[BradHerr]

I can't compete with you all. I am happy to see low 60's on the 25 mile average.





Sent from AutoGuide.com Free App

 

[BradHerr]

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.

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.





Sent from AutoGuide.com Free App

 

[BradHerr]

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...


Sent from AutoGuide.com Free App

 

[BradHerr]

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


Sent from AutoGuide.com Free App

 

[BradHerr]

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.


Sent from AutoGuide.com Free App