Tire pressure - MPG vs Safety
 

I see that many of you run your tires at pretty high PSI to get better mileage. I would like to try this (first 350 miles on my used HCH/CVT averaging 40.1). However, I live in a hot climate and I wonder if I can safely get away with as high tire pressures as you all use outside the New Mexico furnace I live in.

Any other desert drivers do this? What PSI do you use or suggest for here (100 deg and higer.)

Thanks.

Re: Tire pressure - MPG vs Safety
 

I think I would drive the car around in the hot part of the day for awhile, then stop and set the tire pressure to where you want to have it. Drive it some more and check to see that it is still close to what you set it. Use a reliable gauge, not the ones on the air pumps at a service station. No telling how far off they are.

Re: Tire pressure - MPG vs Safety
 

Ok. I am driving to santa fe today and I've pumped them up to 45. I'll check them hot somwhere and see how they're doing. It's a really hilly drive though so I don't know if I'll get a reliable comparison to my previous mpg.

Re: Tire pressure - MPG vs Safety
 

The pressure listed on the sidewall of the tire is a cold pressure and for this reason I think it would be OK to put there even in a hot climate. I would be very interested to see what folks who live in hotter climes say though.

Me personally I have my tires around 50 and I live in Chattanooga, TN. Not the hottest part of the US, but lately it has been staying in the mid 90's.

Billy

Re: Tire pressure - MPG vs Safety
 

I suspect you'll see very little pressure increase. I run the tires on my Civic at 40-44psi depending on my mood, and I've noticed the higher the starting tire pressure, the less of a rise I see from the cold temp to the driven temp. This is of course in Arizona, where the average high temperature was over 110 degrees for a good 3 weeks or so.

I have an infared pyrometer on order right now and intend to take some temperature readings once I receive it because I'm really curious just exactly how much of a temp rise you'll see in typical driving conditions. If the air temp is 105 degrees and the pavement is 140 degrees, I'd have to imagine the tires are 130-160 degrees. I'll know soon enough once it shows up and I can do some experimentation.

Re: Tire pressure - MPG vs Safety
 

Re: Tire pressure - MPG vs Safety
 

Hmmm...interesting question about the warranty concerning suspension damage. I suppose if I ever really knew I just took my HCH out of alignment, and there was even the slightest chance it was due to my driving habits, the first thing I'd do before taking it into the dealer would be to lower my PSI back down from 45 to 30 just to be safe. I doubt there is a way for them to tell what # tires had previously been at.

I also take care in driving - my local streets and most of the greater Jacksonville area have very well-paved roads and of course it's flat as my back, but I work downtown and I have a 5-minute stretch of drive each day passing through the innercity with VERY narrow four lane traffic and LOTS of potholes. The speed limit is 30 MPH and I consistently stay under it when the going gets rough. I also keep a keen eye on my mirrors when driving there to see if it's possible for me to swerve halfway out of my tiny lane in order to avoid the sore spots. Caution caution caution is all I can stress, and I haven't had a problem yet.

Re: Tire pressure - MPG vs Safety
 

Suspension damage as a result of bad roads is not covered by warranty in the first place, so in some sense, you have nothing to worry about.

Re: Tire pressure - MPG vs Safety
 

About running high pressure and hot tempertures, cold too:

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.

Re: Tire pressure - MPG vs Safety
 

PV=nRT

Where P is Pressure and T is Temperature. The other numbers do not really matter or do not change so in this case basically Pressure is directly proportional to Temperature when T is measured in absolute terms ( which Fahrenheit is not). The Rankine scale is basically the same as the Fahrenheit scale except it starts at ablsoute zero. If you add 460 to Fahrenheit you end up with Rankine units. 80 Farhenheit is about 540 Rankine. A 10 degree temp jump at 80 degrees causes a 10/540 * 100 percent (1.8%) in pressure. 1.8% of 32 psi is .57 psi.

In summery at 80 deg a 10 deg rise causes a .57 psi jump. Rich's rule of thumb is pretty close but I like to be a little more precise.