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what tire pressure should i have for max fuel economy? i have all of mine at around 30 psi. should it be higher?
 

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For better economy increase it to 34 to 36 psi and check it regularly. Tire pressure will change about 1 psi for every 10 degrees F change. I find that the ride is not too harsh at those pressures.
 

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kenblasko said:
For better economy increase it to 34 to 36 psi and check it regularly. Tire pressure will change about 1 psi for every 10 degrees F change. I find that the ride is not too harsh at those pressures.
I don't want to start a war, but I don't understand why people always say this. Maybe I'm wrong, but applying the ideal gas law, it seems like it should take about 18 degrees F (or 10 degrees C) to change the pressure by 1 psi. Here's my reasoning.

Say we start at 50 degrees F and a tire pressure of 30 psi. 50 degrees F is 10 degrees C, or 283.15 degrees Kelvin. Applying the noble gas law (Pv = nRT), and pretending the total volume of air inside the tire doesn't change with a small change in temperature, we can simplify the formula to P = T * some constant. At our selected temp and pressure, this constant is 30/283.15. Now reduce the temperature by 10 (which is 18 degrees Fahrenheit). This gives P = 273.15 * 30 / 283.15, which is approximately 28.9, or very close to a one degree drop in pressure.

So as a rule of thumb, it seems to me it's much more accurate to say that pressure change by 1 psi for every 10 degrees C change. If this analysis is wrong, please point out where I messed up. I've been using this rule myself for a while...
 

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I don't want to dig out my thermodynamics book (I think I threw it out many years ago anyway), but it looks okay. I've always thought it was Fahrenheit. I found this with a google search (see link below):
A good rule of thumb is that for every 10 degrees Fahrenheit temperature change, tire pressure changes about 1 psi—higher as temperatures rise, lower as they fall.
http://www.gmgoodwrench.com/TipsAdvice/TirePressure.html
 

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shadow_dad said:
I don't want to start a war, but I don't understand why people always say this. Maybe I'm wrong, but applying the ideal gas law, it seems like it should take about 18 degrees F (or 10 degrees C) to change the pressure by 1 psi. Here's my reasoning.

Say we start at 50 degrees F and a tire pressure of 30 psi. 50 degrees F is 10 degrees C, or 283.15 degrees Kelvin. Applying the noble gas law (Pv = nRT), and pretending the total volume of air inside the tire doesn't change with a small change in temperature, we can simplify the formula to P = T * some constant. At our selected temp and pressure, this constant is 30/283.15. Now reduce the temperature by 10 (which is 18 degrees Fahrenheit). This gives P = 273.15 * 30 / 283.15, which is approximately 28.9, or very close to a one degree drop in pressure.

So as a rule of thumb, it seems to me it's much more accurate to say that pressure change by 1 psi for every 10 degrees C change. If this analysis is wrong, please point out where I messed up. I've been using this rule myself for a while...
You have to base this on absolute pressure (gauge pressure + atmospheric, which is around 14.7 psi). So the pressure change per degree is larger than you calculated, and the 1-psi per degree F may be more accurate.
 

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if you look at the gas cap door sticker ,it will tell you the PSI range you should be running in regards to loads/speeds/ if your not running the correct factory settings to begin with , cold temps will only increase the problems associated with under inflated /incorrect air pressure settings to begin with
 

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shadow_dad said:
I don't want to start a war, but I don't understand why people always say this. Maybe I'm wrong, but applying the ideal gas law, it seems like it should take about 18 degrees F (or 10 degrees C) to change the pressure by 1 psi. Here's my reasoning.

Say we start at 50 degrees F and a tire pressure of 30 psi. 50 degrees F is 10 degrees C, or 283.15 degrees Kelvin. Applying the noble gas law (Pv = nRT), and pretending the total volume of air inside the tire doesn't change with a small change in temperature, we can simplify the formula to P = T * some constant. At our selected temp and pressure, this constant is 30/283.15. Now reduce the temperature by 10 (which is 18 degrees Fahrenheit). This gives P = 273.15 * 30 / 283.15, which is approximately 28.9, or very close to a one degree drop in pressure.

So as a rule of thumb, it seems to me it's much more accurate to say that pressure change by 1 psi for every 10 degrees C change. If this analysis is wrong, please point out where I messed up. I've been using this rule myself for a while...
Yeah, it's the same as with the Arrhenius equation.

k=A*exp(-Ea/R*T)

Stability time is doubled for every 10*C change in temperature.
 

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DaveW said:
You have to base this on absolute pressure (gauge pressure + atmospheric, which is around 14.7 psi). So the pressure change per degree is larger than you calculated, and the 1-psi per degree F may be more accurate.
Oh wow; I didn't know that (obviously)! It looks like you're right; if I start at 34 psi at 32 degrees F and drop 10 degrees F, I get almost exactly a 1 psi drop. Thanks for clearing that up! :bowdown:
 
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