MPG and Tire Pressure

Lesson learned on recent road trip: Tire Pressure Matters.

Duh. Many of you know this, and I should have known better, but here's my story anyway.

I took a little drive from my home at about 7500' above sea level to New Orleans, LA, a few feet below sea level. I carefully set my tires at the recommended 30 psi (the dealer had kindly pumped them up to 36 psi as part of my 5000 mile maintenance), and took off.

Twenty-four hours later, I loaded my outbound mileage into Fuelly and noticed my MPG dropping like a rock. At home, I was humming along nicely above 20 MPG, but I was down to 18 MPG when I got to NOLA. In my burned-out braindead state, I attributed this to buying cheap gas, and resolved to upgrade on the way home.

Sure enough, MPG improved on the way back. Unfortunately, I could find little or no correlation with gas quality or average speed or traffic conditions. It looked like basically my mileage got worse driving downhill, and better driving uphill.

Derp. Air pressure. My tires were basically 3.5 pounds flatter by the time I got to NO, and they 'reinflated' as I drove home. (I did not get a TPMS indicator, though.) Here's a handy chart:


So, for those concerned about optimizing MPG and city vs highway and keeping the speed down to conserve gas and never exceeding 2000 rpm: a pound or two of underpressure will hurt you a lot more than a heavy foot on the interstate.

 

On a 'standard day' the atmospheric PSI where you live is 12.2

Down in New Orleans, it's 14.7 PSI on the same type of day. Pretty cool observation of your MPGs on your part.

 

Thanks!

Standard atmospheric pressure is 12.2 psi at 5000'. That's about the elevation of the Rio Grande valley in Albuquerque. My home is at 7490', so my standard pressure is more like 11.1 psi, and that's where I am when I set my tire pressure. My tire pressure fluctuates about a pound while commuting! The crushing atmosphere in NOLA squished my tires 3.6 pounds flat, although the tropical temperature offset that somewhat.

 

OK so now that you got me thinking...WHAT should my tire pressure be here in South Louisiana for my Double cab tacoma?

 

I thought I saw somewhere on another thread that some members suggested one could improve mpg by bumping up the psi to 35-(37?) as long as one remained on paved roads. I currently run at the recommended 30 front / 33 rear, but I'm thinking of trying to bump all mine up to 35 and see if I get an improvement to mpg. Anyone else run at higher than recommended psi and noticing improved mpg without excessive center tire wear?

 

Whatever your door-frame label says. If you travel anywhere, you'll be going uphill, and your tire pressure will increase unless you bleed some air. The point is that you have to remove or add air as you move up and down in altitude, to maintain the recommended pressure. A few hundred feet is no big deal, but a couple thousand makes a noticeable difference.

 

I don't know the answer about trading tire wear for MPG, and I'm curious what others have to say about it.

If you do bump up your pressure, I think you should maintain that 3 psi front-rear difference.

 

Up in the northern parts we have the same thing with the cold. You can lose 1 to 2 psi per ten degrees of temp drop. Checking your tires on a warm day in late fall does not mean you are set for winter. OF course the tire heats back up with highway driving, but most who shop and work close to home will benefit the most. 70f - 0f = 70d drop * 2 per 10 =14PSI underinflated !

Also the wear on the tires is considerably more on the outside edges of the tires on low pressure (especially front tires during cornering and resultant squealing). too much over recommended will "crown" the tire and you get excessive wear I the center of the tread.

I check mine a few times during the year then at any major temp changes. I did have 2003 Olds Ciera the I got 100k Miles out of the factory tires - the only time I did it though (lost if attention and highway miles)

http://www.goodyear.com/cfmx/web/corporate/media/news/story.cfm?a_id=371

 

Your MPGs and elevation had much more to do with your results than the 3ish PSI difference in your tires.

Travelling from your NM home to a sea level elevation would make your tires feel 'softer' that up home.... not the other way around.

You burn 'less' gas at higher elevations that at sea level because the air is thinner as you go higher... so the mixture is adjusted for the lesser air. That's your observation, not so much the tires pressure.

 

No, you have it backwards. As the outside (ambient) pressure increases, the difference in absolute pressure between inside and outside the tire decreases. The tires therefore get softer as you move to lower elevation. If you've ever seen video of launching a weather balloon, you would notice how little helium they put in it and how it grows enormously as it rises. Tires do the same.

I basically drove the entire trip on underinflated tires. The 3.5 psi pressure loss was about 12% low, and it cost me about 13% in fuel economy. (Not to imply that 1% change in tire pressure always creates a 1% change in MPG, it just worked out that way in this example.)

The computer compensates for higher air density to keep the AFR constant. As long as you stay within the envelope where the computer is able to maintain AFR, elevation should have little effect on gas mileage (all other things, like tire pressure, being equal).

 

I disagree. Yes, I understand and have seen weatherballons 'inflate' as they get into higher altitudes, but that is a very flexible membrane, not a hard capsule.

I know a little about air pressure, density and temperature to say to you that I disagree..

http://commons.wikimedia.org/wiki/F..._feet_and_1000_feet,_sealed_at_14000_feet.png

 

Have always carried 2 lbs. over manufacturer recommended. However, I also check for tire wear to make sure wearing evenly. Never had a problem with uneven tire wear yet.
I check my tires in AM before sun hits them or while in garage and before they have moved. Do this even though two vehicles have TPMS indicating TP's of each tire. Use a handpump to add a few lbs. if needed. Not as much effort as it seems.

 

Your link demonstrates my point exactly. If the bottle got 'harder' as it moved down the mountain, it would not have collapsed. The flexible bottle collapsed because it lacked the structural strength to maintain the pressure differential, so it changed shape to equalize pressure inside and out. A tire is strong enough to support the pressure differential, so rather than getting smaller, it gets softer as it travels downward.

Do the same experiment in reverse: seal the bottle at the bottom of the mountain, then drive to the top. Soda bottles don't stretch like balloons, so heading uphill they act more like tires. At the top of the mountain, your soda bottle will look a little swollen and feel very firm. You started with zero 'tire pressure' in the bottle at the bottom, and now have several pounds of pressure in it at the top. When you return to the bottom of the mountain (assuming you don't open the bottle), the bottle will revert to its original squishiness. Bottles, tires, all same same: they firm up going uphill and soften going downhill.

I also know a thing or two about air pressure, density, and temperature, having studied for years and earned a PhD in physics. You are welcome to continue to disagree.

 

You just made me re-read what I wrote... I can't believe it let that by me. I wrote harder instead of softer regarding the tires I'm super embarrassed right now. ... editing it now.

I'm editing that one word in my post. I meant your tires would be 'harder' at home than in NOLA.

You seem like an intelligent guy... I was wondering why you didn't agree with my statement... now I see why.


The tire science aside, I will argue the ECU will adjust to a lesser fuel amount burned in thinner air than at sea level.

 

I agree with what you said except for one thing. If you still drive the vehicle at elevation the same as at lower elevation i.e. 40 mph - your gas pedal is going to need to be pressed further to compensate by increasing the hp output to give the same speed.

 

OK, now I am just going to have to drive to NOLA again. This time I'll keep the tire pressure as constant as possible, and see what happens.

 

I hear ya, but I'll say no to that. The ECU will determine (IMHO) how much fuel will enter the intake to get the correct burn, not so much as to how far you'll depress the skinny pedal.

Pilots that fly normally aspirated engines (not turbojets) will need to lean the mixture back to get the best fuel economy and RPM/Manifold pressure as altitude is gained... thinner air needs lesser fuel for a good, not too rich, not too lean combustion.

Superior performance for the same engine happens at sea level and with cold air, but you'll empty your tank a little faster because of the mixture required for the pilot to set...... and it's automatically done by the ECU in the case of the automobile's driver.

 

Sorry OP, I was drifting. I didn't mean to cloud up your thread.

 

You're missing the point ...

 

A tortoise and a hair started at point A at 12:01 pm
At 4:37 pm the hair was at point B where was the tortoise?