Weather was decent today so I took the R out. Checked tire pressure with relatively cheap dial guage just after pulling out of garage. Temperature was 43 F. Tires were 37-38 lbs. Drove 3 miles on highway and thought I'd check them using car system. Readings were 42-43 lbs. Did 3 miles run the pressure up or crappy guage on car or hand guage not accurate? I'm looking to buy an accurate guage, preferably USA made. Comments and suggestions on guage welcome.

Also, it was great to run it out fir about an hour. Miss that driving.

 

Once the tires get heated up they gain air pressure. I don't see any reason to think either gauge is wrong.

 

PV=nRT. You can do the math.

 

Damn, you must be an engineer! I did ok in chemistry and biology, but physics and the maths about killed me.

 

 

The trick with PV=nRT:
n and R are constants, and V is approximately a constant, so it's useful to think of it as:
Pressure is proportional to Temperature.
Pressure is absolute but tire pressures are relative. A tire with 30psi actually has an absolute pressure of 45psi. (30 psi above the 15psi atmospheric pressure.)
Temperature is in Kelvin.

That's where the approximation of 1psi per 10 degrees Fahrenheit change comes from.

I know, more than you wanted to know.

 

The answer is 42.

(Of course, that's the answer to life, the universe, and everything)

 

Next time compare the car gauge with the hand gauge simultaneously. This will tell you if they agree but not whether either is accurate. If they disagree then you'll have to wonder which one if off. Or it it both that are wrong?

I believe the answer is 42 as stated above. Great band on all levels. I miss the 80s. I don't remember the 60s though.

 

Yes simultaneous check would be best. I didn't think a 3 mile drive would make a 5 lb difference. I'll check both simultaneously to see what gives, if anything.

 

Just a data point for you. My hand held is a Harbor Freight digital, cheapy. It agrees exactly with the car's indication.

Larry

 

Too young or too high?

 

Yes.

 

Over the weekend at the track, I did both hand checks and TPMS. It appeared that my handheld needle gauge was showing 2lbs less than the computer on all tires. I usually take 2 readings per tire with the hand gauge but even still, there is room for error using the old fashion gauge.

FYI - after several sessions adjusting, I concluded that the best inflation was cold 30 front / 32 rear. This resulted in pressures of 38 all around hot.

I did not receive any low warnings until I dropped below 30 psi. I had read somewhere that there was a street and track setting. I have not found such a setting

 

There is a high speed setting where you are recomended to increase the pressure to (I think) 40 psi (cold tires). If not you might get a low pressure warning. High speed is here defined as faster than 250 km/h over some distance. So this is not ment for track driving, but high speed autobahn driving.

 

And certainly more than I wanted to explain. Thanks for making the effort.

 

Well, it was halftime of the Patriots game. And there is some relevance of the universal gas law...

 

lol!