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My tires keep going down

Feb 15, 2019
380
127
arizona
I get them blown up to 42 at Discount Tire and a week or so later they are down to 35 - 36 and I have low tire warnings.

Thing is they don't go any lower than that. If I don't get them inflated they stay right there.

Which makes for a better ride but I'm not sure about the safety of that. My other cars mostly required 35 except for my truck.

I wonder why they call for 42.
 

camalaio

Active Member
May 28, 2019
1,483
2,181
Vernon, BC, Canada
So you drive somewhere to get your tires filled, and they fill them soon after arrival?

Your tires are being filled while warm due to driving. The stated inflation pressure (42 PSI) is for cold pressure. If they're inflating to 42 while warm, then it will drop some amount when cold.

That said, if it's actually a slow leak on all four wheels (doubtful if it stops at 35, and why my above statement is more likely) then that would be quite the luck streak.
 
Feb 15, 2019
380
127
arizona
So you drive somewhere to get your tires filled, and they fill them soon after arrival?

Your tires are being filled while warm due to driving. The stated inflation pressure (42 PSI) is for cold pressure. If they're inflating to 42 while warm, then it will drop some amount when cold.

That said, if it's actually a slow leak on all four wheels (doubtful if it stops at 35, and why my above statement is more likely) then that would be quite the luck streak.
 
Feb 15, 2019
380
127
arizona
Well it's only about 2 miles to the Discount Tire. I understand driving raises the pressure.

But the pressure stays up the first day even though it's parked. Then they go down to 35 - 36 and STAY there.

They have stayed at 35 for over a week before I took it back for a refill. I wanted to see how low they would go.

I have a compressor and could fill them myself if they got too low. But I'm getting to old to do that if I don't have to :)

With a slow leak they should continue losing air until they are flat, usually.
 

Timbo2

Member
Jun 8, 2019
215
170
USA
I think camalio may be on the right track about cold versus warm tire pressures.

On the 18 inch tires I believe the maximum pressure is 50 PSI. Check the sidewall - inflate at Discount Tire to the maximum and drive home. Check the pressure the next day and see what the cold tire pressure is. it will likely be too high so drop it down to 42 PSI and see how that goes for you.

I'd just use the 12V compressor at home and do it properly while cold, however.
 

coleAK

Member
Oct 23, 2018
895
659
Alaska
First off for streetcars you only ever measure tire pressure cold. Cold is whatever temperature it is where you live that time of year when your car has been stationary for 4 to 6 hours. So yes cold temperature varies throughout the time of the year.

Recommended Tire pressure is based on load tables. Load tables take the vehicle weight and tire load rating to calculate pressure. So You should use the psi cold listed on your door sticker. Running a lower than that psi you will prematurely wear out tires and put you at a much higher risk of blowout. Remember the Ford/firestone legal fiasco from deaths due to a top low PSI recommendation on explorers?
 

camalaio

Active Member
May 28, 2019
1,483
2,181
Vernon, BC, Canada
Well it's only about 2 miles to the Discount Tire. I understand driving raises the pressure.

But the pressure stays up the first day even though it's parked. Then they go down to 35 - 36 and STAY there.

They have stayed at 35 for over a week before I took it back for a refill. I wanted to see how low they would go.

I have a compressor and could fill them myself if they got too low. But I'm getting to old to do that if I don't have to :)

With a slow leak they should continue losing air until they are flat, usually.

The two mile drive can definitely be enough to increase temperatures, especially if the tires are too low since the additional flexing will cause them to warm up faster.

I'm gonna lay out a theory. It might be complete BS, but I'm out of ideas.
  1. Tires are at 35psi. You drive to fill them, which warms them to 40psi.
  2. A mere 2psi is added to give 42psi. You drive away. (effectively now 37 psi cold)
  3. Temperature has dropped over the course of a week, reducing pressure.
  4. Tires are at 35psi. You drive to fill them, which warms them to 40psi.
  5. Another measly 2psi is added to give 42psi. You drive away. (effectively now 37 psi cold)
  6. Temperature has dropped EVEN MORE over the course of a week, reducing pressure.
  7. ... you get the idea.
So basically, I'm suggesting this might be normal seasonal pressure differences and the average temperature fluctuations are lining up in such a way to play games with you, especially since only rather small amounts of air may be added at your fill-ups.

Regarding 42 vs. 45 PSI: I understand this was a change at some point. All used to be 45, now all are 42 (to my understanding). I also stumbled across the fact that XL rated tires (the tires used for the Model 3) are rated to handle their load at a maximum of 42 psi. This may be why Tesla changed it, especially as they moved into more countries with the Model 3. A higher load rating seems to require moving to light truck (LT) tires. This would be problematic for a few reasons.
 
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Reactions: coleAK

coleAK

Member
Oct 23, 2018
895
659
Alaska
The two mile drive can definitely be enough to increase temperatures, especially if the tires are too low since the additional flexing will cause them to warm up faster.

I'm gonna lay out a theory. It might be complete BS, but I'm out of ideas.
  1. Tires are at 35psi. You drive to fill them, which warms them to 40psi.
  2. A mere 2psi is added to give 42psi. You drive away. (effectively now 37 psi cold)
  3. Temperature has dropped over the course of a week, reducing pressure.
  4. Tires are at 35psi. You drive to fill them, which warms them to 40psi.
  5. Another measly 2psi is added to give 42psi. You drive away. (effectively now 37 psi cold)
  6. Temperature has dropped EVEN MORE over the course of a week, reducing pressure.
  7. ... you get the idea.
So basically, I'm suggesting this might be normal seasonal pressure differences and the average temperature fluctuations are lining up in such a way to play games with you, especially since only rather small amounts of air may be added at your fill-ups.

Regarding 42 vs. 45 PSI: I understand this was a change at some point. All used to be 45, now all are 42 (to my understanding). I also stumbled across the fact that XL rated tires (the tires used for the Model 3) are rated to handle their load at a maximum of 42 psi. This may be why Tesla changed it, especially as they moved into more countries with the Model 3. A higher load rating seems to require moving to light truck (LT) tires. This would be problematic for a few reasons.
Good catch on the PSI. Yes you are correct the p metric load tables max out at 42 psi. Above that and yes you need an LT tire.
 

rad1027

Member
Apr 18, 2019
81
54
Fort Worth
You can always use an accurate (i.e. 41.5 psi) displayed tire pressure gauge. I have one and I use it at home every time I add air to my tires. As others have mentioned, I would suggest adding air to the tires when they’ve been sitting for several hours versus driving on them and then filling them up with air. I’m very OCD with this and perhaps having the tire pressure checked regularly is one of the factors that has contributed to my tires lasting longer on al my vehicles.

Where I live it can be anywhere from 110 F to 18 F (different time of the year). So At least twice a year I have to drastically add or remove tire pressure air.

A good estimate is for every 10° fluctuation in air temperature, vehicle tire pressure will adjust by about 1 psi. So if outside air temperature decreases 30° from your last tire pressure adjustment, expect tire pressure to drop about 3 psi.
 

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