Tyre pressures

[kuruma]

Wife out yesterday gets tyre warnings for front tyres that are supposedly at 39 psi. However, car shows that rears are also 39 but no warnings for those. Sticker on door panel says they're all supposed to be 42 so why no warnings for rear tyres?

Plugged in my Ring compressor and set it to 42. The compressor shows that all tyres are around 38-39 psi when I start. It goes to 42 very quickly, like within 15 seconds (which is much faster than I'm used to with other car tyres) and then when the compressor kicks out, it drops back down to 40.

Can't seem to get it higher than that even though the compressor is rated to deliver up to 80 psi. Any ideas and anyone else had these apparently hit and miss warnings?

 

[NorfolkMustard]

I had that too. I presume they are 39.4 and 39.6 or something

I went to the petrol station to get mine pumped up a couple of weeks ago, have been fine ever since

 

[Grymbok]

Wife out yesterday gets tyre warnings for front tyres that are supposedly at 39 psi. However, car shows that rears are also 39 but no warnings for those. Sticker on door panel says they're all supposed to be 42 so why no warnings for rear tyres?

Plugged in my Ring compressor and set it to 42. The compressor shows that all tyres are around 38-39 psi when I start. It goes to 42 very quickly, like within 15 seconds (which is much faster than I'm used to with other car tyres) and then when the compressor kicks out, it drops back down to 40.

Can't seem to get it higher than that even though the compressor is rated to deliver up to 80 psi. Any ideas and anyone else had these apparently hit and miss warnings?

I had the same problem today with a Ring compressor, which then went dead and stopped working completely. I then drove to the garage and filled up there instead. TPMS warning light hasn't gone off yet but hopefully will on next drive.

 

[Adopado]

I had the same problem today with a Ring compressor, which then went dead and stopped working completely. I then drove to the garage and filled up there instead. TPMS warning light hasn't gone off yet but hopefully will on next drive.

Not a good week for tyre compressor pumps then ... mine (just an old Halfords thing) struggled for ages yesterday just trying to get a tyre on my stored motorbike up to 36! It was extraordinarily cold here yesterday though so perhaps low temperature makes pumps less efficient. I could imagine their seals go hard and leak air. I've only used garage air on the Tesla to date.

 

[pgkevet]

You young fit lads should be using a foot pump. Us old farts own a couple of proper Lidl compressors 'cos they're handy for blowing crap off things as well as inflating tyres. Note Lidl have rechargable 12V portable tiddly things in stock at the mo' for the man who has to have everything pocket sized.

 

[MrBadger]

Also watch out on Model 3 12v compressors. Internal supply is 16A peak, 12A continuous and many air pumps exceed this so look for that if buying. Some have said that you can start the pump initially off the wheel then attach whilst ‘inflating’ to keep the current down - never used that technique myself.

 

[Adopado]

You young fit lads should be using a foot pump. Us old farts own a couple of proper Lidl compressors 'cos they're handy for blowing crap off things as well as inflating tyres. Note Lidl have rechargable 12V portable tiddly things in stock at the mo' for the man who has to have everything pocket sized.

I hope nobody is accusing ME of being young and fit ... disgraceful talk! (Anyway you should se the state of my old foot pump(s) ... now there's an upgrade I need to do!)

 

[Cwmwd]

I got TPMS warnings on the way back from collection with similar mix of 39s, stopped at a garage then next afternoon to "fill up" (thought I'd not have to do that again) and it's been fine since.

If I was young(er) and a fit(ter) I'd be going everywhere on my bike not slouching in a comfy Telsa...

 

[Dangerous Fish]

The biggest issue with the 12V compressors is the poor accuracy of the built in gauge.
If you want to stop the TPMS pressure warning messages the best thing to do is inflate the tyres a few PSI over where you want to get to then use a separate good quality gauge to let air out until you get the pressures bang on where you want them.

It's the rounding to whole numbers causing a lot of these error messages where you might have one tyre at 39.4 and another at 38.6 but they both show up as "39".

The door label says 42 but you don't have to run tham at 42. 38-39 will give you slightly less range but a slightly better ride. You can reset the TPMS to whatever pressures you want to run so it doesn't throw the warnings using 'TPMS reset' in the service menu. Just make sure when you reset you have exactly the right pressures in all 4 tyres.

 

[Jason71]

Also bear in mind that those are the cold pressures. Tyres should be pumped to that when the car has been sat on the drive for a while and the outside temp is representative of the time of year.
If you do it when the tyres are warm you will just get warnings again when it cools down.

 

[markharro]

Another tyre pressure question/issue. I finally got round to checking the correct pressures on our MS today on the sticker on the driver side door pillar. Oddly it gave pressures for 21 inch tyres when I have 19s fitted! Just 21 - no mention of 19s which I found very odd. And no guidance on extra pressure if heavily loaded etc. In comparison my old Audi listed about half a dozen different tyre sizes with appropriate pressures and guidance on increased PSI for heavy loading.
Looking at the manual all it appears to say is go by the sticker on your car! So how the hell do I find out what 19 inch tyres are meant to be inflated to on the MS? Hopefully another owner can illuminate me? And it seems odd there is no guidance on higher pressures for higher loading? Thanks any help will be welcome as we are about to start a long journey to France.

 

[pgkevet]

my MS sticker is for 45psi which the tyres heat up using them goes to 49psi on road trips and has even gone to 49psi sat at a charger on a hot day (but stayed at 49 when used after charging) The michelins state not to inflate over 50psi which I find interesting. the car is so heavy that additonal passengers are a small percentage of total mass. kerb weight + driver = appro 2100kg add 3 likely passengers and it's only another 200Kg (unless they're all my weight!)

 

[markharro]

my MS sticker is for 45psi

Thanks but is that for 19 or 21 inch tyres?

 

[pgkevet]

Thanks but is that for 19 or 21 inch tyres?

I have 19's on from new ..sticker says nothing about tyre size

 

[hgmichna]

While you're at it, does anybody know how exactly the tyre pressure sensors work? How precise are they? In my case that's in a Model 3 SR+.

If they are precise, then you could just pump a little higher, then observe the pressures on your next drives and let a little bit of air out of the overpumped tyres, using your fingernail or some other pointed tool. But be careful. Even one second of opening the valve can reduce the pressure by 0.1 bar or even more.

 

[Dangerous Fish]

While you're at it, does anybody know how exactly the tyre pressure sensors work? How precise are they? In my case that's in a Model 3 SR+.

If they are precise, then you could just pump a little higher, then observe the pressures on your next drives and let a little bit of air out of the overpumped tyres, using your fingernail or some other pointed tool. But be careful. Even one second of opening the valve can reduce the pressure by 0.1 bar or even more.

TPMS sensors are usually within + or - 1PSI so fairly accurate. The ones in my M3 tend to under-read by about 1PSI. I know that because I have a very expensive calibrated motorsport pressure gauge I can compare other gauges to. I also have a TMPS scanner, so I can get the precise reading from the TPMS sensor even when the car is not moving.

The problem with the car's built-in TPMS system, is you have to start driving to get the pressure readings and as soon as you start driving there will be some warmth generated in the tyres and the pressure will start to go up (unless you've managed to find a tyre fitter with properly dry Nitrogen to fill your tyres). This is why it's best to set your pressures with the tyres 'cold'.

The other issue is that the car's system rounds up or down the readings it gets from the sensors before displaying the number on screen. So a pressure read as 41.4PSI will be displayed as 41 and a pressure of 41.6 will be displayed as 42. So using the number on the display to set pressures could give you an error of almost 1 PSI in the actual tyre. Not much, but if you want to get them right the only way is to use an accurate pressure gauge to set them cold.

 

[pgkevet]

Ditto to electric dream. I have a decent compressor and also a dirt cheap ebay electronic display gauge and they all correlate well with each other and the tesla readout. The cheapo ebay thing displays 2-tenths of PSI and I use that to finagle tesla display numbers 'cos I'm anal about having them all the same on screen (and it;s a display I like to keep on). If ebay thing puts them all at spot-on 45.0psi then so does tesla albeit with a new tyre or puncture repair you have to fiddle a bit over the next weeks as 02 diffuses out? and you top up a tiny bit (i've lost 1psi after 4 weeks new tyre)

 

[AndrewGR]

It shouldn’t make any difference to pressure changes whether your tyres are filled with damp air or dry nitrogen. Boyles Law tells us that for any gas (or gas mixture) Pressure/(absolute) Temperature is constant if volume is constant. There may be arguments for using nitrogen but I don’t think they have anything to do with pressure. If there is liquid water in the tyre the argument would be different, but under any normal conditions the air in tyres should only contain water vapour - a gas.

 

[markharro]

sticker says nothing about tyre size

Are you sure - mine gives the tyre size code - think it was something like 265/35 R 21 with the "21" indicating a 21 inch tyre

 

[pgkevet]

Are you sure - mine gives the tyre size code - think it was something like 265/35 R 21 with the "21" indicating a 21 inch tyre

I had to walk all the way out to the barn again <g>.. you're right. 19" stated.