Tire Pressure on 19" Wheels

[BerTX]

I have been running at 50 PSI for a few days on my 19" Primacy tires. No TPMS warnings and no noticeable difference in ride quality on air suspension. I know of a Honda Insight owner who runs his tires at 90 PSI and has done so for years without any harm. He does this for range, but I'd hate to think what it does for his ride quality and comfort.

I run my Insight at 60 psi. That's one of the nice things about a mature platform -- lots of testing has been done by the Insight user community and shared via their forum. The the testing has shown exactly which tire gives the best fuel economy for that car. Given that everybody who concentrates on fuel economy runs that particular tire, testing was done at various tire pressures to determine the optimum pressure (60 psi). Sure, there are minimal gains from running the pressure higher, but most consider it insignificant relative to the ride and handling trade-offs.

Hopefully soon we can get away from the growing pains of Tesla and its products on this forum and start learning and sharing more of the fun stuff.

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Unless you really really really overinflate, the dangers aren't really around tire failure but reduced traction (smaller contact patch) and uneven treadwear which can shorten the life of your tires.

I think this is somewhat mythical, held over from the days of bias-ply tires. Probably the wider the tire, the more likely there is still some distortion from moderate overinflation, but not much.

 

[jerry33]

Unless you really really really overinflate, the dangers aren't really around tire failure but reduced traction (smaller contact patch) and uneven treadwear which can shorten the life of your tires.

Actually, you will have better traction in the wet and on packed snow and ice. There can be slightly less traction on dry pavement, but that's generally where you need it the least.

When we used bias-ply tires forty years ago, higher pressure lead to uneven wear. Today most, not all, steel belted radial tires have an almost flat tread area (aka large crown radius), which isn't affected by higher pressures.

 

[gtimbers]

My Model S85 was run at 45 for 20000 miles until i sold it for an 85D. I would be afraid of accelerated center tread wear at 50 psi. Since I got nearly perfectly even wear (except for inside rears due to camber) I would be afraid of the extra 5 psi. I am surprised at Tesla "approving" both ratings. Only one of them will be optimum for wear. I certainly would expect better range with 50, but the ride will most certainly be a bit harsher. 2 or 3 psi is pretty easy to feel.

 

[jerry33]

My Model S85 was run at 45 for 20000 miles until i sold it for an 85D. I would be afraid of accelerated center tread wear at 50 psi. Since I got nearly perfectly even wear (except for inside rears due to camber) I would be afraid of the extra 5 psi. I am surprised at Tesla "approving" both ratings. Only one of them will be optimum for wear. I certainly would expect better range with 50, but the ride will most certainly be a bit harsher. 2 or 3 psi is pretty easy to feel.

If Tesla fitted bias-ply tires, you'd be right. Unfortunately, when it comes to tires, many things that were true in bias-ply tire days are not applicable to the majority of radial tires. No centre wear for me in two years at 51 psi. Heat is the enemy of tires, not pressure.

 

[Nevek]

They don't have different stickers for different tire sizes. They print the correct info on the sticker. So unless they managed to get 50 psi in for 19" wheels incorrectly I don't think so.

Here is a picture of the sticker.

View attachment 73915

When setting my 85D tire pressure, I remembered this thread and the 50 psi number. My sticker from an April build has 45 psi. I saw your later note that Tesla says both are OK, which I find a bit confusing. To compound it, the service center had set my tires at 42 psi!

Other cars I've owned gave a range based on load levels. I think I'll go with 50 psi for now and keep tabs on it.

 

[jerry33]

When setting my 85D tire pressure, I remembered this thread and the 50 psi number. My sticker from an April build has 45 psi. I saw your later note that Tesla says both are OK, which I find a bit confusing. To compound it, the service center had set my tires at 42 psi.

Well, the vehicle placard pressure is based on a set of assumptions, so it's only a starting point, but in most situations you never want the tires to be lower than the vehicle placard pressure.

Whenever anyone works on your tires, the first order of business the next morning is to adjust them with your own good pressure gauge. (Using your own good gauge will at least be consistent, won't have been dropped on the shop floor, and will only have a limited number of use cycles).

There are several ways to get the correct pressure for the tire. Load is certainly one of them, static loaded radius is another, but in high performance tires, the load rating stops incrementing before the maximum pressure is reached. The pressure range between the where the load stops increasing and the maximum are to give the tire the speed rating.

Note that whatever pressure works best for you normally, and even if you don't drive particularly fast, it's always a good idea to increase the pressure when going on a trip with a carload of family and luggage.

 

[sorka]

I lowered my P85D's 19" Primacy's from 50 to 45 today. The ride was way to harsh at 50 psi.

 

[SomeJoe7777]

I believe the change in the 85D's placard pressure is the result of the software upgrade that raised the top speed. I believe when the 85D was first announced/released that the spec'd top speed was 130 MPH (I may not be remembering correctly). But one of the software updates increased that to 155 MPH.

Looking at the bottom of this page regarding the Michelin Primacy MXM4 19" tires, an increase in speed for "W" rated tires from 130 to 155 requires an additional inflation pressure of 4.5 psi, and a derating in load to 95% of sidewall max load.

Thus, what happened is that Tesla determined the minimum tire pressure that can adequately handle a fully loaded 85D at 130 MPH because minimum pressure for this load and speed would result in the softest ride. This was determined to be 45 psi and that's what was put on the placard. After cars began shipping with the software update that allowed 155 MPH speeds, the tire manufacturer's recommendation was followed to increase the pressure by 4.5 psi to accommodate the same load at 155 MPH, and thus the placard was changed to 50 psi.

You can run an 85D at 45 psi even if your placard says 50 psi if you want the softer ride, but you cannot safely go faster than 130 MPH if you're fully loaded. If you're lightly loaded you can probably go to 155 MPH with 45 psi, but most people do not accurately track their vehicle + passengers + cargo weight to make that determination.

Note that this analysis is for Michelin Primacy MXM4 245/45R19 W98 tires. A similar analysis would apply for other manufacturers or 21" tires.

 

[thecloud]

This chart, posted in an earlier thread, shows that 50 psi is only recommended for AWD models with 19" wheels. On pre-'D' cars, the chart shows different values for air vs. coil suspension, and Primacy vs. other tires. Depending on your configuration, the recommended inflation pressure might be 50 psi, 45 psi, 42 psi, or even 40 psi!

 

[NorCalSJ]

Slightly off topic, but as I was driving down CA 85 just south of I-280 today I noticed the car didn't seem to want to stay going straight. I think it was following the rain grooves. It's been a long time since I had a car that did that (though it's common on motorcycles). Do you suppose it has anything to do with the 50psi in my tires? Does anyone else here in Silicon Valley feel the same behavior?

I drive that road all the time and the same happens to me every time, that is following the rain grooves

 

[David99]

As "SomeJoe" said, the higher pressure is due to the increased top speed of the new models. It's not a weight change or something similar.

The old way of thinking that tires would bulge when there is "too much pressure" is nonsense. There is no uneven thread wear when the pressure is at maximum. As always, the maximum set my a manufacturer is always very much on the safe side. It is never the breaking point or the true maximum a system can handle.

I have been driving my Model S 19 inch with 50 PSI for a long time and it's fine. There is no uneven thread wear. The higher the pressure, the lower the rolling resistance. You will get better range with higher pressure. Although the effect is diminishing and not linear. Going 5 over the recommended will give you maybe 5%. Going 10 over recommended will give you 6-7%. So there is no point in going way over board.
The main difference you will notice is ride quality and noise from the tires. Higher pressure causes a rougher, harder ride quality and more noise. Lower pressure gives you a smoother and quieter ride. Higher pressure causes less tire wear due to less rolling resistance.
On my 2014 85 I will get a "tire pressure too high" warning here and there when I go with 50 PSI. Maybe with the new firmware they changed it.

In California it is the law that a service center has to inflate the tires to what the sticker says.

 

[breser]

Thus, what happened is that Tesla determined the minimum tire pressure that can adequately handle a fully loaded 85D at 130 MPH because minimum pressure for this load and speed would result in the softest ride. This was determined to be 45 psi and that's what was put on the placard. After cars began shipping with the software update that allowed 155 MPH speeds, the tire manufacturer's recommendation was followed to increase the pressure by 4.5 psi to accommodate the same load at 155 MPH, and thus the placard was changed to 50 psi.

You're probably right about the top speed increase being the reason for the change. However, I don't think you're right about cars shipped after the software update having 50 psi. I had one of the earliest customer 85D built and I was only the second delivery because of a drive unit issue that delayed my delivery by a week (and ended up resulting in my rear drive unit being replaced before delivery). Pretty sure 85D vehicles had 50 psi right from the beginning. Based on Nevak's post on the previous page, it seems that they are now putting 45 psi on the the 85D. So the change appears to be the opposite of what you're suggesting. They seem to have started at 50 psi and reverted to back 45 psi.

 

[SomeJoe7777]

You're probably right about the top speed increase being the reason for the change. However, I don't think you're right about cars shipped after the software update having 50 psi. I had one of the earliest customer 85D built and I was only the second delivery because of a drive unit issue that delayed my delivery by a week (and ended up resulting in my rear drive unit being replaced before delivery). Pretty sure 85D vehicles had 50 psi right from the beginning. Based on Nevak's post on the previous page, it seems that they are now putting 45 psi on the the 85D. So the change appears to be the opposite of what you're suggesting. They seem to have started at 50 psi and reverted to back 45 psi.

Yes, interesting. My 85D was delivered 4/23 and shows 45 psi on the placard. Perhaps they raised the pressure recommendation on early models anticipating the higher speed increase via software, but subsequent load analysis showed that 45 psi was sufficient at 155 MPH even when loaded?

The placard is supposed to recommend what is required for safe operation at the vehicle rated load and speed. If 45 psi for recent 85D's is the norm, then they must have done some additional analysis to lower it from 50 psi.

 

[jerry33]

The placard is supposed to recommend what is required for safe operation at the vehicle rated load and speed.

The vehicle placard pressure is what the engineers estimated the car would need for the typical driving they envision for that car. It's also, unfortunately, modified by the marketing department (they always want a softer ride). If the engineer thought that normally one or two people would be riding in the car, and that the typical speed was 80 mph, that's what they would set it for. (If this wasn't the case, every vehicle placard pressure would be at the tire's maximum because the tire's maximum is what's required for the tire's maximum speed.)

The best approach is to use the vehicle placard pressure as the pressure which you never want to go below unless circumstances dictate otherwise (mostly these are rare exceptions for passenger cars).

Notes:
This applies for passenger cars only, light trucks are another matter where the vehicle placard pressure is based on a full load--even if only one person is riding in the vehicle.

Tesla may do it differently, but until there is an official announcement about what assumptions they make (in addition to the 18C ambient and daily pressure check which is part of every single tire pressure recommendation), it's best to err (and air) on the side of caution and assume the vehicle placard pressure is the lowest you ever want.

 

[shadowinstallz]

S85D March 2015 Build/delivery. My door jam sticker shows 50psi. Also my tires were delivered inflated to 50psi.

 

[Nevek]

I had my 85D in the Bellevue WA service center for some delivery issue fixes this week and asked about the 45 vs 50 psi labeling on the dual motor cars. This was after my car was given back to me and the paperwork said that my tire pressure had been set. I was told that they were awaiting clarification and perhaps some replacement stickers would be issued at some point, although they were uncertain what psi would be on the stickers.

The next day, with cold tires in my garage, I checked the pressure with a very accurate gauge. They were each set to a precise 47.5 psi. (My car is labeled for 45 psi but I had previously set them myself to 50)

Full marks to to the tech for solving the problem as best s/he could!

 

[Andyw2100]

I had my 85D in the Bellevue WA service center for some delivery issue fixes this week and asked about the 45 vs 50 psi labeling on the dual motor cars. This was after my car was given back to me and the paperwork said that my tire pressure had been set. I was told that they were awaiting clarification and perhaps some replacement stickers would be issued at some point, although they were uncertain what psi would be on the stickers.

The next day, with cold tires in my garage, I checked the pressure with a very accurate gauge. They were each set to a precise 47.5 psi. (My car is labeled for 45 psi but I had previously set them myself to 50)

Full marks to to the tech for solving the problem as best s/he could!

That's a creative solution by the tech, but it highlights how ridiculous the situation is: Tesla's own techs don't know what the current correct recommendation for tire pressure is, so the creative ones are taking an average of the two possibilities!

 

[jerry33]

That's a creative solution by the tech, but it highlights how ridiculous the situation is: Tesla's own techs don't know what the current correct recommendation for tire pressure is, so the creative ones are taking an average of the two possibilities!

The vehicle placard pressure is just a starting point. It needs to change (or not) based on operating conditions.

 

[Nevek]

The vehicle placard pressure is just a starting point. It needs to change (or not) based on operating conditions.

I think most of us understand that there isn't a single "right" answer other than a minimum pressure. I find it amusing that it causes angst, but it would be good to eventually get the ranges and criteria from the mothership.

Most of us are data hounds, and we must be fed!

 

[Andyw2100]

That's a creative solution by the tech, but it highlights how ridiculous the situation is: Tesla's own techs don't know what the current correct recommendation for tire pressure is, so the creative ones are taking an average of the two possibilities!

The vehicle placard pressure is just a starting point. It needs to change (or not) based on operating conditions.

My point was that if the Tesla service techs don't know what Tesla is saying the starting point should be, how are we supposed to?

I realize there isn't a heck of a lot of difference between 45, 47.5, and 50. The problem is that Tesla can't get their ducks in a row on something this basic. To me that's indicative of a much bigger problem.