 25” flat screen in the dashboard; it should prioritize useful information over cheesy animations.
b1gb0n312@reddit
I bought a used oem wheel off ebay was detected by my car tpms
therin_88@reddit
The spare doesn't spin so there's no way for TPMS to work.
TTTomaniac@reddit
Not all TPMS uses a pressure sensor to begin with. For example, my 2015 Skoda Octavia uses the spin rate signal from ABS control to determine whether a wheel is losing circumference compared to the others. Doesn't catch outright punctures but it did catch that time I caught a wood screw and had a slow leak in time to get it patched.
ThatOldEngineerGuy@reddit
I just wish spare tires were standard these days.
Common-Application56@reddit
For a manufacturer a sensor may be around $5 per sensor at the most. However at scale it makes no sense to add it to a wheel for temporary use.
They totally could and come up with a system to keep an eye on the spare tire donut or full size. But nope
BillyRubenJoeBob@reddit
About 6 months ago, I paid to have the sensor in my spare replaced because it triggered the warning light on the dash.
Sweet_Speech_9054@reddit
Many do. But the system works based on tire movement so the spare needs a special system to make it work. It’s just a receiver that can send a signal to wake it up but it’s still more work. They also don’t last as long in the spare because the sensor can actually charge itself when moving.
Galopigos@reddit
They have no way to charge themselves, they are a very simple unit really. A pressure sensor chip on a transmitter circuit board that is based on the key fobs powered by a lithium battery. The reason the spare units don't last as long is simply because the sensor is kept active as long as the ignition is on.
Heavy_Gap_5047@reddit
The sensor uses tire rotation to provide the energy it needs to wirelessly communicate with the car. The spare doesn't rotate.
Galopigos@reddit
WHAT?? Virtually every TPMS uses a 2032 or 2045 lithium battery inside it to power the sensor. They do use a sensor to wake them up when the vehicle starts moving.
Longjumping_Cow_5856@reddit
Plenty of spares are part of the system?
ReversEclipse1018@reddit
Only Toyota SUV’s as far as I’ve encountered. I work at a tire shop, and the only spare sensors that are included in the reset are Toyota. I’ve also had plenty of Toyotas that come in for a flat, and it turned out the spare sensor was dead.
Longjumping_Cow_5856@reddit
Makes sense most of mine have been Toyota!
ChrisGear101@reddit
Vehicles with full size spares usually have sensors. Donuts won't, because they are temporary use only.
ReversEclipse1018@reddit
Not necessarily. I’ve had a handful of Toyotas come in to check TPMS, and none of the tires be flat. Remember it’s a Toyota, check the spare, sensor’s dead.
CauliflowerTop2464@reddit
Our 2009 Lexus LS460 had one. It was both convenient and inconvenient because it didn't know which position each tire was in. If one tire was low, we had to manually check each tire including the one the trunk. We usually had stuff in the trunk for some odd reason.
Late-Button-6559@reddit
My vehicles with it do have the spare with a sensor. BUT, the sensor is only working when it’s spinning.
It’DLL be a battery life choice - fair.
ReversEclipse1018@reddit
Toyota SUV’s have a sensor in the spare that is required to work in order for the tire pressure light to be reset. Only vehicles I’ve encountered that have a sensor in the spare.
Civil-Departure-512@reddit
It’s smart till you have to deal with it. There’s a sensor in the spare on the FJ Cruiser. Typically will go off 3-4 times a year. Always momentary panic when it beeps and you rush out to check all the tires. Only for it to be the spare 9/10 times
No_Geologist_3690@reddit
I would say because it’s because it’s one of the most neglected maintenance checks on a vehicle. Mechanics aren’t lowering tires on trucks / pulling trunks apart on oil changes to check them.
Customers would get annoyed having to check that every 6 months to keep the light off, and honestly it’s not something I’m doing for free as a mechanic.
TableDowntown3082@reddit
It was fairly common practice for toyota to use a tpms sensor in the spare for a good while. When programming the sensors, you had the option to include or exclude the spare in normal operation. They'd also typically have a second set of hex keys for primary and secondary wheel sets.
otterland@reddit
This is actually an insanely brilliant idea. I'm a maintenance dork and mine are always inflated, my fluids are fresh and so on. But it took someone reminding me of it a few years ago to even think about checking it a couple times yearly.
Unfortunately spares are basically obsolete. Roadside service and goo in a can have replaced spares. I carry my traditional spare and a plugging kit. I got a screw in my tire the other day and plugged it in a parking lot and lost one psi.
JPhi1618@reddit
I think the real answer is that it doesn’t affect your gas mileage. One of the main reasons (along with safety) was to improve gas mileage.
adjusterjackc@reddit
Amazon.com: Pencil Tire Pressure Gauge 2 Pack with Valve Caps, Stainless Steel Body Tire Gauges for Tire Pressure with Deflator Valve, Metal Nozzle Tire Gauge for Cars, Pickups, RVs, Bicycles : Automotive
Thereelgerg@reddit
$
Anachronism--@reddit
Some systems use the abs sensors to detect a wheel that has a smaller diameter due to lower pressure. It only works when the wheel is on the car.
Most in wheel sensors wake up when the wheel spins, otherwise they would run down the battery.
My jeep has a full size spare with a sensor and it learns its location when you rotate the tires.
zinsser@reddit
I am an idiot sometimes. I changed a flat and threw the bad tire in the bed of the truck. Later I went to take the tire to the shop to get it repaired, only to have a momentary panic when I started the truck and it showed a tire pressure warning light on the dash. Yeah, I guess that one with the sensor is kind of low.