Tire Pressure and Range - One Mans Test Results

Objective:
I wondered just how significant air pressure really is to EV range and decided to test it.


Background:
Car, (MYLR) left a 60f heated garage to 40f ambient and was driven 25 miles to warm up. Air pressures were validated between 2 digital gauges (which match to within ¼ psi, and tire TPMS. 11 mile interstate runs with speed matched, trip odo reset @ identical start and stop points, autopilot and self-driving managing the car, and all climate/HVAC off. Runs are back to back, at night, same direction, with traffic a non-factor. Tire pressures were checked via TPMS before and after each run. The 35 psi run did show a ½ PSI increase from start to, so there was some heat increase.

I used large gaps in speed and pressure so we’d get obvious results.

As the 50mph (lower wind resistance) was repeatable it suggested the process didn’t require two passes @ 35 psi. (I was also low on battery. Got home at 2%.)


Results:

Thoughts:

• The 45 psi runs showed no tire pressure changes. 35 psi run did show a ½ PSI increase from start to finish so there was some heat increase.
• I don’t get these lowish power demand number in real life. Climate control has a big impact!
• I expected tire rolling resistance to play a reduced role at higher speeds, but I wasn’t so sure we’d see it, nor that it would be this significant.
• I’m a little bummed the two 75mph runs don’t match more, but at higher speeds it doesn’t take much for even minor wind changes to affect things.
• Ride difference was significant and immediately noticeable.
• I was surprised at the how significant air pressure changes are at lower speeds
• Although that usually means I’m local and care not at all.
• On a real-life trip where I’m expecting something like 250 miles of range at most? I’m not likely to worry about what is likely to be less than 5 miles of range difference.
• Range/pressure is not linear. Delta drops as pressure increases. Meaning the increase in 35-40 is larger than the increase from 40-45.
• I don’t run 35 PSI either. I normally inflate to 37-38 and add when I hit the 35 PSI warning light.

YMMV.

Note: This will be added to the "Air Pressure" document I've shared/linked to elsewhere.

 

[fiehlsport]

I got battery powered inflator and so far it’s rocking. Gauge is accurate and the battery can add 4psi to all four tires for 35 psi and still have 3/4 charge
Fanttik X8 APEX Tire Inflator Portable Air Compressor, 2X Faster Inflation, 150PSI Cordless Tire Inflator with LCD Dual Screen, Suitable for Cars, E-Bikes, Motorcycles, For Up to 26.3 Inch Tires https://a.co/d/2vyH7Gw

+1 For fantiik. I use mine all the time and it rocks. The mode for filling up sports balls makes it go really slow so you don't pop them, too. Easy to pop in the trunk for road trips, and charges via USB-C which the Tesla has natively.

It uses 18650 cells, fun fact.

Also seems to set pressure 1:1 with the TPMS sensors, seems very accurate.

 

[andaconda]

I bet I could use the same gauge and measure 3 times I would not not get same results

Sounds like my Blood Pressure!

 

[CMPT PAD]

Sounds like my Blood Pressure!

my blood pressure gauge behave the same way. I eliminate the first 2 readouts and take the average of the the next 3

 

[dafish]

We've drifted a bit, but to bring us back - I've done some more testing. Yep, air pressure, once we're past a reasonable level, lets say 35 PSI, is negligible in range affect. Let's not overlook handling and bent rim risks, but range is not a factor.

For your consideration:

 

[jcanoe]

Increasing cold tire pressure above the value indicated on the manufacturer's label on the vehicle's B pillar or in the owner's manual will improve efficiency, range by approximately 1% for every 2 to 3 PSI. Running at 44 or 45 PSI instead of 42 PSI is an easy way to achieve 1% lower consumption at highway speeds. I.e., 300 Wh/mile would drop to ~290 Wh/mile.

 

[RichAZ/CapeCod]

View attachment 949430

If I'm reading the above chart correctly it shows:

At 78 mph, with 37 PSI range was 313 and 341 miles

At 73 mph, with 37 PSI, range was 286 and 309 miles

OK, what am I missing here??

Rich

 

[CyberGus]

If I'm reading the above chart correctly it shows:

At 78 mph, with 37 PSI range was 313 and 341 miles

At 73 mph, with 37 PSI, range was 286 and 309 miles

OK, what am I missing here??

Rich

Those values are not "miles", they are "watts per mile".

 

[dafish]

Those values are not "miles", they are "watts per mile".

Yep!

I suppose I expected it to be inferred, but there was mild tailwind on leg A, while leg B a reversed interstate route, had a comparable headwind.

 

[dafish]

Increasing cold tire pressure above the value indicated on the manufacturer's label on the vehicle's B pillar or in the owner's manual will improve efficiency, range by approximately 1% for every 2 to 3 PSI. Running at 44 or 45 PSI instead of 42 PSI is an easy way to achieve 1% lower consumption at highway speeds. I.e., 300 Wh/mile would drop to ~290 Wh/mile.

No sir, with sincere respect due you, that's not correct. It grant you it's an oft quoted myth. The data you're using is only true at substantially lower air pressures. I've tested twice now, and have seen test results on this same thing. All clearly show air pressure has a significantly diminished role in rolling resistance as air pressure increases. Let me share this table I saved from, as I recall, an SAE publication addressing same.

As you can see, 25 to 35 PSI has a significant impact. 35 to 45 PSI is negligible. Also note that while RR increases as speed does (I'd not expected that personally), the variation at 35 vs 45psi as speed goes up is quite linear.

Finally, you can test this yourself. My MYLR generated a warning on low air pressure when it dropped to 35psi. That Tesla factory warning stated a .2% range loss incurrred as a result. My tests show a bit more than that, still below 1% for 5 PSI, but at some point we're within a margin of error/ acceptable deviation.

My respects to you,

-d

 

[CMPT PAD]

No sir, with sincere respect due you, that's not correct. It grant you it's an oft quoted myth. The data you're using is only true at substantially lower air pressures. I've tested twice now, and have seen test results on this same thing. All clearly show air pressure has a significantly diminished role in rolling resistance as air pressure increases. Let me share this table I saved from, as I recall, an SAE publication addressing same.

View attachment 950020


As you can see, 25 to 35 PSI has a significant impact. 35 to 45 PSI is negligible. Also note that while RR increases as speed does (I'd not expected that personally), the variation at 35 vs 45psi as speed goes up is quite linear.

Finally, you can test this yourself. My MYLR generated a warning on low air pressure when it dropped to 35psi. That Tesla factory warning stated a .2% range loss incurrred as a result. My tests show a bit more than that, still below 1% for 5 PSI, but at some point we're within a margin of error/ acceptable deviation.

My respects to you,

-d

I love the science and empirical data

 

[jcanoe]

No sir, with sincere respect due you, that's not correct. It grant you it's an oft quoted myth. The data you're using is only true at substantially lower air pressures. I've tested twice now, and have seen test results on this same thing. All clearly show air pressure has a significantly diminished role in rolling resistance as air pressure increases. Let me share this table I saved from, as I recall, an SAE publication addressing same.

View attachment 950020


As you can see, 25 to 35 PSI has a significant impact. 35 to 45 PSI is negligible. Also note that while RR increases as speed does (I'd not expected that personally), the variation at 35 vs 45psi as speed goes up is quite linear.

Finally, you can test this yourself. My MYLR generated a warning on low air pressure when it dropped to 35psi. That Tesla factory warning stated a .2% range loss incurrred as a result. My tests show a bit more than that, still below 1% for 5 PSI, but at some point we're within a margin of error/ acceptable deviation.

My respects to you,

-d

It would be interesting to test the Tesla Model Y with the factory 19" wheels, tires at 70 MPH to compare the Wh/mi consumption at the Tesla recommended tire pressure of 42 PSI and also at 45 PSI.

 

[kelpcar]

Increasing cold tire pressure above the value indicated on the manufacturer's label on the vehicle's B pillar or in the owner's manual will improve efficiency, range by approximately 1% for every 2 to 3 PSI. Running at 44 or 45 PSI instead of 42 PSI is an easy way to achieve 1% lower consumption at highway speeds. I.e., 300 Wh/mile would drop to ~290 Wh/mile.

1% savings from 300 would be 297, not 290.

 

[Goobers]

While I will date myself and take us even further off topic, I was in IT when this first made the rounds.

http://wise-obs.tau.ac.il/~friedel/computer_fun/jokes/help-desk.html

A bit late to this... but I did work as tech support at a community college a while back... one teacher had me come to see why the computer wasn't working and they couldn't see anything on the monitor.

Solution... press power button on monitor.

 

[jadatis]

This shows that tirepressure has minimal effect on energy use.

At lower speed lesser windforce, so energyloss by tires is relatively more then at higher speed.

At 75mph the tire produces a little less heat ( so energyloss) per rotation. per 100miles/kilometers then a little lower energyloss, then at 50 mph, because warmer tire, so lesser deflection, so lesser heatproduction per rotation.
Windforce thoug is quadratical with speed, but also more miles /h, so 75/50= 1.5 times as much distance, and 1.5^2= 2.25 x as much energy needed per hour, but 1.5 times as much distance, so 1.5 times as much energy needed for 100miles/km
That gives the lower percentage energyloss by higher speed, energyloss of rolling resistance stays about the same, but energyloss by windforce gets 1.5 times as much

That in your tabel energy use is not 1.5 times as much, is because at 50mph the energyloss of rolling resistence is about 20% of total so 80% for windforce ( if I use the right word)

That 80% becomes about 120% at 75mph, but the 20% even a bit lower, so estimated 18%. So total 138%.

This will count for about the 35 psi cold filled.
I will do some calculations with your data, to see if theory is in line with your findings.

But no or 0.5 psi pressure rising seems low to me, I expected about 2 to 4 psi.
Mayby tmps calculates back the pressure given by sensors to anindex temperature, like so motor tmps do, and also found circumstantial evidence that Porche tmps also do that.
Then 0.5 psi is yust because of roundings.

And you went from 60 degrF in the garage to 40 degr outside, also influences that.

 

[SalisburySam]

Service set mine at 44 psi. I believe years ago the standard pressure for model 3 was 44. I was told Tesla reduced it.

My 2018 Model 3 shows 45psi on the B pillar sticker, later changed (2019?) to 42psi. I know of one owner of a 2018 who had the lower pressure sticker placed over his original sticker at a service center while in for the HW3 upgrade. Mine remains original at 45. I have mobile service replace my cabin air filters annually (they get really nasty in a year) and they always check and set tire pressure and tread depth for possible rotation. They always set to 45psi.

 

[nimbuzz]

The rule of thumb is whatever works for you. The vehicle manufacturer is required to provide a sticker, located on the B pillar by the driver's seat, that provides the maximum vehicle load and the recommended tire pressure. The Tesla Model Y has this sticker; 42 PSI for front and rear tires. The fact that Tesla Service set 45 PSI may mean they assume the vehicle owner won't regularly check the tire pressure, a few extra pounds of air to account for slow loss of air from the tire.

You can vary from the recommended tire pressure by 2 to 4 PSI with no issue. Setting the tire pressure too low can be unsafe as the tire will not be able to carry its rated load and may heat up while driving leading to tire blowouts (this is why every new vehicle is required to have a TPMS.) Too low tire pressure will cause cupping where the outer edges of the tire will wear faster with the center part of the tread.

Setting the tire pressure too high may cause the ride may become too harsh. Too much pressure will limit the tread area contact patch with the road and reduce grip. The center part of the tread will wear faster than the outer tread.

The maximum tire pressure for tire inflation is molded into the side of the tire. Note: All tire pressures are when measured cold before being driven more than 1 mile.

After trying different tire pressure setting I have set the OE Continental ProContact RX 19" tires to 44 PSI.

Can I assume your “44 PSI” is cold?

 

[Jim R]

While I will date myself and take us even further off topic, I was in IT when this first made the rounds.

http://wise-obs.tau.ac.il/~friedel/computer_fun/jokes/help-desk.html

A classic. Thanks for the recall.

 

[andaconda]

While I will date myself and take us even further off topic, I was in IT when this first made the rounds.

http://wise-obs.tau.ac.il/~friedel/computer_fun/jokes/help-desk.html

Now, that gave me a good laugh!

 

[jcanoe]

Can I assume your “44 PSI” is cold?

Yes. 44 PSI measured in the A.M. (not in direct sun) before driving. I would add that this winter I set the tire pressure at 43 PSI at an A.M. temperature of 43F (easy to remember). I left the tire pressure this way from December '23 to March '24, only recently lowered the pressure by 1.5 PSI as it was getting warmer (especially at the beginning of March.) Currently I observe 42 PSI in the A.M. as I start out.

 

[dafish]

This shows that tirepressure has minimal effect on energy use.

At lower speed lesser windforce, so energyloss by tires is relatively more then at higher speed.

At 75mph the tire produces a little less heat ( so energyloss) per rotation. per 100miles/kilometers then a little lower energyloss, then at 50 mph, because warmer tire, so lesser deflection, so lesser heatproduction per rotation.
Windforce thoug is quadratical with speed, but also more miles /h, so 75/50= 1.5 times as much distance, and 1.5^2= 2.25 x as much energy needed per hour, but 1.5 times as much distance, so 1.5 times as much energy needed for 100miles/km
That gives the lower percentage energyloss by higher speed, energyloss of rolling resistance stays about the same, but energyloss by windforce gets 1.5 times as much

That in your tabel energy use is not 1.5 times as much, is because at 50mph the energyloss of rolling resistence is about 20% of total so 80% for windforce ( if I use the right word)

That 80% becomes about 120% at 75mph, but the 20% even a bit lower, so estimated 18%. So total 138%.

This will count for about the 35 psi cold filled.
I will do some calculations with your data, to see if theory is in line with your findings.

But no or 0.5 psi pressure rising seems low to me, I expected about 2 to 4 psi.
Mayby tmps calculates back the pressure given by sensors to anindex temperature, like so motor tmps do, and also found circumstantial evidence that Porche tmps also do that.
Then 0.5 psi is yust because of roundings.

And you went from 60 degrF in the garage to 40 degr outside, also influences that.

I encourage everybody to evaluate the validity on their own, so have at it. A few thoughts:

1). While one could, as you did, note drag change as speed changes, the exponential relationship of drag to speed is relatively well known. Few will debate it. The idea of the test is to observe increased power demands at different air pressure at the same speeds. Tests run at two different speeds merely to see if results varied as speed increased. As we see, speed really doesn’t change the nature of the results: Tire pressure changes have insignificant, and decreasing, impact above roughly 35psi.

2). Don’t forget tire pressure was checked before and after each run. Nothing significant was noted.

3). Did you notice the car was driven 25 miles to stabilize tire temp?


That said, that I was able to see data largely consistent with the SAE graph shown was assuring to me.