I’ll be frank, I’m mathematically challenged. Don’t get me wrong, I can read a formula and plunk numbers into a calculator I just don’t “think” mathematically. So my question is how many cubic feet of air does one of my 235/85R16 tires hold when empty and when filled to 70psi?I’ve googled the question already and found numerous responses that eventually turned to gibberish in my non-mathematical brain, plus posters would argue with one another about their calculations. It didn’t give me a lot of confidence.FWIW I solve most physics type problems by 3D visualization. I mentally construct a working 3D model of the problem. I’m nearly always correct in my assumptions; I just fail when it comes to supplying the math needed to prove it.Steve

Don't take this the wrong way...but how come do you want to know?Just interested and wondering if this is something I should know also about my tires...

Wow, that's probably a tough one. But when it comes to dealing with pie, I'd rather just eat it than mathematically try to figure it out! :B

The cubic feet of air remains the same with the tire, inflated or not, only the pressure changes. Now, if you want to know how many cubic feet of air must run through an air compressor to inflate the tire to 70 psi, that is an entirely different question.To add 70 psi is equal to 4.76 atmospheres so what ever the actual cubic feet of your empty tire is, (Guessing it to be somewhere around 2.2 cu. ft.) than it would take 2.2 x 4.76 or about 10.5 cu. ft of air compressed in to your tire to equal 70 psi.

Geocritter wrote:I’ll be frank, I’m mathematically challenged. Don’t get me wrong, I can read a formula and plunk numbers into a calculator I just don’t “think” mathematically. So my question is how many cubic feet of air does one of my 235/85R16 tires hold when empty and when filled to 70psi?I’ve googled the question already and found numerous responses that eventually turned to gibberish in my non-mathematical brain, plus posters would argue with one another about their calculations. It didn’t give me a lot of confidence.FWIW I solve most physics type problems by 3D visualization. I mentally construct a working 3D model of the problem. I’m nearly always correct in my assumptions; I just fail when it comes to supplying the math needed to prove it.SteveThe size(cubic feet) would be the same empty and under pressure. It does not change, just becomes pressurized. A cubic foot is a cubic foot at 10 PSI and also a cubic foot at 100 PSI.The math to determine how MANY cubic feet you have, I can't help you help there, I join you in the non-math part.

rgatijnet1 wrote:The cubic feet of air remains the same with the tire, inflated or not, only the pressure changes. Now, if you want to know how many cubic feet of air must run through an air compressor to inflate the tire to 70 psi, that is an entirely different question.Excellent point and better expressed than the way I did. I'll restate.What is the 235/85R16 tires cubic foot volume at 14psi (atmospheric pressure at sea level)and next how many cubic feet of air would be required to fill one of the tires to 70psi?Steve

How many cubic feet of air does my 16" tire hold?

40 Replies

Geocritter
Explorer
Apr 23, 2014
I have a very nice fairly large air compressor sitting in storage in Austin Texas. I'm currently in Huachuca City AZ headed for Fresno and Port Angeles next. The problem is I've been spoiled, I tried using a small RV size air compressor and hated it almost immediately. That's when my thoughts drifted to compressed nitrogen. Anyhow I purchased a cylinder of nitrogen this morning, I purchased a two stage regulator a while back, and I'll use the hose off my air compressor (which I've taken with me on my journey). Time will tell if I made a good decision or not.

Steve
BrianinMichigan
Explorer
Apr 23, 2014
Sorry,I was just thinking of my own preferences. A long time ago when I went out west my front air bags developed a slow leak causing an alignment issue. After I changed the tire I discovered the low air bags. I had to fill them every day to keep the front tires from wearing.
Geocritter
Explorer
Apr 23, 2014
Brian in Michigan wrote:
If a tree falls in the woods and there is nobody around does it make a sound? :BI am a math dufus. But what would happen to me is that the tank would run out and I would need the air to keep the tire topped off till I could get it fixed. I vote for a compressor.

Or you could mount the spare...BTW I didn't know the matter was up for vote, I thought it was simply a topic involving a personal preference.
BrianinMichigan
Explorer
Apr 23, 2014
If a tree falls in the woods and there is nobody around does it make a sound? :BI am a math dufus. But what would happen to me is that the tank would run out and I would need the air to keep the tire topped off till I could get it fixed. I vote for a compressor.
Geocritter
Explorer
Apr 23, 2014
Francesca Knowles wrote:
Since the question is "how much air does the tire hold", I think something's being overlooked.

How does one correct for the fact that our atmosphere is actually 78% nitrogen already?

Seems to me that means that only 22% of the volume one puts into a tire when "airing up" is actually air.

:h

Francesca, "air" is generally a mixture of 78% Nitrogen 21% oxygen and 1% dis&dat. When you fill up a tire with compressed air it is that mixture of gases you're filling it up with that constitutes "air". If you wanted to fill it up with pure oxygen (which you wouldn't want to do for many safety reasons)you'd need to get a tank of oxygen, same thing goes for nitrogen as well as the various components of dis&dat such as argon, CO2 etc.

Steve
oilslick
Explorer
Apr 23, 2014
I think the most important thing to come out of this thread is use a pressure regulator on the cylinder.
Francesca_Knowl
Explorer
Apr 23, 2014
Since the question is "how much air does the tire hold", I think something's being overlooked.

How does one correct for the fact that our atmosphere is actually 78% nitrogen already?

Seems to me that means that only 22% of the volume one puts into a tire when "airing up" is actually air.

:h
Geocritter
Explorer
Apr 23, 2014
Kaz wrote:
Geocritter wrote:
I think I’m slowly getting there. If atmospheric pressure at sea level is 14.7psi (empty tire) and the tire holds 2.37cu ft empty, then solving for X where X equals the volume of gas in the tire at 70psi using this formula:

2.37/14.7 = X/70

resulting in X = 11.3cu ft of gas (nitrogen) in the tire to achieve a pressure of 70psi.

I absolutely don’t trust my math, can someone verify?

Steve

Not exactly, but close. You're confusing "absolute" pressure (psia) with gauge pressure (psig). If you put a tire gauge on an empty tire (only atmospheric pressure), it will read 0 psi, not 14.7 psi. So, 70 psig is actually 84.7 psia. Otherwise, you are correct. By the ideal gas law, pressure is proportional to the amount of gas in the container.

Good point I hadn't thought of that. However, the only concern I have with pressure is how it affects volume. Now I know that with the smaller 50cu ft cylinder, if I needed to I could fill at least 4 deflated tires (not that I ever expect to). It appears that the small tank will easily meet my probable compressed gas needs for years. I also learned that I can exchange the cylinders at Napa auto parts stores which makes it very convenient too.

Once again, I want to thank everyone for their input.

Steve
Kaz
Explorer
Apr 23, 2014
Geocritter wrote:
I think I’m slowly getting there. If atmospheric pressure at sea level is 14.7psi (empty tire) and the tire holds 2.37cu ft empty, then solving for X where X equals the volume of gas in the tire at 70psi using this formula:

2.37/14.7 = X/70

resulting in X = 11.3cu ft of gas (nitrogen) in the tire to achieve a pressure of 70psi.

I absolutely don’t trust my math, can someone verify?

Steve

Not exactly, but close. You're confusing "absolute" pressure (psia) with gauge pressure (psig). If you put a tire gauge on an empty tire (only atmospheric pressure), it will read 0 psi, not 14.7 psi. So, 70 psig is actually 84.7 psia. Otherwise, you are correct. By the ideal gas law, pressure is proportional to the amount of gas in the container.
Tinstar
Explorer
Apr 23, 2014
You're making it too complicataed. Here you go: