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Discussion Starter #1
When I owned a Prius and was on the Prius forum there were quite a few folks that were hypermiling and running tire pressures of 50 psi or higher. The results were usually a 2-4 mpg increase. My personal experience was the same.

Many of the LRR tires have maximum pressure of 50 psi.

Is anyone doing this and what, if any, benefit are you seeing.

These are US measurements.

Thanks.
 

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Not me. I stay with the manufacturer's recommended tire pressure listed on the door jamb label...36 lbs, no more, no less. Inflating tires to their max listed on the sidewall means they will be running a few pounds higher when hot, which personally, I wouldn't feel safe doing....not to mention premature center tread wear, rougher ride, safety issues in the form of higher susceptibility to blowout, and possible compromised handling. Others can hypermile all they want, but for me it's not all about MPG, and a possible slight increase of a couple mpg isn't worth compromising a balance of economy, comfort, handling, tire longevity, and safety. But, that's just me.
 

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When I owned a Prius and was on the Prius forum there were quite a few folks that were hypermiling and running tire pressures of 50 psi or higher. The results were usually a 2-4 mpg increase. My personal experience was the same.

Many of the LRR tires have maximum pressure of 50 psi.

Is anyone doing this and what, if any, benefit are you seeing.

These are US measurements.

Thanks.
NEVER run anything other than the recommended pressures UNLESS you are running at extremes of temperature (<-30°c, >50°c) or abnormal load.
Handling, braking and comfort are affected.
 

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why compromise comfort handling and safety ?
Handling is a lot better with the tires a bit more inflated, for the same reason it spends less energy, tyres deform less... You do loose a bit of traction because the area of contact with the floor is decreased. You also loose traction if you underinflate the tyres.

Unless you drive very fast or over sand, you usually have a lot of spare traction.

NEVER run anything other than the recommended pressures
100% agree, but learn to read the recommended pressure. In Europe, according to the authority which establishes the rules, ETRTO, the number on the car door and manual, is to be interpreted as a minimum. The pressure should never be lower in any moment. Plus, the pressure should be increased if you are going to drive in harsh conditions (high speed, heavy charge, towing, long trip time).

So, if you adhere 100% to the recommendations in Europe (using the values for the EV):
  • Keep tyre pressure above 2.5 bar (36 psi).
  • Increase pressure by 0.2-0.5 bar (3 to 7 psi) if driving at sustained high speed, heavier charge... resulting in 2.5 to 3.0 bar (36 to 43 psi).
  • Respect tyre cold pressure maximum inscribed on the tyre, 3.5 bar (50 psi, pressure reading when cold).
This is the document the European Tyre and Rim Technical Organisation publishes for consumers for free, you want to read page 22. This is a new edition, September/2017, may have some differences from what I said.

Now, the risks: over-inflation increases the risk of impact damage, particularly puncture (on the other hand, it protects the rims better). Under-inflation is dangerous for driving, and increases the risk of tyre break-up (tyre deforms more).

I drive a lot on motorway, so I have my tyres usually inflated 0.3 bar over the number on the door (2.8 bar = 40 psi). I think this is in perfect accordance with ETRTO recommendations. I thank the extra range. I have no problems with that driving in Lisbon on sett paved roads and tram lines.

PS: About heat: don't worry. When you see a letter V tyre, in Europe, it automatically means that it is rated for a maximum cold pressure of 3.5 bar (50 psi). A V tyre is rated for driving at 240km/h. Testing is performed at that speed and at its maximum pressure. You're not heating it anywhere near that.
 

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For starters, 2 - 4 mpg with higher pressures seems unlikely to me. Lots of smart people on the Prius forum but without coast down tests to confirm a decrease in rolling resistance, those anecdotal reports are highly suspect. It would take thousands of miles with the same driver in the same season with say two different pressures to be reasonably sure of results.

That small gain, perhaps 3% for hypermilers, is vastly outweighed by the cost of the tire life, both tread wear and increased likelihood of damage. Not to mention the other comments here about noise, comfort, handling, and braking. Better all round (although also with tradeoffs) is to fit a narrower tire that is made for efficiency.

Just to point out the issues with verifying efficiency gains, I've made two minor aero mods on my car that should increase efficiency on the order of 5%. No way can I confirm such a number, but the mods are such that efficiency should be increased by some number (just like increasing tire pressure to reduce the contact patch on smooth roads). I'm currently about 20% over the EPA mpg ranking but of course that is not due only to my mods. Driving style, speed, and especially weather conditions (I routinely check wind speed and direction on trips) far exceed my small aero gains. I will say that I made similar modifications on my last car and also exceeded by a good margin most owners on Fuelly over 200,000 lifetime miles with no attempts to drive especially efficiently. But that is not the same as testing, just an anecdote that is highly influenced by driving style.
 

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Discussion Starter #9
As a former Prius driver with more than 150K miles driven I can tell you that 2-4 mpg difference with the tire pressure at 50psi vs 36psi is achievable. The ride is a little rougher.


Recommended tire pressure by the manufacturer is set for the best combination of comfort and handling. However many LRR tires are rated to 50 psi or higher. I found my tires to last the appropriate amount of time as long as they were rotated properly. Often I got an additional 10-20K miles for the proper care.

I applaud your efficiency. 20% over the EPA estimate is to be lauded.
 

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I played with higher pressure in my Prius and did see a little gain, but also felt I was wearing tires faster. The cost of that increased tire wear offset the gas savings so I stopped the practice. Maybe in Europe where gas prices are much higher the tradeoff comes out ahead, but in the USA it did not seem to. Nevermind all the points in the rest of this discussion about safety, handling, etc..

That raises an follow-on question. The pressures on the door label certainly apply to the factory tires. However once the tires have been replaced, the new tires may have different characteristics from the factory original tires. That tire manufacturer may recommend running at different pressures. Which numbers do folks follow? The one in the door and the ones on the tires?
 

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Also isn't there an increased risk of an over inflated tyre suffering of a blow-out when tyre is stressed.
An over inflated tyre is not a tyre simply inflated above the pressure indicated on the door. As per ETRTO recommendations, 3 to 7 psi above that is still proper tyre inflation. Anything below that is under inflation.

Over inflated tyres do not have increased risk of blow out. Under inflated tyres do. Blow out happens on the lateral part of the tyre, when tyre is deformed more than it can take, even may touch the rim. Inflating the tyre more reduces deformation and reduces the risk of blow out.

Over inflated tyres are more subject to puncture.
 

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I played with higher pressure in my Prius and did see a little gain, but also felt I was wearing tires faster.
No, properly inflated tyres (equal or above the indication on the door) last longer. For the same reason they save fuel: they deform less and therefore heat less. Because they heat less, the wear is reduced and is more uniform.

Look, I'm not an expert in tyres or even cars. Besides the small document from the ETRTO, I hope I can find again another one from the NHTSA (a big one, with many chapters on specific issues with tyres), and then I'll post here.

However once the tires have been replaced, the new tires may have different characteristics from the factory original tires. That tire manufacturer may recommend running at different pressures.
No, the tire manufacture does not recommend pressures, because that depends solely on the car, its weight and weight distribution mostly.

If you don't change measure, the pressure is the same. In Europe we are not allowed to change measure, you probably are. A thinner tyre requires higher pressure to withstand the same weight (pressure = force/area). Temporary tyres are usually filled to 4.2 bar and above (60 psi). If the measure does not change, pressure doesn't too.

Disclaimer: I too am a Prius driver (Prius+), so you may find me biased towards maximum energy efficiency.
 

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People, to make it clear, the following are quotations from the ETRTO, the European authority on tyres and rims. This tells us how to interpret recommended pressures:
  • "Over-inflation causes the tyre to be more susceptible to impact damage. Under-inflation causes over-heating and can greatly shorten the life of a tyre. It reduces road holding and can cause irregular wear, internal damage and, ultimately, even tyre break-up." So, over-inflation: impact damage (also punctures), but while it may damage the tyre, it may save the rim. Under-inflation: all other risks, particular tyre break-up = blow up (caused by extreme deformation) and shortened life.
  • "The pressures (cold) recommended by tyre manufacturers in their technical documents should be regarded as minima."
  • "When the car is subjected to hard driving conditions [...] it is recommended that cold inflation pressure be increased by between 20 and 50kPa while respecting the maximum inflation pressure of the tyre ([...] 350kPa for sizes having a Speed Symbol H, V, W or Y, [...])"
The only downside to more inflated tyres is feeling more the roughness of the road (handling is definitely better), and, on dirty roads, decreased traction. Out of the tarmac or on a very, very rough road, always decrease the tyre pressure.
 

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Most Niros other than the base model come with roof rails, including mine. I removed them and the passenger side outside mirror. Each is worth 2.5% efficiency gain. The first one comes from the difference in EPA rating between the base model and mine, the second one is commonly quoted online including ecomodder.com (you'll find other ones there). The mirror mod is very cheap. I find passenger side mirrors almost useless, but I do use a larger inside first surface mirror that is slightly curved and gives me a back and full passenger side view.

I'd love to remove the rear door handles, and even modify the front ones, but the cost will never be recovered by the minor aero improvements. Seems crazy to me that manufacturers go to great lengths in all cars to get the windows flush but do nothing on the door handles. So simple, and would cost nothing to do OEM over current part.

As a final note, I also removed a net 40 pounds from the weight of my car. I can now carry my bicycle inside upright and have enough room to sleep beside it. Can't do that in the Ioniq unfortunately. Otherwise I would have much preferred it for the mpg. Like the way it looks too - couldn't even consider the current Prius. I've only seen two on the road, one a couple days ago parked in Montreal (not sure I looked at the license plate). Black and beautiful.

My mpg is completely anecdotal. However, I'm expecting average annual to match or exceed EPA ratings (on the base model) in four season driving without any extreme mods or driving.
 

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Under-inflation: all other risks, particular tyre break-up = blow up (caused by extreme deformation) and shortened life.
Of course a deformed tire is subject to "blow up", and blow off as well. However the tire manufacturer specified maximum pressure is based on what the carcass can handle, either belt destruction or blow off pressure or both. Most specifications like this are conservative, usually with a 10% or even more margin of safety. So from simply a safety standpoint, you might be able to run 10% over maximum pressure. Besides engineering estimates, there are also liability concerns, especially within the United States legal system.

BTW, your English is great. However in your prior post, you probably meant weight instead of measure. Measure is a general word that includes all sorts of stuff like weight and dimensions.
 

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Inside my door it says 36 fronts, 34 rears I do 36 rears and 38 fronts and find I have a smoother ride, get a touch more range and it doesn't hurt the handling. But that's just me. Tire pressure is taken early in the morning before the car is used and before the sun hits the rubbers.
 

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Inside my door it says 36 fronts, 34 rears I do 36 rears and 38 fronts and find I have a smoother ride
That goes against pretty much all conventional wisdom, although +2 psi is probably within the error of the measuring device, so may not be statistically significant.
 

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Are the recommended psi numbers different for the different tire sizes, depending on which model? My SEL...and I would imagine the Blue with 15" wheels...would most likely share the same suggested 36psi. But what about 16" and 17" tires on the PHEV, EV, and the HEV Limited? ....and is the suggested psi different for different countries?
 

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Inside my door it says 36 fronts, 34 rears I do 36 rears and 38 fronts and find I have a smoother ride
That goes against pretty much all conventional wisdom, although +2 psi is probably within the error of the measuring device, so may not be statistically significant.
To clarify a smoother ride I find the recommended psi in the door makes the car feel mushy, it feels slightly bouncy and not as rigid on turns. High winds also seem to push the car. It sounds crazy but my wife actually feels it too.
 
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