First Major Road Trip - After Update

[boilermax]

Drove Max from Raleigh, NC to Rochester, NY and back for a total of 1625 miles.  Overall FE was almost exactly 42 mpg (from actual gas usage, not indicated mpg).  Most speed limits were 65 or 70 mph.

 

Going north we drove through VA and WVa (all mountain highways) to Pittsburgh, PA.  Drove around Pittsburgh then up to Erie, Buffalo and on to Rochester.  Drove around Rochester for 4 days.

 

Going south we drove through NY & central PA (more mountain highways) to Harrisburg then on to VA and home to Raleigh.

 

I had a hard time keeping Max in the sweet spot while on the highway.  Maybe that was due to the lengthy sections of mountain driving.  Anyway, I was still pleased with my overall average.  It certainly beats the revised 40 mpg highway rating!

 

Bob

 

[Tdefny]

Nice trip this time of the year. I am a little jelly.

[ptjones]

Drove Max from Raleigh, NC to Rochester, NY and back for a total of 1625 miles.  Overall FE was almost exactly 42 mpg (from actual gas usage, not indicated mpg).  Most speed limits were 65 or 70 mph.

 

Going north we drove through VA and WVa (all mountain highways) to Pittsburgh, PA.  Drove around Pittsburgh then up to Erie, Buffalo and on to Rochester.  Drove around Rochester for 4 days.

 

Going south we drove through NY & central PA (more mountain highways) to Harrisburg then on to VA and home to Raleigh.

 

I had a hard time keeping Max in the sweet spot while on the highway.  Maybe that was due to the lengthy sections of mountain driving.  Anyway, I was still pleased with my overall average.  It certainly beats the revised 40 mpg highway rating!

 

Bob

How many miles on CMAX? I driven those routes several times, but not with CMAX. Divide hours into miles driven to get average MPH. What tire PSI were you using 38 or 50?

Paul

Edited October 26, 2013 by ptjones

[boilermax]

Max has about 6600 miles total at this point.

 

I keep the tires at 40 psi. 

 

I didn't keep track of the driving time, so I can't compute the average mph.  When the speed limit was 65 or 70 (which it was for almost all of the time), I set the cruise control to about 68 in hopes of hitting the sweet spot.  Sometimes I did, sometimes I didn't.

[ptjones]

Max has about 6600 miles total at this point.

 

I keep the tires at 40 psi. 

 

I didn't keep track of the driving time, so I can't compute the average mph.  When the speed limit was 65 or 70 (which it was for almost all of the time), I set the cruise control to about 68 in hopes of hitting the sweet spot.  Sometimes I did, sometimes I didn't.

I was thinking about the time on your trip gauge when you filled up. I take a pic with IPhone every time I fill up for future study if I want too. Eventually I want to make a graph of MPH vs MPG's with mine. Several members are using 50psi(tires rated for 51psi Max) and I'm trying it now with my trip to Phoenix this week to see if it makes a difference.

Paul

[boilermax]

I've read the thread on tire pressure, including the suggestion to exceed the max pressure rating.  I choose to not grossly over-inflate my tires in an attempt to chase higher FE.  I feel that I get the best tread wear at or slightly above the recommended pressure.  High pressure can cause the tires to wear prematurely in the center of the tread.  At about $200 per tire, it would take quite a fuel savings to pay for early tire replacement.

 

I see by your fuelly rating your current average FE is 45.0.  Right before our trip that was exactly what ours was - 45.0.  The last tankful before leaving on the trip was an indicated 47.9 mpg for about 400 miles of local driving.

[ChuckJr]

My CMAX came from the dealer having the tires filled with Nitrogen. I am no rocket scientist or engineer but the two benefits of using Nitrogen versus standard air was bigger particles means less loss of pressure overtime and the tires run cooler during longer drives. Now the crappy points, I don't have any way to play with my psi aside from going back to the dealer. I don't even want to check the current pressure they loaded cause I don't want to loose any psi (silly, I know).

 

Is there any ill effects of increasing the psi with my home air compressor? Will it pretty much negate the benefits of the nitrogen?

[fotomoto]

My CMAX came from the dealer having the tires filled with Nitrogen. I am no rocket scientist or engineer but the two benefits of using Nitrogen versus standard air was bigger particles means less loss of pressure overtime and the tires run cooler during longer drives. Now the crappy points, I don't have any way to play with my psi aside from going back to the dealer. I don't even want to check the current pressure they loaded cause I don't want to loose any psi (silly, I know).

 

Is there any ill effects of increasing the psi with my home air compressor? Will it pretty much negate the benefits of the nitrogen?

 

Air is 78% nitrogen.  While pure nitrogen in tires does have benefits, you have pointed out the futility of real world usage.   IMO, it's just another money maker for dealerships and a lure to keep you coming back to them for tire service.

[ptjones]

I've read the thread on tire pressure, including the suggestion to exceed the max pressure rating.  I choose to not grossly over-inflate my tires in an attempt to chase higher FE.  I feel that I get the best tread wear at or slightly above the recommended pressure.  High pressure can cause the tires to wear prematurely in the center of the tread.  At about $200 per tire, it would take quite a fuel savings to pay for early tire replacement.

 

I see by your fuelly rating your current average FE is 45.0.  Right before our trip that was exactly what ours was - 45.0.  The last tankful before leaving on the trip was an indicated 47.9 mpg for about 400 miles of local driving.

I have 38K on my CMAX and just checked my tires with tread depth gauge. All tires are very close in wear with 6/32 outside and slightly more on center and inside. It would appear running higher PSI is a good thing from tire wear stand point.  BTW my Master Gauge Lifetime score is 45.9mpg with 79% HWY and my last trip two weeks ago I got 63.4MPG and 875mi on that tank. I have been averaging about 52MPG's without that tank for the last couple months. I'm always looking for ways to improve MPG's and my latest is wheel covers. :)

Paul

[ptjones]

My CMAX came from the dealer having the tires filled with Nitrogen. I am no rocket scientist or engineer but the two benefits of using Nitrogen versus standard air was bigger particles means less loss of pressure overtime and the tires run cooler during longer drives. Now the crappy points, I don't have any way to play with my psi aside from going back to the dealer. I don't even want to check the current pressure they loaded cause I don't want to loose any psi (silly, I know).

 

Is there any ill effects of increasing the psi with my home air compressor? Will it pretty much negate the benefits of the nitrogen?

Air has 78%Nitrogen in it already so the difference is Pure Nitrogen has no water in it.

Paul

[Jmonty]

We used pure nitrogen in aviation maintenance, mainly to avoid oxidation I believe.

[fotomoto]

We used pure nitrogen in aviation maintenance, mainly to avoid oxidation I believe.

 

I would suspect it also because it isn't flammable too.   Back in the old days, I used it in photography labs.  It would displace air in large film developing chemical containers to retard deterioration from oxygen.  

[Jmonty]

I would suspect it also because it isn't flammable too. Back in the old days, I used it in photography labs. It would displace air in large film developing chemical containers to retard deterioration from oxygen.

 

Hmm

 

It was just in the struts and tires. Would it expand less when changing altitudes?

[fbov]

Paul had the right answer - the only advantage to nitrogen in tires is the absence of moisture. If you use a small compressor on a cold, dry day (low moisture content air) you can avoid most of that.

 

The only reason to avoid water vapor is pressure consistency. It's well known that gas expands when heated, but it's a linear relationship (PV=nRT, ideal gas law). However, the pressure resulting from water vapor is exponential with temperature. You get a far greater pressure increase when the tire warms up than if you used a pure gas.

 

It also doesn't hurt that N2 is more stable than O2 molecules... our oxygen-nitrogen atmosphere is unique in the known universe because O2 doesn't like to stay O2. It's far happier as H2O or CO2. Think of it this way; plants ingest nitrogen (fertilizer, respiration) and give off oxygen. We mammals ingest oxygen and give off nitrogen (albeit in solid and liquid form rather than gas). Without plants, we'd likely have a CO2 atmosphere like Mars and Venus do now, and we once did.

 

But I digress...

 

Regarding altitude, keep in mind that tire pressure is expressed as "pressure above ambient atmospheric pressure." If you go up in altitude, atmospheric pressure drops, and tire pressure increases by a comparable amount.

 

Regarding temperature changes, PV=nRT works as long as you express T in Kelvins (absolute temperature). Kelvin scale is simply Celcius + 273, as absolute 0 is -273C. Going from 273K to 300K is about like Winter to Summer in temperate zones. You should see a ~1/12 loss in pressure, or ~3psi at 36psi.  Not huge, but real.

 

Have fun,

Frank

[JAZ]

Will this be on the final?

[fbov]

No.