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2015 Toyota Tundra DC SR5 5.7L
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Harbor Freight has a 5 gallon portable air tank with a 3ft hose, air chuck attachment, and a 125psi max pressure on sale for $20.

Does anyone know how many LT265/75R16 tires this tank can fill, or how I can calculate this approximate pressure/volume requirement?

Thanks! :D

 

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I have one in aluminum (very light) and its plenty for one tire.
 

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2015 Toyota Tundra DC SR5 5.7L
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Discussion Starter · #3 ·
I have one in aluminum (very light) and its plenty for one tire.
Thanks Gene. I saw that HF has a 7gal one with a 160psi max rating that looks like it would be better. It's $80, but probably worth it.

I just don't want to get into the project of installing a compressor, but I need the ability to air up/down during my Death Valley trips. A tank like this would probably do the job.
 

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Gas Equation: PV = nRT
where,
P = pressure,
V = volume,
n = moles of gas,
T = temperature,
R = 8.314 J K-1 mol-1, ideal gas constant.

See this and this for more info.


To do the calculation you will need to find out the volume of the tires. This can be estimated by taking the volume of a cylinder the size of the whole wheel-tire assembly and subtracting the volume of a cylinder the size of just the wheel. This will be a bit small because of the dropped center in the wheel (which makes it possible to mount the tire) but it will get you in the ballpark. Here is an example:

205/60/15

area of a circle= pi(r)^2

volume= pi(r)^2(width)

Diameter of tire= (section width)(aspect ratio times 2) + wheel diameter= 62.7 cm

15x7 wheel volume= 20269 cubic cm

volume of tire/wheel assembly= pi(31.35)^2(20.5)= 63296 cubic cm - 20269= 43027 cubic cm

205/60/15 on 15x7 wheel holds 43027 cubic cm



Here is another calculator that will allow you to use whatever units you want. You need to figure how many moles of gas the tank holds at max pressure and then how many moles of gas the tires hold at the pressure you run them. If the tank holds 6 times as many moles than the tires it should be enough to fill them. (I say six because there are 4 tires plus the tank-it won't go down to zero but will only drop till the pressure is equal to the tire-and then another tires worth in the capacity of the 4 rims, margin for error, etc.)

Remember when you are figuring the moles of gas in the tire you only need the moles required to go from your aired down pressure to your street pressure. When all is said and done you will have calculated the moles of 3 things: what the tank holds, what the tire holds at street pressure and what the tire holds at off road pressure. If the difference of the last 2 times 6 is less than the moles in the tank you should be good to go.

Also (more an issue on the higher pressure tank) make sure you have something that can fill the tank to that pressure.

All told it would probably be almost as cheap (if you build the system yourself) and a much better/longer lasting system to just get a CO2 setup. This will air your tires up many many times before refilling.
 
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