Tire Pressure 42PSI front 40PSI Back - Why? - Toyota Prius Forum : Prius Online Toyota Forums
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#1 (permalink) Old 07-25-2008, 03:20 AM
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Default Tire Pressure 42PSI front 40PSI Back - Why?

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
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#2 (permalink) Old 07-25-2008, 08:46 AM
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Default Re: Tire Pressure 42PSI front 40PSI Back - Why?

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

Don't worry. Be faithful.
-- - - - - - - - - - - - --
2004 • Lifetime mpg: 52.3 actual / 53.3 MFD • 60.7cs
ScanGuage II • Sharkfin • Amsoil 0/30 • 56/54
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#3 (permalink) Old 07-25-2008, 08:50 AM
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Default Re: Tire Pressure 42PSI front 40PSI Back - Why?

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.
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#4 (permalink) Old 07-25-2008, 09:03 AM
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Default Re: Tire Pressure 42PSI front 40PSI Back - Why?

why not the same psi all tires say 42?
what are your thoughts on using nitrogen?
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#5 (permalink) Old 07-25-2008, 10:01 AM
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Default Re: Tire Pressure 42PSI front 40PSI Back - Why?

Quote:
Originally Posted by JCPrius2008
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.
Quote:
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
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#6 (permalink) Old 07-25-2008, 10:32 AM
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Default Re: Tire Pressure 42PSI front 40PSI Back - Why?

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!!

Ken (in Bolton,Ct) 2005 Pkg #4 (AM)
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#7 (permalink) Old 07-25-2008, 02:10 PM
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Default Re: Tire Pressure 42PSI front 40PSI Back - Why?

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
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Default Re: Tire Pressure 42PSI front 40PSI Back - Why?

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
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#9 (permalink) Old 07-29-2008, 07:43 AM
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Default Re: Tire Pressure 42PSI front 40PSI Back - Why?

Quote:
Originally Posted by 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
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

Don't worry. Be faithful.
-- - - - - - - - - - - - --
2004 • Lifetime mpg: 52.3 actual / 53.3 MFD • 60.7cs
ScanGuage II • Sharkfin • Amsoil 0/30 • 56/54
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#10 (permalink) Old 07-29-2008, 08:22 AM
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Default Re: Tire Pressure 42PSI front 40PSI Back - Why?

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?
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