Join Date: Sep 2005
Location: Bolton, Connecticut
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Re: Tire Pressure 42PSI front 40PSI Back - Why?
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.
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.
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)