10mpg increase

I am assuming that nitrogen filled tires hold their psi longer because the nitrogen is a larger gas particle. This makes me think that the smaller gas particles of oxygen will leak out faster because it is easier for them to escape the tire. so it would be logical that you would have a larger concentration of nitrogen in the tire over time due to the different leakage rate, and at some point the nitrogen should be highly concentrated. The elevation should not stop this affect, I guess if the air at higher elevation has a smaller % oxygen then that would speed up the process.

I think the benifit of filling tires with nitrogen is make believe. air is mostly nitrogen and over time you tire is going to be have a higher concentration anyway. Do what you like but I wont have a hard time sleeping because I just use air to fill my tires. Like I said before all my tires seem to fail because of what happend on exterior of the tire.

 

Yes, nitrogen will tend to become more concentrated, if the diffusion rates mentioned in previous threads are correct. But those figures are not hugely different, so the rate of concentration should not be high. And the terminal point with infinite time should be well short of 99%.

Elevation factors in because it sets the partial pressure outside the tire, thus limiting how low the inside partial pressure can fall.

I'm also challenging KhaPhoRa's suggestion that, even with a drastic difference in diffusion rates, 99% could be achieved in just two top-offs. Not a chance.

 

Wow that took me back.. Partial pressures, Dalton's law.. You are right. So remind me, would that also mean that even using pure nitrogen eventually oxygen would diffuse in? Either way the result would be the same?

 

Relax, son. It was just a for-fun napkin calculation. You are correct about partial pressures.

 

It would be highly unusual to obtain +10 MPG (or about 25% improvement) from the states pressure increase alone. Fuzzy1 is absolutely right to be looking for other factors.

My hunch is that the dealer disconnected the 12V battery during the service, which often alters the reported MPG for a some times afterwards. If you search the forms here there are literally dozens of posts complaining of large drops in MPG immediately after a dealer service. In most of these cases the tires and the oil check out fine and no other problem can be seen, but mysteriously the MPG in significantly reduced. Then in a week to a few weeks the problem magically sorts itself out as the ECU re-establish their historical data.

 

I've never seen anyone else report such a large increase in fuel economy from tire pressure increase of that magnitude (30 PSI to 40 PSI).

 

Easy enough to test - just drop the pressure back to 30psi and run the car that way for another tank.

 

I agree with you. I live in a suburb of Chicago. I have 140 miles on my car since the last fill. I am getting 57 mpg. No highway miles. I have no explanation for it. It normally does not do this good. Must be the weather.

 

Re: N2 - Yes if tires filled with air (78% N2), over the years your %N2 will climb possibly to about 90% N2. So that's part of the reason why its hard to make a stronger recommendation for N2 in car tires. I like Costco approach of putting N2 in new car tires....but I'd just refill with regular air after that.

Re: Effect of Tire Pressure- I have previously shown this chart (below), based on some equations, but I did not have actual Prius MPG data so it's the shape of the plot that's important. As you go to too low pressures, the MPG decrease worsens. FuelEconomy.gov say impacts is: "You can improve your gas mileage by up to 3.3% by keeping your tires inflated to the proper pressure. Under-inflated tires can lower gas mileage by 0.3% for every 1 psi drop in pressure of all four tires." But other sources say the effect much larger than EPA says it is...so I do not know.

I've noticed with my Gen2 sometimes I am getting 50 MPG on the display, then stop to get gas... then it won't go over 45 MPG on the display. I tend to feel real MPG is best calc'ed via hand at fill-up. But then we have (in Gen2) that flexible guess tank so we have a hard time with these sorts of measurements.

Here's some MPG data from Edmunds:

 

the only reason i mention it is, i started out running recommended, then went to 42/40 and never perceived much improvement.

 

mff-mff-mff

 

wait wait AFTER you paid off the car THEN they sent you a title w/them as the leinholder? So which title is most current? Would you have to get them to sign off if you sell the car?

 

After I paid off the car they sent me a title that listed them as the lien holder, they of course signed the line releasing the lien as the loan was paid off. I used that one when I moved to texas and did a title transfer..

 

wow! 4.5 mpg increase over a 30 psi change! amazing!