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I improved my 25 mile mile a bit. For the record, I'm using Interstate 19 between Tubac and Tucson, which is has an elevation drop of about 450 feet spread over 40 miles. Rarely is there any wind of note. I set the cruise control to 60 and pumped of those tires to 43 psi.
Vehicle Speedometer Car Gauge Odometer
I have been running the sidewall maximum pressure. I haven't done any driving at lower pressures to check mileage differences.

I usually run 71 mph on the interstate (about 55 miles) and 59 mph on the highway (about 35 miles) during my daily commute.





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What are the pros and cons of running such high pressure?
In theory higher pressures yield, lower rolling resistance, which equals higher fuel economy. It also creates a stiffer sidewall, that prevents the tire from "leaning" when turning. This improves handling.

There is a point of diminishing returns though. At some pressure, the tire becomes so stiff that imperfections in the road are no longer absorbed by the tire, but instead transferred into the suspension. Harmonics sets up in the suspension and actually causes higher rolling resistance.

Then you also get other negatives of higher pressure. The ride quality suffers, as does straight line traction.

This is definitely not for everyone. Like I said, I understand the theory behind higher pressures and also the consequences of running them. There is definitely a point north of the vehicle manufacturers recommended tire pressure that will give better mileage and handling and a point south of their recommendation that will improve ride quality and straight line traction. It is up to the individual to find what compromises they want to make to get what they want from the car; higher fuel mileage or softer ride.

I have left my tires at the tire manufacturers maximum limits for 85,000 miles and haven't had any complaints. I haven't done any side-by-side mileage tests at lower pressures to verify that I am running the optimum pressure for economy though, so your mileage may vary...


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Interesting, the super-high economy in the mountains. I noted this too, on my 2001 Sentra 5MT, 1.8 naturally-aspirated port-injected engine. At home at 600 ft, most tankfuls are 35-40. 40 and better if it's summer and if I'm really trying with 60 mph highway speeds, etc. When on trips in the mountain west, was seeing 45 mpg and better without really trying, iirc.

Guesses: 1. Gasoline may be straight, no ethanol. 2. Mountain driving simulates the pulse-coast strategy, with full-throttle hauls upgrade, and fuel cutoff coasting downgrate. 3. Less wind resistance at altitude 4. Less pumping loss because at altitude, more of the driving is done with wider throttle settings due to the thin atmosphere.

Edit: Guess #5: Higher effective tire pressure as you ascend
 

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Interesting, the super-high economy in the mountains. I noted this too, on my 2001 Sentra 5MT, 1.8 naturally-aspirated port-injected engine. At home at 600 ft, most tankfuls are 35-40. 40 and better if it's summer and if I'm really trying with 60 mph highway speeds, etc. When on trips in the mountain west, was seeing 45 mpg and better without really trying, iirc.

Guesses: 1. Gasoline may be straight, no ethanol. 2. Mountain driving simulates the pulse-coast strategy, with full-throttle hauls upgrade, and fuel cutoff coasting downgrate. 3. Less wind resistance at altitude 4. Less pumping loss because at altitude, more of the driving is done with wider throttle settings due to the thin atmosphere.
All these factors contribute to higher economy at higher altitudes. To me it seems the "thinner" air is the key. Lower air density contributes to lower wind resistance. The lower oxygen levels in a given volume of air also requires less fuel to keep the optimum air-to-fuel ratio


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I find the tire pressure issue interesting. In summer time doesn't tire pressures go up with heat? I would guess one would have to monitor pressures to make sure you don't exceed max pressure?

I just run 38-40 and don't worry about it. When running factory pressures, I am normally monitoring to make sure I haven't lost too much pressure.
 

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One thing to keep in mind: Tire pressure is really a 'gauge pressure' (relative pressure) unless you measure it with a (rare, lab only I suppose) absolute pressure gauge (and if you do this, add ~14 psi to get the proper or expected reading). If you ascend in the mountains, as the atmospheric pressure drops, the tire pressure increases. Worth about 5 psi at 10,000 ft. relative to sea level.

Yes, tire pressure goes up as temperature goes up. You're supposed to measure at ambient temperature. Yes, you'll have to add air in the wintertime, and take it out in the summer (aside from leaks and slow diffusion), to maintain the desired pressure.
 

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I find the tire pressure issue interesting. In summer time doesn't tire pressures go up with heat? I would guess one would have to monitor pressures to make sure you don't exceed max pressure?

I just run 38-40 and don't worry about it. When running factory pressures, I am normally monitoring to make sure I haven't lost too much pressure.
This is what I do. I do not recommend others trying it.

From what I've experienced, at 51 psi "cold" tire pressure at 70°F, I see 56-58 psi once I get some heat in the tires. To me, this seems acceptable as less than catastrophic tire pressure. Using 56-58 psi as "safe" operating range, I will put 53-55 psi in my tires when cold during the winter to get the tires to 56-58 psi while driving on the interstate.

The problem is when the ambient air temps fluctuate daily during the fall and spring. It makes it harder to keep the tires at a consistent "warm" pressure.


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Interesting, the super-high economy in the mountains. I noted this too, on my 2001 Sentra 5MT, 1.8 naturally-aspirated port-injected engine. At home at 600 ft, most tankfuls are 35-40. 40 and better if it's summer and if I'm really trying with 60 mph highway speeds, etc. When on trips in the mountain west, was seeing 45 mpg and better without really trying, iirc.
Guesses: 1. Gasoline may be straight, no ethanol. 2. Mountain driving simulates the pulse-coast strategy, with full-throttle hauls upgrade, and fuel cutoff coasting downgrate. 3. Less wind resistance at altitude 4. Less pumping loss because at altitude, more of the driving is done with wider throttle settings due to the thin atmosphere.
Edit: Guess #5: Higher effective tire pressure as you ascend
Guess 1 is wrong - our gasoline is generally 10-15% ethanol.


Guess 2, 3, & 5 are probably the reason with guess 2 being the seriously important one. P&G can boost fuel economy by up to 80% when done properly.

Guess 4 is correct for NA engines. The 1.4T engine in the Cruze LT, ECO, and LTZ uses the turbo-charger to keep the air/fuel mixture at the optimum for clean burn.
 

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Guess 1 is wrong - our gasoline is generally 10-15% ethanol.


Guess 2, 3, & 5 are probably the reason with guess 2 being the seriously important one. P&G can boost fuel economy by up to 80% when done properly.


Guess 4 is correct for NA engines. The 1.4T engine in the Cruze LT, ECO, and LTZ uses the turbo-charger to keep the air/fuel mixture at the optimum for clean burn.

Agreed. I would go with 2. Lots of downhill miles with very little uphill once crossing over the divide. Basically going from 11,000 feet (or about that) down to 5500 feet. All downhills except for a few stretches. One the other hand, going the other way, I may only get 32 mpg.
 

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OK. You guys made me go out and look. I always have mine set on the 500 mile best trip. Here is my best 25 miles . . and yes, mountains play a huge part in it.
View attachment 214786
That was the run obermd had me check from the Johnson tunnel (around 11,100 feet) down to the twin tunnels at Idaho Springs (probably around 6700 feet or so). I was actually still pegged passed 99.9, so I drove it to the top of Floyd Hill (probably around 7600 feet)
 

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In theory higher pressures yield, lower rolling resistance, which equals higher fuel economy. It also creates a stiffer sidewall, that prevents the tire from "leaning" when turning. This improves handling.

There is a point of diminishing returns though. At some pressure, the tire becomes so stiff that imperfections in the road are no longer absorbed by the tire, but instead transferred into the suspension. Harmonics sets up in the suspension and actually causes higher rolling resistance.

Then you also get other negatives of higher pressure. The ride quality suffers, as does straight line traction.

This is definitely not for everyone. Like I said, I understand the theory behind higher pressures and also the consequences of running them. There is definitely a point north of the vehicle manufacturers recommended tire pressure that will give better mileage and handling and a point south of their recommendation that will improve ride quality and straight line traction. It is up to the individual to find what compromises they want to make to get what they want from the car; higher fuel mileage or softer ride.

I have left my tires at the tire manufacturers maximum limits for 85,000 miles and haven't had any complaints. I haven't done any side-by-side mileage tests at lower pressures to verify that I am running the optimum pressure for economy though, so your mileage may vary...


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Bradherr, looks like we are both averaging the same MPG. I run my tires around 45 PSI, in the summer. I tried it around 50 PSI, but I didn't care for the ride and didn't notice much difference in MPG's.
 

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I also have dropped back to 40 PSI from 51. I like the ride better. I have seen a very slight drop in MPG but that could also be attributed to new tires.
 

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Personally, I would never touch 50 psi...the ride quality sucks balls. I like my teeth fillings left in my head. I usually run between 36-39 psi. On my last trip to Iowa over Christmas, I averaged 58 mpg on the way back, with a few 25 mile loops way over 60 mpg. There was a bit of a tail wind which helped.
 

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my mpg is stable year round

didnt change with tire pressure either, have run 40 to 51 psi, mpg stable

didnt change with stock tires or studded tires, or 90f summer temps or -40f winter temps

bare road or snow covered, just same mpg like clockwork
 
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