Tyre Pressures

[Bortig]

I have a question I think I know the answer but wanted to check, my tyre pressures are 38 psi front and 34 psi back which I set with a compressor and separate gauge, however on the mercedes me it shows them as being 40 psi and 36 psi, is this correct know the tyres warm up is that the difference, but why does the mercedes me show ghat as its not really very accurate, thoughts.

 

[markjay]

I have a question I think I know the answer but wanted to check, my tyre pressures are 38 psi front and 34 psi back which I set with a compressor and separate gauge, however on the mercedes me it shows them as being 40 psi and 36 psi, is this correct know the tyres warm up is that the difference, but why does the mercedes me show ghat as its not really very accurate, thoughts.

The Mercedes me figures are measured in real-time, or based on periodic sampling? The pressures you're seeing now may have been measured earlier when the tyres were warm.

 

[Bortig]

The Mercedes me figures are measured in real-time, or based on periodic sampling? The pressures you're seeing now may have been measured earlier when the tyres were warm.

So it really still means best to check tyres manually rather than rely on the me app, can't even check my oil as there's no dip stick and this is progress, not so sure.

 

[Chrishazle]

I've found from experience that I'm better running my tyres at 1-2psi below what MB recommend for my S204 (30 front, 33 rear) - I normally do my tyres with a cordless compressor, but when I first got it I checked it agains my old fashioned manual gauge so know it reads 1psi low so set the cordless one the same 28 front and 31 rear, cold tyres.

 

[markjay]

So it really still means best to check tyres manually rather than rely on the me app, can't even check my oil as there's no dip stick and this is progress, not so sure.

Can't you see the tyre pressures in real-time on the instrument cluster while driving? That's generally good enough.

 

[Bortig]

Yes I can see that in the display, thought it was connected to the me app, will have another look and compare them.

 

[BigJase88]

Neither the air compressor or the tpms themselves will be that accurate. They are more of a ballpark. But you can see a rise of about 5psi between cold and hot running. I would not be concerned

 

[PhilP68]

The TPMS shows psi 2 - 3 higher than what I measure compared to what I measure manually; I think the car's calibration is out since I have 3 guages that all align plus align to where I get mmy tyres fitted. Therefore, I always set to my guages and I don't worry about the difference since I assume that a pressure drop will stil be shown on the car's system and alert me to an issue when driving. Plus I always have my eyes, eyes and seat of the pants to go by.

 

[Bortig]

Yeah hadn't thought of the alert angle, so it'll still tell me if loss of pressure, good point.

 

[markjay]

Yeah hadn't thought of the alert angle, so it'll still tell me if loss of pressure, good point.

It will only alert you when the loss of pressure is significant. I am not sure what the threshold is exactly, but it's around 8-10psi loss before the alert is displays. It's meant to warn you about a flat tyre, but it's not a substitute for regularly checking the pressures.

 

[BfgSLKMan]

Manufacturers tyre pressures are normally quoted at 20c ambient temp, so if it says 2.2bar, then thats when the air in the tyre is 20c. Roughly tyre pressure decrease/increase 0.1 bar for every 10c. So when you look on the onboard computer and it say 2.1bar at 12c then the pressure is about right if it is supposed to be 2.2bar. I know that having used a calibrated air line (at work) that the on board TPMS is very accurate, so I suspect that discrepancies will likely be in your pump, so you will be better taking the reading from the car than the pump guage (and then you know how much to adjust your pump guage by).

 

[PhilP68]

It will only alert you when the loss of pressure is significant. I am not sure what the threshold is exactly, but it's around 8-10psi loss before the alert is displays. It's meant to warn you about a flat tyre, but it's not a substitute for regularly checking the pressures.

Eggs and suck!

 

[PhilP68]

Manufacturers tyre pressures are normally quoted at 20c ambient temp, so if it says 2.2bar, then thats when the air in the tyre is 20c. Roughly tyre pressure decrease/increase 0.1 bar for every 10c. So when you look on the onboard computer and it say 2.1bar at 12c then the pressure is about right if it is supposed to be 2.2bar. I know that having used a calibrated air line (at work) that the on board TPMS is very accurate, so I suspect that discrepancies will likely be in your pump, so you will be better taking the reading from the car than the pump guage (and then you know how much to adjust your pump guage by).

Don't know about the guage of the OP but my guages are the ones I used with my race bikes; all expensive, calibrated and accurate. The TPMS on my previous Audi and BMW cars always matched with my guages; i assumed the lack of accuracy of the MB system was just yet another characteristic of these beautifully designed but ultimately, lower quality cars (IMHO).

 

[BigJase88]

Don't know about the guage of the OP but my guages are the ones I used with my race bikes; all expensive, calibrated and accurate. The TPMS on my previous Audi and BMW cars always matched with my guages; i assumed the lack of accuracy of the MB system was just yet another characteristic of these beautifully designed but ultimately, lower quality cars (IMHO).

It's like anything if you don't keep them calibrated then how accurate are they

I am an Instrument Tech and i do calibrate gauges. Unless it is calibrated on a certified piece of equipment like a Druck Pressure Calibrator then how would you ever know.

A couple of p.s.i drift on a Bourdon Gauge is not uncommon. Generally these things have a +/- 10% tolerance

 

[XFJagman]

I've found from experience that I'm better running my tyres at 1-2psi below what MB recommend for my S204 (30 front, 33 rear) - I normally do my tyres with a cordless compressor, but when I first got it I checked it agains my old fashioned manual gauge so know it reads 1psi low so set the cordless one the same 28 front and 31 rear, cold tyres.

Did you know that the police can fine a driver incrementally for each 1 PSI below the recommended value? (But not for being over) It's unlikely to happen, but the option is there.
I haven't yet checked how many PSI my tyres gain when hot, but I will follow the advice in another reply and check the dashboard reading at start and then on the move and compare it with the actual cold reading using a gauge.

 

[PhilP68]

It's like anything if you don't keep them calibrated then how accurate are they

I am an Instrument Tech and i do calibrate gauges. Unless it is calibrated on a certified piece of equipment like a Druck Pressure Calibrator then how would you ever know.

A couple of p.s.i drift on a Bourdon Gauge is not uncommon. Generally these things have a +/- 10% tolerance

I'll know where to send my guages then, if they ever start to drift and so misalign to numerous other guages
Oh, but maybe they have all drifted but still perfectly synchronised, just like all the woman in my house

 

[BigJase88]

I'll know where to send my guages then, if they ever start to drift and so misalign to numerous other guages
Oh, but maybe they have all drifted but still perfectly synchronised, just like all the woman in my house

#Gauges

 

[PhilP68]

#Gauges

lol Gages in my world

 

[Bortig]

I've got a Michelin digital one it seems as accurate to what my indy uses

 

[tonysmb]

Did you know that the police can fine a driver incrementally for each 1 PSI below the recommended value? (But not for being over) It's unlikely to happen, but the option is there.
I haven't yet checked how many PSI my tyres gain when hot, but I will follow the advice in another reply and check the dashboard reading at start and then on the move and compare it with the actual cold reading using a gauge.

What's your source for the statement about pressures / incremental fines?

To stand any chance of being of any conclusive evidential value it would require a calibrated gauge, tested before and after the check, a knowledge of the ambient temperature and the temperature of the tyres. Clearly, a seriously under-inflated tyre is dangerous but varying levels of fines for -1psi, -2psi etc. sounds like creative reporting to me.

In S. France last year my 'cold' pressures were 2-3psi higher than when I checked them before leaving the UK, simply due to the warmer air temperature.