Higher Tire Pressures for Fuel Economy?

[hateshino]

i was told running 40 was WAY too high by the tire guys when they did my pressures. they told me most cars run 32-34psi.. someone educate me? lol

[Loren]

i was told running 40 was WAY too high by the tire guys when they did my pressures. they told me most cars run 32-34psi.. someone educate me? lol

32-34 is pretty typical, and even that is higher than most car makers specify. Tire guys are "sheep". They do what they're told. They're not keen on giving good performance alignments with lots of negative camber, either. "You'll wear your tires out, son!"

This deserves it's own topic. (maybe I'll get motivated and split it out when I'm done)

Edumacation:
Car makers specify tire pressures based on the specific tire size that they put on the car and the weight and balance of the car. For almost all cars, this pressure is not set for best performance or for best economy, but for best COMFORT. They're doing all they can to fight NVH (noise, vibration and harshness), and tires are the first line of defense against NVH from the road. So, they set pressures usually in the 26-32 range.

Tire makers specify tire pressures based on what their tires can safely run at for extended periods of time. For most modern car tires, this will be either 44-45 or something like 52 for some tires. The tire maker is saying you can run this pressure, and they wouldn't tell you that if there was ANY danger of them getting sued as a result. You can bet that there's a margin of safety built into that number.

So, what you'll find is that a lot of people will run 32 psi because "that's what they were always told to do". It's been the sage "car guy" advice for years, and generally it gets the job done for most people.

Car makers want you to run what they want you to run, and it might be 32, it might be less... sometimes even more. They've not done any testing outside of that range, so they won't recommend anything else.

Tire makers will usually suggest running what the car maker said, but you "can" run up to their max pressure.

Now, people in the know who are willing to sacrifice some ride quality and NVH for a couple MPG will often run max sidewall pressure. It's deemed by most people to be safe, as the tire maker has specified it. It works.

The adventurous hardcore hypermilers have experimented and learned that you can usually get even more of an MPG increase by going even higher than the sidewall rating. Some will run a tire rated at 44 up to 55-60. That's a 30% increase... not sure I'd be comfortable with it, but I'm pretty confident that tires are designed with at least 20% "fudge factor" in the specs.

Understand, too, that hypermilers aren't stressing their tires by running them at high speeds, either. So, a tire that's designed to handle the stress of running at 110 mph at 44 psi for hours probably won't be stressed any MORE than that by running it at a somewhat higher pressure at half the speed.

I wouldn't try super-high pressures with OLD tires. If you've got dryrot in the sidewalls, you could be asking for a blowout. But, if you've got tires in good condition, do some experimenting. You've tried 32 psi, next, try 40. Then 45. Then 50. And if you like the results, maybe even try 55. There will be a point of diminishing returns, that's what you want to find. Where do the gains from increasing tire pressure start decreasing? A little below that is where you want to be... for a good balance between FE, comfort and safety. For ultimate FE, just keep raising the pressures until it doesn't make any difference any more!

[Empire]

I scanned through ^ so forgive me if I repeat any

but in my experience, fighting sidewall flex and all of that jazz, I've found you can run a lot more pressure than what is printed on the side wall. It all depends on the tire itself. Lots of trial and error. I ran a pair of Dayton's at 52 psi and they were rated at 36 I believe. They had even wear, good traction, little sidewall flex. Made for a solid tire. Not bad for junkyard used tires.
As far as new tires go, my general rule of thumb is start at 5 psi over and just play with it. You don't want to over inflate and cause the tread to bulge out and blow out the belt.
Another little tid bit to keep in mind is that the tire pressure on the sidewall is for when the tire is on the rack, not under a ton of car.

[Loren]

Another little tid bit to keep in mind is that the tire pressure on the sidewall is for when the tire is on the rack, not under a ton of car.

Not sure I would put it quite that way, but max sidewall pressure is "cold pressure", which will actually be a higher pressure when the tire is up to operating temp.

That max pressure is at the max rated weight for that tire. Why would they bother specifying a maximum pressure for the tire NOT installed on a vehicle?

Max rated weight is defined by the load index of the tire.
Tire Tech Information - How to Read Speed Rating, Load Index & Service Descriptions

[hateshino]

im running 40/40 right now and it feels pretty good on the streets. traction is still good and my ride feels more "firm". i might try to up it a lil more and see how that goes..

[Loren]

As you go higher, you might want to start setting the rear a little lower than the front. It will reduce the likelihood of oversteer in an emergency maneuver. Go to like 45/40, and then 50/45, etc.

But, did you check your MPG yet? Any improvement?

It's REALLY hard to quantify because it's hard to get consistent mpg unless you do a lot of highway driving.

[hateshino]

is this only beneficial for highway driving? ive been doing more city driving as of late..

[Loren]

Should help all around. There are just so many more variables in city driving that your MPG could fluctuate from one tank to the next by as much or more than the difference this change makes.

To really know that it worked for you, you'd need to do some "scientific" testing. A lengthy highway trip (at least half a tank) with each setting, at the same speeds, ideally on the same roads in the same conditions.

[Wicked1]

Setting the air pressure higher in the front tires than in the back is good advice. There is more weight over the front tires that will keep the tread pressed down more uniformly to the road. The rear of the car doesn't press down so much so a highly inflated tire will tend to "bulge" and you will ride just on the center of the tire's tread, causing early wear.

We have all seen that diagram in a tire shop of the wear caused by overinflation, underinflation, and camber and toe misalignments.

Basically, I run 40 psi front (cold) amd 38 psi rear (cold) in my tires. My TPM says that my tires gain about 5 psi driving down the highway for an hour, with no additional wear. However, If I inflate them to 32 psi in all 4 (cold), they increase by 8 psi and I notice a reduction in fuel economy.

[fuelie327]

I love when people run too much air pressure in there tires. Mainly because when they ask me to warranty their 60,000 mile tires that only got 30 k , I can explain to them that the warranty doesnt cover a tire with the center tread worn out before the edges. When they ask why that happened I let them look at the air gauge , it is typicaly 10 to 15 pounds or more overinflated. " Tire guys" tell you to inflate your tires to what the vehicle manufacture specifies because that is the right way to do it. As far as " tire guys " not improving performance by doing alignments with more negative camber is a pretty ignorant statement. The whole point of the alignment is to improve tire life. Handling is secondary only because if all of your wearing angles are correct , the handling will be fine. Go ahead and run more air in your tires , the cost of new tires will ofset your fuel mileage. Feel free to stop in to Regal Tire when the time comes

[got_bst?]

it goes both way there has to be a good balance. I run 8-12 pounds over and have no issue with my tires lasting what they are meant to last

[sxysweed]

It's REALLY hard to quantify because it's hard to get consistent mpg unless you do a lot of highway driving.

And even if you do alot of highway driving (I sometimes do 2000 highway miles a month), running a higher tire pressure won't make as a big of a differance as slowing down a bit (doing 70 vs 80), or accelerating slowly, or doing whatever form of hypermilling. As long as your tires aren't under inflated, you'll hardly notice a differance.

Random fyi, various cities have laws that state that gas stations must give you free air.