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Discussion Starter · #1 ·
I live in sunny southern California, I have roadside assistance, and other than an occasional trip to Big Bear or Mammoth or Arizona, help is only minutes away, so I really don't need the run flats on my GLA250 (2wd).

When the RF's finally wear out, I'd like to change over to regular tires, but wondering what happens to the TPMS that wants to see 44psi? Can it be reset to something like 35psi?
 

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I live in sunny southern California, I have roadside assistance, and other than an occasional trip to Big Bear or Mammoth or Arizona, help is only minutes away, so I really don't need the run flats on my GLA250 (2wd).

When the RF's finally wear out, I'd like to change over to regular tires, but wondering what happens to the TPMS that wants to see 44psi? Can it be reset to something like 35psi?
The TPMS threshold can be reset using the car's settings menus. Basically you set your car's tires pressure to the correct values then reset TPMS. TPMS then uses the new values for its baseline.

However, 44 psi is way too high for your current tires it should be 32 psi front/29 psi rear (at least those are the correct values for my 2016 GLA 250 4Matic). 44 psi is the maximum pressure rating for the tire. The correct values for your car can be found on the fuel filler flap.
I bet you have a very harsh ride as a result and poor tire wear, too.
 

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Discussion Starter · #3 ·
The TPMS threshold can be reset using the car's settings menus. Basically you set your car's tires pressure to the correct values then reset TPMS. TPMS then uses the new values for its baseline.

However, 44 psi is way too high for your current tires it should be 32 psi front/29 psi rear (at least those are the correct values for my 2016 GLA 250 4Matic). 44 psi is the maximum pressure rating for the tire. The correct values for your car can be found on the fuel filler flap.
I bet you have a very harsh ride as a result and poor tire wear, too.
Yeah ride has always been harsh, but I assumed they were correctly set at the factory! (Foolish of me, I guess). I've never heard of a tire at that pressure before, but assumed it was because of the RF.

Seems to me that this incorrect setting (for a year now) should be a warranty issue, as well as tire wear. Doubt I'll get MBZ to agree with that!
 

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Yeah ride has always been harsh, but I assumed they were correctly set at the factory! (Foolish of me, I guess). I've never heard of a tire at that pressure before, but assumed it was because of the RF.

Seems to me that this incorrect setting (for a year now) should be a warranty issue, as well as tire wear. Doubt I'll get MBZ to agree with that!

Tire pressure is one of those things that I check right away when ever i get a new car, a long with removing dealership lic frames and stickers
 

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Hi WhiteRabbit-You surely are right. Thanks. The sticker on the driver side door jamb says only 44, the fuel filler flap says 34 front and 29 rear. I will fix that first thing tomorrow. My suggestion to Oceansol would be, if you are thinking of changing from the runflats, and you still have some tread, do the change sooner rather than later, and save the runflats for when you get rid of the car. This could be especially important if you lease, as some of the lease folks want to see runflats when the car is turned in, and if you trade it, not having runflats could affect the trade value.
 
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Discussion Starter · #6 ·
Hi WhiteRabbit-You surely are right. Thanks. The sticker on the driver side door jamb says only 44, the fuel filler flap says 34 front and 29 rear. I will fix that first thing tomorrow. My suggestion to Oceansol would be, if you are thinking of changing from the runflats, and you still have some tread, do the change sooner rather than later, and save the runflats for when you get rid of the car. This could be especially important if you lease, as some of the lease folks want to see runflats when the car is turned in, and if you trade it, not having runflats could affect the trade value.
Thanks for the tip, but my car is purchased and I keep my cars a long time, so I'll just run them till their done.

Thanks so much for this forum, too! I've learned a lot and continue to do so.
 
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I changed front pressure, cold, to 34 and rear to 32. The TPS didn't have a problem with this, without doing a reset. Will see if it feels different.
 

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I changed front pressure, cold, to 34 and rear to 32. The TPS didn't have a problem with this, without doing a reset. Will see if it feels different.
I would still reset the TPMS since you don't know what baseline it is using.
If you do reset it, do so when the tires are cold.
 

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Good advice-will do. I noted that all 4 tires were at 37, cold, this morning before I changed pressures. As mentioned I put fronts at 34 and rears at 32. The pressures seen on the TPMS system were 1 or 2 pounds higher than at the tires-I checked while adjusting air with two different gauges, both agreed. I kept the system on while driving, and the TPMS went up to 37 pounds, all 4 wheels, after a few minutes of 60 mph driving.
 
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since we are on the topic of tires, this is a public service reminding people to rotate their tires, all four are the same size even for the AMG,

The service manual is very vague about this, my advisor said every 5000 miles should do
 

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Good advice-will do. I noted that all 4 tires were at 37, cold, this morning before I changed pressures. As mentioned I put fronts at 34 and rears at 32. The pressures seen on the TPMS system were 1 or 2 pounds higher than at the tires-I checked while adjusting air with two different gauges, both agreed. I kept the system on while driving, and the TPMS went up to 37 pounds, all 4 wheels, after a few minutes of 60 mph driving.
TPMS on both my Mercedes also reads 1 - 2 lbs higher than my tire pressure gauge does. And I use pro type gauge.
So I go by the gauge and that gives me a little cushion. :)
 

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Discussion Starter · #12 ·
I emailed my service writer to discuss this situation, and he said 'no problem, bring it in we'll fix it.' So I stopped this morning and a young man came and took the car to the service area for a few minutes, then when he brought it back he said, "all fixed, all 4 tires at 44psi and the TPMS is reset!" He showed me the gas filler cap with the recommended inflation pressures, and said that they always inflate to the max allowable! I was very surprised by that and told him why do they go the the max when most of the time I don't have max load. He said he could set it anywhere I wanted.

So we discussed, and settled on 35psi front, and 33psi rear. I know recommended is 31F/29R, but since they do this all the time I decided a compromise was in order.

Any thoughts?
 

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Since you had driven in and they took the car right away the tires would have warmed up quite a bit. The pressure should be checked and set when the tires are cold.
Check what TPMS says the pressures are after the car has set over night without being driven. I bet the pressure will be lower in every tire.
Personally I never rely on the dealer or any garage to set the tire pressures correctly. They invariably set them too high.
 

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When they delivered my car, the dealer had pumped up the tires as well (above 40 psi). Lowering them to what the gas cap says significantly improved the ride (very harsh -> manageable).

Also, keep in mind that what your TPMS shows could be off due to altitude. I live at 6,000 ft and travel to 8,500 ft and the car generally shows 4-5 psi over where it is with a good gauge. All cars I have had with TPMS do the same thing at altitude.
 
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Discussion Starter · #15 ·
Thanks guys, I thought about the fact that the tires were warm when I brought it in, so likely they'll be lower when I head home tonight. In fact, that's why I had him go 4psi over, because I figured they'd be about right when cool. I guess I have to figure out how to reset the TPMS to current pressures.

Thanks!
 

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Discussion Starter · #16 ·
Rayman, I live at sea level, but when I go up to Big Bear (6750ft) in the winter, the car in the cold mornings always show low pressure. So I hit a gas station and bring it up a bit, but just a bit.
 

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Thanks guys, I thought about the fact that the tires were warm when I brought it in, so likely they'll be lower when I head home tonight. In fact, that's why I had him go 4psi over, because I figured they'd be about right when cool. I guess I have to figure out how to reset the TPMS to current pressures.

Thanks!
To reset the TPMS monitor values, from page 335 of the 2015 GLA Owner's Manual (pdf version):
Set the tire pressure to the value recommended for the corresponding driving situation on the Tire and Loading Information placard on the driver's side B-pillar (Y page 328).
Additional tire pressure values for different loads can also be found on the tire pressure table on the inside of the fuel filler flap (Y page 328).
Make sure that the tire pressure is correct on all four wheels.
Make sure that the SmartKey is in position 2 in the ignition lock.
Press the Left or Right Arrow button on the steering wheel to select the Serv. Menu.
Press the Up or Down button to select Tire Pressure.
Press the OK button.
The multifunction display shows the current tire pressure for each tire or the Tire pressure will be displayed after driving a few minutes message.
Press Down arrow button.
The multifunction display shows the Use current pressures as new reference values message.
If you wish to confirm the restart:
Press the OK button.
The Tire Press. Monitor Restarted message appears in the multifunction display.
After driving for a few minutes, the system checks whether the current tire pressures are within the specified range. The new tire pressures are then accepted as reference values and monitored.
If you wish to cancel the restart:
Press the Back/Cancel button.
The tire pressure values stored at the last restart will continue to be monitored.


Its not as complicated to do as it seems from the above directions. ;)
 

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FWIW, here is the temperature compensation chart for filling tires. The colder it is the less pressure you supply (moles of air support the car, not psi). This chart also helps detail air loss with colder temperatures which can occur at higher elevations. Of course, there is natural leakage through the tire itself, so monthly checks are a good thing. I use the TPMS on my 997 more to check for potential nails than anything else. I count on my tire gauge fill in the details.

PS: The table image was resized when downloaded. Hopefully it is still readable.
 

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compact spare?

We are considering a GLA250 4matic as my wife's daily driver, downsizing from an ML bluetec. I'm leery of this pocket benz, but she likes it. One concern from a test drive [19" wheels, Conti SSR MOE tires] is the rough ride. If we reach a deal on this certified used [a dealer demo, 5k on the odo], one factor will likely be there willingness to sub back the 18" 5 spoke 7"w wheels and out of the MOE tires into a Michelin LTX or similar regular tire. I don't see a compact spare listed for the GLAs. Anyone have one/know what will fit? I see ML compact spares, an 18x4 wheel on the same 5x112 bolt pattern - anyone know for sure this will work, or what would? Thanks.
 

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Welcome to the GLA forum Riggodeaux. If you search on "GLA spare tire" in the Search box on the upper right, you'll find a lot of discussion on how to do a spare. Good luck in your decision.
 
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