[JCPrius2008]

Hi All,

Got a questions for you guys out there...

I own a Year08 Prius. I do try to make sure the tires are inflated according to the values recommended by Toyota. This means that I'm going by the values indicated on the sticker found on the door (is it the driver side or front passenger side? I can't remember which).

Having read other posts earlier, it seems to me that many are claiming 42front/40back would boost MPG even more! So questions are:

1) Is it save to inflate higher than the manufacturer's recommended values? I'm also worried the tire would burst on me all of a sudden...
2) Why 42/40? Did somebody out there do some kind of comprehensive tire pressure test and conclude that 42/40 is the sweet spot?

Thanks all for your help! Look forward to hear from you guys!

Joe

 

[PA Prius]

Bursting pressure is said to be four to five times that of the maximum listed on the sidewall. Nevertheless, it is your choice if you go over that number. I am currently running 50/48. I suppose some choose 42/40 because they want to make sure they are not going over the sidewall maximum. I did a 1400 mile trip from PA to IN to PA last week-- 61.0 mpg first tank, 62.0 second tank, and finished with 305 miles on the third tank at 63.7. I chose to drive 60 mph. AC was on. I've found speed and tire pressure to be the two most significant factors in gas mileage.

PA P

 

[GKarschnick]

The higher inflation seems to "cup" the tires resulting in shorter tire wear. It's a trade off, mpg vs tire wear. I do the 42/40 and get about 4 mpg over the manufacturers recommended pressure.

 

[fireboss]

why not the same psi all tires say 42?
what are your thoughts on using nitrogen?

 

[jdenenberg]

1) Is it save to inflate higher than the manufacturer's recommended values? I'm also worried the tire would burst on me all of a sudden...

Your tires have a Maximum Cold Presssure Rating on the sidewall. It is 44 for the original Goodyear Integrity tires. Stay at or below that PSI and your tires won't burst.

2) Why 42/40? Did somebody out there do some kind of comprehensive tire pressure test and conclude that 42/40 is the sweet spot?

The two pound difference is because your Prius is heavier in the front than in the rear. The Toyota recomendation is 35/33 which is their compromise for a softer ride. The OE tires have soft sidewalls and tend to scrub the tread edges on turns at that pressure. They also tend to track unstably at highway speeds (they are not very good tires). The higher pressure of 42/40 reduces both of these problems as well as slightly improving MPG.

JeffD

 

[Ct. Ken V]

Hi, JCPrius2008,

I'm not sure what the max tire pressure (specified on the tire sidewall) is for the different brand of tire that is shipped on the touring edition, but for the Goodyear Integrities the come on the standard version of the Prius, their max pressure is 44 PSI. So many of us use 42 PSI on the front to allow a margin for the expected rise in pressure as the tire heats up after driving a few miles (I'm sure they would be safe to use even if you were to check the pressure after having driven a little & found the pressure to be as much as 46 PSI).

I've used 42 & 40 for 2 years with no problems, but some like to keep it down to 40 & 38. Always maintain the 2 pound differential between the front & rear with the higher value on the front (where the extra weight of the engine is). We do run these higher pressures to get some slight increase in MPG [because harder (higher pressure) tires roll easier], but mostly to try to make our tires last longer. If you stick with the 35 & 33 recommended pressure you find on the door or doorpost, you will probably find your tread start'g to wear off on both the inside & outside edges of all your tires but mostly untouched in the middles. This type of wear is usually from under-inflation if it appears smooth, but if it looks cupped then you've got a suspension part/alignment problem. Toyota allows one free alignment in the 1st 20,000 miles of ownership or within one year from your "in-service date" [not necessarily your delivery date because some dealers may move that date back a few days (if you bought on the 1st or 2nd day of a month) to count in their quota for the previous month if that month's sales were slow)]. You should ask your dealer for a printout of the info for your VIN # that includes your "in-service date" & keep it with your warranty booklet since your warranty is also based on that same date.

Also, to get that free alignment, I think you can't just ask for it. I think you have to complain of a handling (wandering) or pulling to one side problem or show some unusual tire wear pattern.

Edit #1 : While I was cook'g & eat'g my breakfast (in the middle of typing my post) it looks like some others got in her before me. I will offer brief responses to some of them here.

GKarschnick :
Higher pressures do not cause cupping. It simply wears more of the center of the tread off because the cross section of the tire becomes rounder when it's harder & the middle is the only portion of the tire that is in contact with the road surface. For the Prius 42 front & 40 rear seems to be the happy medium that keeps the entire tread cross section of the tire in even contact with the road surface for more even tire wear. If you've got "cupping" then you've got a suspension part/alignment problem.

fireboss :
Because of the added weight of the engine in the front, if you were to run the same pressure there as on the rear, most likely the weight ditribution would be more near the edges of the tires (where the sidewall brings the load weight down towards the road surface) leaving the tread middle bear'g less of the load (& wear'g less as in an under-inflated condition. As far as nitrogen, the air we breathe is almost 80 % nitrogen already. Pure nitrogen is important in racing tires because it less affected by temperature increases as the tires heat up due to the high speeds. If the tire place you use offers free nitrogen for your tires, go for it. I wouldn't pay extra for it though.

If you do opt for the nitrogen, what happens if you find one tire a little soft/low on pressure? If you can't find a gas station offer'g nitrogen (on a trip away from your tire place) you're stuck add'g just AIR. You can still achieve the same pressure as the other 3 when you add the AIR, but in a short time of driving the other 20 % of the AIR that isn't nitrogen will be affected by the tire temperature & probably increase the pressure of just that tire (maybe making the handling of you car a little different). Now, joking : personally I'd go for Heium to make my car lighter to get better MPG.

Edit #2 : P.S. Wow, even Jeff got in here before my post went through & his answers are about the same as mine but briefer. Way to go, Jeff!!

 

[JCPrius2008]

Wow! Thx guys! So many helpful tips! I'll start spending more time here from now on. I'm an obssessed with hyper-mile-ing.

But what about this?
Ok here is how I measured my tire pressure:

The nearest gas station from my house that offers free air to customers is 10 miles away and I usually get there by freeway by driving 60 miles/hour.
The reason for all these trip details is that, I've heard that tires get hot and as a result you can get higher pressure reading some period of driving.

Questions:
1) So when you guys said 42front/40back, do you mean *cold* pressure or *hot* pressure?
2) If 42/40 were the recommended *cold* pressure numbers, maybe I should go above these numbers because the tires are hotter by the time I get to the gas station. If so, how much higher?


By the way, wondering what MPG you guys are getting out there?
I have 2008 Prius:
My last 1000 mile roadtrip in northern california near Lassen Volcanic National Park and Lava Beds, I averaged 60mpg but I drove slow around 40~55mpg.
Usual freeway driving: 62miles/hour I get 52mpg
City driving: short trips ~ 25mpg, longer trips 45 mpg

 

[JCPrius2008]

Hi PA Prius,

So I'm wondering how you determined the pressure of your tires? Do you do what I do, drive to gas station and then fill them up there? Or you do it when tires are cold, when you car's been sitting in your garage for a long time?

50/48 sounds kind of high...would you say your ride is more bumpy then?
Also harder tires tend to lose air more quickly right? Don't you have to keep putting air in them every once in a while?

Just curious...cuz getting 60MPG by driving 60MPH is really quite impressive.

thanks!
JCPrius2008

 

[PA Prius]

Hi PA Prius,

So I'm wondering how you determined the pressure of your tires? Do you do what I do, drive to gas station and then fill them up there? Or you do it when tires are cold, when you car's been sitting in your garage for a long time?

50/48 sounds kind of high...would you say your ride is more bumpy then?
Also harder tires tend to lose air more quickly right? Don't you have to keep putting air in them every once in a while?

Just curious...cuz getting 60MPG by driving 60MPH is really quite impressive.

thanks!
JCPrius2008

I check the tires after the car has been sitting in the garage, so it is a cold-check. I use a bicycle floor pump (10 strokes to increase 1 psi) and a good pressure guage. I'd recommend both.

I suppose the ride is a bit harsher than at a lower pressure, but all four in my family are fine with it. It may depend on the conditions of the roads you often travel. I don't notice any additional air leakage at higher pressure.

This past weekend the four of us went to the shore for the weekend, so we were packed pretty full. Our average was 60.5 mpg for the 270 mile trip. Again we stayed at 60, but also at 55 where that was the speed limit, which was at least half the distance. There was also heavy traffic, so looking far ahead and adjusting speed accordingly is also important.

PA P

 

[hyperion]

Rather than ask this question on a Prius site, I would put the question to Toyota on the reasons they selected the unusually high pressures for this size car? (I'm speaking of the high 35/33 figure.) I'm sure you will find it is because of the design features of the suspension system. Tire pressures are extremely important in this. You will be changing much more than the "possibly" higher milage figure, if you do even that. I did not find it so!
What next? a solid rubber tire?

 

[PA Prius]

Are you seriously questioning whether increasing tire pressure increases mpg?!?

PA P

 

[fireboss]

i have up the tire pressure driving 30 miles 55 60 am getting 38 42 mpg took trip 2 weekends ago mfd stated 60 mpg did the math 33 mpg. am driving light foot looking ahead coasting not breaking millage less than i would have expected

 

[JCPrius2008]

Hi PA Prius,

I really continue to be impressed by your number 60MPG/60Miles/Hour especially that your car seems really loaded (4 people) and still you are getting these impressive numbers.

Questions are:
Now I wonder if there is manufacturing spread, meaning that Toyota will ship the product if the car efficiency is greater than 48MPG. For me, I always get 52 MPG on the freeway which I normally drive for an hour to get to San Francisco. So I had always considered myself "lucky" in a sense that I might have gotten a car at the superior side of the spectrum.

I put in more air this weekend, increaseed the pressure to roughly mid 40's for all 4 tires. I wasn't very precise...so that's why I've said "mid 40's"
And I didnt notice that my MPG went up at all, in fact I actually think it dropped a bit. And those tires are original tires that came when I bought the car brand new. What tires are you using?

Thanks!
JCPrius2008

 

[PA Prius]

Hi PA Prius,

I really continue to be impressed by your number 60MPG/60Miles/Hour especially that your car seems really loaded (4 people) and still you are getting these impressive numbers.

Questions are:
Now I wonder if there is manufacturing spread, meaning that Toyota will ship the product if the car efficiency is greater than 48MPG. For me, I always get 52 MPG on the freeway which I normally drive for an hour to get to San Francisco. So I had always considered myself "lucky" in a sense that I might have gotten a car at the superior side of the spectrum.

I put in more air this weekend, increaseed the pressure to roughly mid 40's for all 4 tires. I wasn't very precise...so that's why I've said "mid 40's"
And I didnt notice that my MPG went up at all, in fact I actually think it dropped a bit. And those tires are original tires that came when I bought the car brand new. What tires are you using?

Thanks!
JCPrius2008

I am using Goodyear Integrities, which I'm sure is the same you have. Mine are about at the end of their life. I'm guessing they'll make it until I put the snow tires on this winter. That to say that my tires are getting pretty smooth, which decreases their rolling resistance. I read on Michelin's website that tires improve their rolling resistance by 25% through the life of the tire.

I see you have a 2008. How many miles are on it? If the car is still young I'm sure your gas mileage will improve as you get more miles on it and as the tires become worn.

One weekend's driving likely won't give you a good picture of whether your mileage changes by increasing tire pressure. There are too many variables-- weather, length and type of trips made, etc. My next door neighbor got an '08 and was on a trip from PA to FL. I e-mailed him mid-trip and told him about my tire pressure. He put his from mid-30's up to 50 and then reported an increase of 5 mpg. That sounds a bit high to me. I haven't checked back with him for how that is looking longer-term.

I do drive very conservativly. I use the cruise control on the highway if the roads are fairly flat and not too much traffic. If not, I think I can do better without. Or sometimes on hills I'll leave the cruise on but tap it down two or three mph as I approach the crest of a hill and then increase it again going down.

PA P

 

[JCPrius2008]

Hi PA Prius,

Oh yeah, it's a new car....I bought it in January 2008 and currently I have 7500 miles on it already, yeah not much I know.
My tires came with the car when I bought it and yes I guess I could label them as still "brand new" since I only covered 7500 miles roughly so far.


What year of Prius do you and how your mileage so far?

Oh yeah, what gas are you using? I recently went from Chevron 87 to Shell 87 and I notice I do a little better with Shell.

So, I'm also wondering where you are from. I guess the places where you drive your car to really make a difference.
I'm from the San Francisco Bay Area. Traffic is bad here and the roads ain't all that flat and smooth even on freeway.
However I think I'm a good driver, accelerate slowly, pulse/glide if I can, ....etc.

Thanks!
Joe

 

[PA Prius]

Hi PA Prius,

Oh yeah, it's a new car....I bought it in January 2008 and currently I have 7500 miles on it already, yeah not much I know.
My tires came with the car when I bought it and yes I guess I could label them as still "brand new" since I only covered 7500 miles roughly so far.


What year of Prius do you and how your mileage so far?

Oh yeah, what gas are you using? I recently went from Chevron 87 to Shell 87 and I notice I do a little better with Shell.

So, I'm also wondering where you are from. I guess the places where you drive your car to really make a difference.
I'm from the San Francisco Bay Area. Traffic is bad here and the roads ain't all that flat and smooth even on freeway.
However I think I'm a good driver, accelerate slowly, pulse/glide if I can, ....etc.

Thanks!
Joe

Mine is an '04. I got it used in '06 with 41,000 miles on it for $16,500. It now has 92,000 miles on it and I could probably get the same price for it!

I'm in southeast PA, rolling hills and fairly decent roads. I'm sure the traffic is less than yours.

I use whatever cheap gas I can find.

PA P

 

[hyperion]

Are you seriously questioning whether increasing tire pressure increases mpg?!?

PA P

.
.Not in the least!
I'm simply trying to point out that tires are very important in how a car handles and Toyota has done an awfull lot of testing before coming to the figures selected and printed on the sticker on the door posts.
We all know there are a thousand "gimmic's" that can be used to get better gas milage. And we all often find circumstances when we do something (like "stomping" on it to get around that little old lady) that we really know is foolish and going to cost us at the pump. When I read posts about folks getting 60 MPG in a Prius loaded to max I sort of smile and wonder where that awufully long hill is!
I've driven my car (an "04") what I would realalistically call normally for over three years and have been very happy with my "consistant" 44 MPG.
Sixty MPG, I have never seen. And I do recognise what type of small, "relatively as light as the designer can make it" type car I am driving. It already goes "hippiot- hop" enough on washboard type streets for me to ever make it worse just to save a couple miles per gallon.
I'll grant you that air preasure is a cheap way of designing a suspension system but these cars are definately not Ferrari's, Beemers or Jaguars. Toyota settled on what I consider a higher than usual tire pressure for an important reason and I hope the one that slides into me attempting to stop on a wet street isn't doing so because someone looking for a few extra MPG hasn't "over inflated" his tires.
Changing designed tire pressures definately does change things. Some for the better and some for the worse. I'll take Toyota's word that they have chosen the "all around" better figure since it has remained the same for the past four years.

 

[PA Prius]

Hyperion, I agree that Toyota had the big picture in mind when making their psi recommendations. As I stated earlier I drive very conservatively and especially so if the road is wet. From your comment, "Tire pressures are extremely important in this. You will be changing much more than the 'possibly' higher milage figure, if you do even that. I did not find it so!" it sounded like you were thinking that raising psi does not raise mpg, which I don't think is questionable. The amount can surely be questioned.

The mileage I've reported is from my MFD. As you can see by my taglines, my actual is exactly 1 mpg less than the MFD, so the numbers in my earlier posts here are slightly overstated.

I wouldn't consider raising tire pressure a gimmick. Block heaters would come closer to that category for me because those who use them are raising their mpg with the use of electricity which does not show up in their mpg. It could be it pays off, I don't know.

As far as smiling and the awfully long hill.... I'd be smiling too if I could find a down hill that stretched both ways on a round trip, such as ours this past weekend to the shore and back, or three weeks ago from PA to IN and back. You'll probably smile even more when I say our family of four got 49.74 mpg (actual) average on our 7,500 mile cross country camping trip last summer- with roof rack, and yes the AC was on.

PA P

 

[tochatihu]

Darn good numbers with a roof rack



DAS

 

[wstander]

Hyperion noted:

"I'll grant you that air preasure is a cheap way of designing a suspension system but these cars are definately not Ferrari's, Beemers or Jaguars. Toyota settled on what I consider a higher than usual tire pressure for an important reason and I hope the one that slides into me attempting to stop on a wet street isn't doing so because someone looking for a few extra MPG hasn't "over inflated" his tires."

Hyperion,

I do not know what your experience is when assessing 'normal tire pressures', but my 3 Porsches all stated 36F/44R, my Pontiac TransAM GT was 38/38, ....all manufacturers' recommendations.

Large commercial truck tires regularly run 65PSI light, and 100PSI loaded..

(In fact, I have not seen tire pressures as low as 25PSI since the bias belt days of the 1950/1960s.)

As to Toyota (or any manufacturer for that matter), most set recommended pressures to compromise between performance, mileage, safety, and comfort/noise. I very much doubt that any Toyota engineer or Goodyear engineer will ever state the reasons or recommend any different settings for fear of being sued, so relying on 'official' representation is moot.

In my opinion, tire pressure alone has little to do with suspension changes, but tire size modification does and should be factored in. Going to 'Plus one or Plus two' usually requires alignment and castor/camber adjustments to compensate for vertical suspension movement and toe-in/toe-out.

Most of us have discovered that the current 'recommended pressures' are too low to even place a flat footprint to the road; tire wear seems to occur on both inner and outer edges, implying underinflation at 35/33. Tire fatigue occurs, typically, due to sidewall flex more than center tread failure. In your wet weather scenario, low pressure is just as evil as over pressure; in fact, low pressures can trap water and increase the likelihood of aquaplaning because the tread cannot properly sipe the water away from the tread.