Tire Pressure Changes

Over the past many months, I have been running tire pressures at front 39 psi and rear 37 psi. At those pressures I have seen the expected increased mpg (generally low 47's to high 49's) as most others have along with expected changes in ride and handling.

What is occurring is the loss of air pressure over a month's time. Since I keep monthly tabs on another vehicle's lease milage, I also at the same time keep track of the Prius details. Since the OnStar monitors the other vehicle's parametrics including tire pressure in a monthly e-m to me, I have also begun to monitor the Prius's pressures. To my astonishment, the past two months in particular, it has lost nearly 2 lbs of pressure over 30 days. The pressures were lowering to ~ 37 and 35, +/- 0.5 lbs.

Since the other car came from the dealer with N2 as a no charge, the OnStar monitoring shows, since new in Nov '11, no change in pressure over the past many months since new. My other experience with N2 (in East TN) with my wife's Volvo, was also a positive one with limited changes in airpressure from summer / winter transitions. I know there are some negative views here in regard to N2, but my experiences have been positive. I know the details in the atmosphere's % makeup, Boyles Law, etc (!) and myself originally was not a believer.

I am concerned that the Avid S33's are loosing that much pressure over 30 days. With only ~5400 miles on them, that is bothersome to me. Usually buying 2 tanks of gas per month per average, it does appear that a pattern is showing with higher mpg with the early month tank (after raising pressures to 39/37) and the later month tank with the lowered pressures. I've tried the water test to the valve cores and don't seem to see any bubbling of air escaping; although the amount of air needed to drop 2 psi over 30 days might be too low to see with the water test.

This has been long winded (!), but has anyone else had this dramatic changes in pressures as noted above? I may ultimately go with N2 to see if this can change the molecular escape of da air from the S33's ....

Regards,
Radmangto

 

FL8 .... Yes, the pressures are checked early AM (~7AM) with no sun to partially heat tires, etc.. The Ambient temps here in Fla at that time of the day over the past many many months has been routinely ~75/77 degrees with highs routinely 90-92 degrees. So I am at a loose at the pressure decrease. It seems that each time has the preverable "slow leak" due to to nail, etc. :>))

Thanks for your input,
Radman

 

Yes, the tires seem to be loosing ~ 2psi in front and ~1.5 psi in rear the past 2 monthly checks. I only began the scrutiny the past two months with the mpg pattern being noticed. They were probably loosing psi prior to the monitoring, but can't say with specific pressures.
Thanks,
Radman

 

It has been nearly 20+ days since I wrote the original post. I checked the pressures 7 days ago .... the fronts are down nearly 2 psi and the rear down ~ 1 psi. I would expect in a few more days, it will be down again. So, why .... I guess a tire engineer will know or have better ideas than us/me.... :>))

What I would guess, with the advent of LRR tires, the sidewalls being made so thin and the tread area also being somewhat thinner, that the natural issues with gas molecules passing slowly through the different materials in the tire makeup more than non-LRR tires. I do know, using N2 in my Continential DWS tires on my Corolla and Volvo V70 last year and the year before, the pressures were rock solid ... even with the temp changes from TN winters vs very very hot summers. So, I can only say that the S-33's are not tight in regards to permeability to the air molecules. That is a pure guess ....

What brand and miliage of tires are you currently running ...? It seems that your mpg change is quite radical for such pressure changes. I might consider there might be something else in the mix. No sure what ... change in gas, gas formulation, ICE need injector cleaner, yadda yadda ...

It is a mystery though .... why some and not others .... yoi .....

Regards,
Radman
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Update ... it has been nearly 30 days since the Original Post and like clockword, I checked the pressures this morning and they have again dropped 2 lbs in each tire ... F/R were 39/37 (Aug 13) ... this morning, F/R were 37/35 (Sept 12) .... and the 2nd tank this month was 46.8mpg (yesterday's fillup and manual calc) vs 51.8mpg mid month with higher pressures during that period .... go figure .... :>)) Similiar feather pedal-type driving ... AND, stayed under local driving 55 mph as a test of my driving endurance ... !!!

Seems like too much change in mpg for such few lbs of pressure change. I wonder what the mpg would be with normal suggested pressures? Guess that is one question I probably not try to answer .....

So, I'll just fill up and drive .... !!!
Regards,
Radman
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