Tire pressure: Max - 10% = better mileage?

I found the below article on tire pressure, etc. on this forum but wanted to see if any of you can confirm or deny the below excerpt. http://www.carbibles.com/tyre_bible.html

 

Tyre pressure and gas-mileage.

 

For the first two years of our new life in America, I'd take our Subaru for its service, and it would come back with the tyres pumped up to 40psi. Each time, I'd check the door pillar sticker which informed me that they should be 32psi front and 28psi rear, and let the air out to get to those values. Eventually, seeing odd tyre wear and getting fed up of doing this, I asked one of the mechanics "why do you always over-inflate the tyres?" I got a very long and technical response which basically indicated that Subaru are one of the manufacturers who've never really adjusted their recommended tyre pressures in line with new technology. It seems that the numbers they put in their manuals and door stickers are a little out of date. I'm a bit of a skeptic so I researched this on the Internet in some of the Impreza forums and chat rooms and it turns out to be true. So I pumped up the tyres to 40psi front and rear, as the garage had been doing, and as my research indicated. The result, of course, is a much stiffer ride. But the odd tyre wear has gone, and my gas-mileage has changed from a meagre 15.7mpg (U.S) to a slightly more respectable 20.32 mpg (U.S). That's with mostly stop-start in-town driving. Compare that to the official quoted Subaru figures of 21mpg (city) and 27mpg (freeway) and you'll see that by changing the tyre pressures to not match the manual and door sticker, I've basically achieved their quoted figures.

 

So what does this prove? Well for one it proves that tyre pressure is absolutely linked to your car's economy. I can get an extra 50 miles between fill-ups now. It also proves that it's worth researching things if you think something is a little odd. It does also add weight to the above motto about not trusting forecourt pressure gauges. Imagine if you're underfilling your tyres because of a dodgy pressure gauge - not only is it dangerous, but it's costing you at the pump too.

What's the "correct" tyre pressure?

 

How long is a piece of string?

Seriously though, you'll be more likely to get a sensible answer to the length of a piece of string than you will to the question of tyres pressures. Lets just say a good starting point is the pressure indicated in the owner's manual, or the sticker inside the driver's side door pillar.I say 'starting point' because on every car I've owned, I've ended up deviating from those figures for one reason or another. On my Subaru Impreza, as outlined above, I got much better gas mileage and no difference in tyre wear by increasing my pressures to 40psi. On my Honda Element, I cured the vague handling and outer-tyre-edge wear by increasing the pressures from the manufacturer-recommended 32/34psi front and rear respectively, to 37psi all round. On my Audi Coupe I cured some squirrelly braking problems by increasing the pressure at the front from 32psi to 36psi. On my really old VW Golf, I cured bad fuel economy and vague steering by increasing the pressures all-round to 33psi.

So what can you, dear reader, learn from my anecdotes? Not much really. It's pub-science. Ask ten Subaru Impreza owners what they run their tyres at and you'll get ten different answers. It depends on how they drive, what size wheels they have, what type of tyres they have, the required comfort vs. handling levels and so on and so forth. That's why I said the sticker in the door pillar is a good starting point. It's really up to you to search the internet and ask around for information specific to your car.

The Max. Pressure -10% theory.

 

Every tyre has a maximum inflation pressure stamped on the side somewhere. This is the maximum pressure the tyre can safely achieve under load. It is not the pressure you should inflate them to.

Having said this, I've given up using the door pillar sticker as my starting point and instead use the max.pressure-10% theory. According to the wags on many internet forums you can get the best performance by inflating them to 10% less than their recommended maximum pressure (the tyres, not the wags - they already haves inflated egos). It's a vague rule of thumb, and given that every car is different in weight and handling, it's a bit of a sledgehammer approach. But from my experience it does seem to provide a better starting point for adjusting tyre pressures. So to go back to my Subaru Impreza example, the maximum pressure on my Yokohama tyres is 44psi. 10% of that is 4.4, so 44-4.4=39.6psi which is about where I ended up. On my Element, the maximum pressure is 40psi so the 10% rule started me out at 36psi. I added one more to see what happened and it got better. Going up to 38psi and it definitely went off the boil, so for my vehicle and my driving style, 37psi on the Element was the sweet spot.

Higher tire pressures decrease rolling resistance. Decrease rolling resistance increases mileage. Decrease rolling resistance also results in decrease grip. It comes down to whether you want better gas mileage or more grip. You can't have your cake and eat it too.

there's also the other factor of premature wear on the tires as well..

there's also the other factor of premature wear on the tires as well..

 

The article said the odd tire wear was gone. I can see that handling would be decreased but then again, maybe not. If the sidewalls are stiffer because there is more air pressure pushing against them...

it's just when you push them too far in pressures, you'll end up wearing the center part of the tread, which effective braking could be lessen..

Wow 28psi seems dangerously low.

^^^ You're probably used to high performance tires.

 

The P205/70R15 tires on my '98 Outback were max load rated at 35 psi. The SOA recommended tire pressure was 29/28. I would run 32/30 on them. Living on the edge!

That would suggest running my RE960's (stock size) at 45 PSI! That seems like a pretty harsh ride. It would remove the tire from any shock absorption for the car. I'm already running them at 37F/35R (except when they come back from teh dealer that deflates them to 32PSI).

 

Cheers,

Mike

^^^ You're probably used to high performance tires.

 

The P205/70R15 tires on my '98 Outback were max load rated at 35 psi. The SOA recommended tire pressure was 29/28. I would run 32/30 on them. Living on the edge!

 

 

even so with high performance tires, when I get them hot on track or autocrossing, I dont normally go past the point where the entire's tread heat is even across the tread (used a pyrometer to figure that out).

 

But because of daily driving of not being as aggressive on the shoulders of the tires, I run the tire pressures higher to prevent cupping of the center tread.. which actually holds true when you start carrying about 400 to 600 lbs of gear in the the car to a track event at long distances.. you want the tire to always be operating with even heat across the entire tread to have even tread wear AND even grip performance (as you dont want to over heat any part of the tire more so than the other).

So if I normally run 32/30 and I want best possible gas mileage (taking a road trip on Sunday) what should I inflate to? 36 psi ya think?

I do 35 front 33 rear...isn't that factory spec? Isn't that what the door pillar says?

Nah, I got the 16 inchers so 32 front, 30 rear.

Keefe, how much air do you put in when u autocross?

Yesterday, 10:21 AM psucaptainkickass

I do 35 front 33 rear...isn't that factory spec? Isn't that what the door pillar says?

 

 

Remember those amounts are based on ambient temps of 70deg. Below that like 30 to 40deg they should go up to 38 front and 36 rear psi....

Just my .02

 

My tires read a 51psi max. My tires are set to 47psi front 45psi rear.

 

From TireRack

 

"The test tires were inflated to 15 psi, 20 psi, 25 psi and 30 psi. Running them all under the same load, the air pressure in all of the tires went up about 1 psi during every 5 minutes of use for the first 20 minutes of operation. Then the air pressures stabilized, typically gaining no more than 1 psi of additional pressure during the next 20 minutes.

 

For me it's performance over ride comfort. Less rolling resistance, stiffest sidewall, and better response. I have never had any adverse tire wear (always nice and even).

 

Drew

^ Holy crap...too risky (and too many potholes here) for me

What's a pot hole?... lol

I wish I could find a pot hole, I'm sure it's filled with some pleasant flowers... do you have to answer a troll's three questions before you are allowed to enter?

Wow, resurrection of a 3 year old thread.

Back to the topic... I've got the vanilla 2.5i with 16 inch tires and the dealer always inflates to 35psi front and back although the door sticker reads 32 front 30 back. I have never understood why the dealer never followed the door sticker recommended inflation pressures.

Wow, resurrection of a 3 year old thread.

Back to the topic... I've got the vanilla 2.5i with 16 inch tires and the dealer always inflates to 35psi front and back although the door sticker reads 32 front 30 back. I have never understood why the dealer never followed the door sticker recommended inflation pressures.

 

Because 35 all around is considered a safe bet for shops too lazy to read the damn info off the door (at least that's what i've found)

I do 35 all around, which is a hair below max -10%. It rides a little rough, but I can use all the help I can get with mileage.

Your cold max tire pressure is 40 psi?!?! What kind of tires are you using?

Your cold max tire pressure is 40 psi?!?! What kind of tires are you using?

 

Continental ProContact SSR

 

I bought the car with them on there, and they've got a ton of tread so I'm not trashing them till next year. They're a runflat, yet I have no TPMS

 

It says 40PSI max cold inflation on the tire, but TR lists it as 51PSI?

51 sounds a lot more accurate, I'd go with TR's specs.

51 sounds a lot more accurate, I'd go with TR's specs.

 

 

So the max pressure molded into the tire has a safety margin already built in? It states "max inflation pressure 40PSI" or something like that.

 

I'll try adjusting to the door jamb tire pressures and see how the ride does.