Tex, I found more info on California gasoline's. http://www.arb.ca.gov/msprog/levprog...1/alliance.pdf

 

Hey Fitfreaks,

Here in NM, regular gas is 86 Octane. I have run it in my fit for over 3 yrs with great success. 87 Octane is plenty for this car.

But....to get good gas mileage, a simple adjustment of your air pressure up to 45 psi will make a very big difference.

We live high in the mountains (7500 ft), and therefore climb up and down regularly. Over the whole 3.3 yrs of driving, we are averaging a true 41 mpg. This is hand calculated on a 2008 manual Fit.

The increase in air pressure does not make the ride rougher for us, but you can really notice the diff in the glide of the car.

And it is much cheaper than buying high test.

 

At the altitude you drive the air has less oxygen content than at sea level so you have a rich burn condition that cools the combustion chamber and deters pinging and knock.. Back in the days of carburetors there were places along the road where people would exchange the carburetor fuel jets for smaller ones to prevent fouling out the spark plugs.. On the return trip they would take the small jets out and re install the ones they had removed.

 

nmfit - 45psi is way high. Interested in knowing how it handles for you on the highway @70mph. I've tried bumping pressure and over 38 it's just too touchy for my likes. I've settled on 37HOT - that is starting w/ 34cold for me in the summer months. I'll check it during one of my 400 mile trips to confirm. I guess I could go that high local driving, but safe handling would still be an issue, though I can understand the lower rolling resistance making a difference. You be a hypermiller!

 

I found that too much pressure can have a negative effect on fuel mileage.. Radial ply tires were found to get better fuel mileage than conventional bias ply tires due to the resilience of the sidewalls that also allow the belted tread to maintain contact without lifting of the tread from the road when cornering.. Over inflating them defeats these features and increases rolling resistance.

 

Wouldn't the true pressure be lower even though the gauge reads 45 psi up at 7000 feet?

 

True pressure? Do you mean absolute pressure? That would be the ambient atmospheric pressure plus the pressure of the air in the tires.

At sea level you have 14.7PSI ambient pressure (the pressure of the air around us). Tires inflated to 45PSI above ambient at sea level (using a normal pressure gauge) would be 59.7PSI absolute. If so then yes, the absolute pressure would be less at 7,000 feet with the same 45PSI above ambient. At 7,000 feet the ambient air pressure is 11.3PSI so the absolute is 56.3PSI.

However this would not make a difference in how the tires perform. What's important is the difference between ambient pressure and internal tire pressure. A tire pressure of 45PSI measured at 7000 feet performs the same as 45PSI measured at sea level since the pressure gauge does not measure absolute pressure.

Now if you drove a tire with 45PSI from sea level to high altitude, and the tire does not expand at all with the lower ambient pressure, the internal pressure will remain at 59.7PSI, but a pressure gauge that measures pressure difference above ambient would now show that the relative pressure inside the tires was now above 45PSI. For practical purposes, I think temperature has a greater effect on tire pressure.

45PSI is too much unless you're driving a 3/4ton pickup or a school bus.