TPMS - Low Pressure - Why Not Tell Us Which Tire?

[billarnett]

No, No, No, No, No!

!. Heat, not pressure, kills tires. I don't know where people get the idea that putting some air in the tires is harmful--it's not. Well, fifty years ago, when we used bias-ply tires, if you put too much pressure in relative to the load, the centre of the tread would wear out quickly. Radial tires typically don't have this problem because most have a very large crown radius and the belts stabilize the tread. For almost all on-road use, you never want the pressure to be less than the vehicle placard pressure. If the day is hot you want it to be higher. If the day is cold and you inflate with indoor air you also want more pressure to compensate.

2. If you put 25 psi in one tire and 45 in another and drive for a couple of hours at highway speeds, both tires will have about the same pressure due to thermal equilibrium. However, the tire starting at 25 psi will be far hotter and may even debond. At the very least the compounds in the tire will have been substantially degraded.

3. The maximum pressure on the sidewall is for cold inflation first thing in the morning. It's not a problem (and in fact is perfectly normal) for the tire to have higher pressure after driving. Ideally the pressure increase is under 10% if the day remains relatively the same temperature, or 15% if the day grows significantly warmer. If it increases by 25% you need to find out what the cause is.

4. You never want to adjust pressure in a warm/hot tire. (This doesn't mean not adding air to a leaky tire to get to the repair shop.)

5. All tire pressure recommendations are based on a set of assumptions. One of them is that the ambient temperature is 18 C (65 F), another is that the pressure check is done cold, first thing in the morning, on every driving day. Change the assumptions and you need to change the pressure. The change is almost always upwards.

Well, I mostly agree. But sometimes there's no alternative to adjusting the tires hot. And it rarely the case that the conditions are identical to the standard ones. So doesn't it make sense for the cars computer to compute the right gauge pressure to use based on the actual ambient and tire temperature at the time?

As an experiment I tried taking one of my tires down to 30 and driving around the block. No TPMS warnings :-(. I didn't want to drive any farther like that.

Aside: my P85D with the stock 19" Michelins says 50 psi all around on the vehicle placard while the tire sidewalls say 51 psi max. That's an awfully small allowed range. My car was delivered with 46 psi all around. Should I go up to 50?

 

[jerry33]

Well, I mostly agree. But sometimes there's no alternative to adjusting the tires hot. And it rarely the case that the conditions are identical to the standard ones. So doesn't it make sense for the cars computer to compute the right gauge pressure to use based on the actual ambient and tire temperature at the time?

No. Tires are expected to increase pressure as the ambient temperature rises during the day. In hot climates you need to increase pressure to reduce the heat buildup. For example on a tire that has a 45 psi vehicle placard pressure, at 40 C (104 F) you need to increase the pressure by 4 psi to compensate (there are tables).

As an experiment I tried taking one of my tires down to 30 and driving around the block. No TPMS warnings :-(. I didn't want to drive any farther like that.

I don't see what that's supposed to show, other than that the tire has to be very low for the TPMS to alert. Less than a mile slowly is still considered cold.

The TMPS, as implemented by Tesla, is there only to warn you of severe underinflation. It's not a substitute for regular checking. Even with systems that tell you the pressure, you need to check sometimes to insure the sensors are actually showing the correct pressure.

Aside: my P85D with the stock 19" Michelins says 50 psi all around on the vehicle placard while the tire sidewalls say 51 psi max. That's an awfully small allowed range. My car was delivered with 46 psi all around. Should I go up to 50?

The most likely reason for this is that Tesla assumes you will be driving the car at high speeds, so they've already put in a bit of pressure to compensate for expected driver behaviour. As far as what you should run, it mainly depends on the climate and how often you check your tires. If you are in the Pacific Northwest, 45-48 seems a good range. If you're in Arizona in summer, 51. Then watch the wear and adjust if needed.

 

[billarnett]

No. Tires are expected to increase pressure as the ambient temperature rises during the day. In hot climates you need to increase pressure to reduce the heat buildup. For example on a tire that has a 45 psi vehicle placard pressure, at 40 C (104 F) you need to increase the pressure by 4 psi to compensate (there are tables).


I don't see what that's supposed to show, other than that the tire has to be very low for the TPMS to alert. Less than a mile slowly is still considered cold.



The most likely reason for this is that Tesla assumes you will be driving the car at high speeds, so they've already put in a bit of pressure to compensate for expected driver behaviour. As far as what you should run, it mainly depends on the climate and how often you check your tires. If you are in the Pacific Northwest, 45-48 seems a good range. If you're in Arizona in summer, 51. Then watch the wear and adjust if needed.

Are those tables different for different tires? If not, or if they're almost the same, then the computer could easily include them. I'll bet there's not one driver in a thousand that has access to that data. Having the cars computer tell you the right pressure at all times would make it a lot easier to not screw up.

My experiment was to see if the car reported which tire was low.

I live in the SF Bay Area. I guess I'll go with 50 psi for now. But it makes me a bit nervous, that's much higher than I've ever used before. I guessing tesla recommends such a high pressure to improve range.

 

[mknox]

I've read that some owners are able to contact service who can bring up their individual tire pressures on their computers. So the car provides the data to the mothership, it's just not being displayed to the owner. These particular owners that had posted this, if memory serves, were pre autopilot.

Yep. I was having TPMS errors and over a period of several visits, Tesla replaced the receiver unit, the antenna and eventually all 4 wheel sensors. When the problem came back, they were able to remotely diagnose the car and pinpoint which of the new wheel sensors had gone bad. Had it replaced yesterday (along with my rearview camera which had failed in a manner unrelated to the TPMS).

 

[FlasherZ]

Yep. I was having TPMS errors and over a period of several visits, Tesla replaced the receiver unit, the antenna and eventually all 4 wheel sensors. When the problem came back, they were able to remotely diagnose the car and pinpoint which of the new wheel sensors had gone bad. Had it replaced yesterday (along with my rearview camera which had failed in a manner unrelated to the TPMS).

That doesn't necessarily mean it supplies tire pressures, though - they can see which sensor hasn't updated in a while, but the specific pressures?

 

[mknox]

That doesn't necessarily mean it supplies tire pressures, though - they can see which sensor hasn't updated in a while, but the specific pressures?

Fair enough, but I would bet that they are getting values in the telemetry.

 

[smilepak]

My TPMS does tell me which tire needing air. Currently mine say rear right.

 

[Cyclone]

My TPMS does tell me which tire needing air. Currently mine say rear right.

Could you share a pic please? Do you have the old or new TPMS sensors?

 

[smilepak]

Could you share a pic please? Do you have the old or new TPMS sensors?

Attached. It came with the 70D

 

[Cyclone]

Attached. It came with the 70D

Nice. It won't tell you how low, but at least you aren't hunting for all four (though I'd still check all 4). Thanks!

 

[mknox]

Nice. It won't tell you how low, but at least you aren't hunting for all four (though I'd still check all 4). Thanks!

Yeah, mine just says something like "Check Tire Pressures". With my slow leak last week (nail in tire) it ended up being the 4th tire I got to with my pressure gauge!

 

[RAM_Eh]

Not sure this has been said but V7 will tell you the tire pressure of each.

 

[MassModel3]

Not sure this has been said but V7 will tell you the tire pressure of each.

Oh, very nice. I wonder if we'll need the newer TPS sensors and controller, or if it will be compatible with the original hardware.

 

[blanche]

Mine indicates which tire is low, for example "Check front left tire" or "Low tire pressure x2" to indicate multiple tires.

 

[Khatsalano]

In current 6.2, there is a text message on the dash that tells you which tire it is. It does not state pressure, but that's coming in 7.0.

- K

 

[Cyclone]

In current 6.2, there is a text message on the dash that tells you which tire it is. It does not state pressure, but that's coming in 7.0.

- K

Some feel only the newer TPMS system (which includes all 2015s) tells which tire. However, some have indicated that the service center can see the individual tire pressures and that may be on the older TPMS system. If the latter, I would hope both give readings on v7. If the latter only applies to the newer TPMS system, then I worry the older cars won't get that feature in v7. I hope I am wrong and we all get it!

 

[dsm363]

In current 6.2, there is a text message on the dash that tells you which tire it is. It does not state pressure, but that's coming in 7.0.

- K

Even older cars? I don't think I've even seen that on my Signature car. It just says 'tire pressure low' or something similar.