(Updated 4/22/12)

(Under construction: check back soon)

Rolling resistance, the drag force from a car’s tires, may account for 10-40% of a vehicle’s energy dissipation outside of the engine (probably 20% typically). Minimizing it is easy, unless you really need the best snow traction all year round.

(Here is an efficiency calculator including rolling resistance, if you’re curious about resistance: http://ecomodder.com/forum/tool-aero-rolling-resistance.php)

First some Definitions (measured cold, i.e. first thing in the morning):

Over Inflation: higher than the maximum pressure rating stamped on the tire sidewall Standard Inflation: lower than the sidewall maximum pressure but above the door jamb or glove box placard pressures Under Inflation: lower than the placard pressure.

Max Sidewall vs Placard Pressure

A lot of cars, mine included, have around 30 PSI printed on the door jamb or glove box placard, while the tire sidewall says 35 or 44 PSI max. Personally I inflate 6 of my tires to the max printed on the tire at 44 PSI, and I think my cars have seen a 5% improvement in fuel economy by doing this and nothing else. Efficiency typically increases up to around 40-50 PSI, after that the returns diminish quickly (heavier cars and narrower tires will see diminishing returns at higher PSI).

Max Sidewall Pressure:

  1. Higher efficiency.

  2. Improved turn handling on dry pavement.

  3. (Slightly) reduced risk of hydroplaning (maybe).

  4. Less Wear, (probably) with the same pattern: It’s probably too early to speak based on my own experience, but so far with 35,000 miles between two sedans after switching from 30 PSI to 44, I haven’t seen a perceptible change in wear. I know I’m not alone. (But wider tires may be more prone to center wear than my somewhat narrow sedan tires. Do pay attention to your wear as you go.)

Placard Pressure:

  1. Quieter smoother ride: I don’t seem to notice the difference much, maybe it depends on your seats or roads. Maybe I just don’t care.

  2. Best snow traction, supposedly.

  3. Wide tires may see more even wear at placard pressure.

Why do my cars specify 29-30 PSI on the placard, while my tires specify higher max pressures on the side-wall? I believe the tires are saying “you MUST use max pressure for the max weight,” but I’m not sure what the car maker is trying to tell me. Probably it means: “for best snow traction, set to 30 PSI,” or maybe “this is where the ride over railroad tracks is sweetest”, but I don’t know.

Better Tires; for next time

Next time you buy tires, be sure to get at least 44 or 51 PSI, and lower rolling resistance (less than 0.010), but also pay attention to tread life.

For reviews on competing low rolling resistance brands, you can search for threads like this, or ask for specific help from the friendly folks at cleanmpg.com.

Danger; blowout and tire damage

First Disclaimer: The following info seems probably accurate, but I haven’t been able to dig down to primary sources yet. This is provided as-is. I’ll update as I go.

If you have high rolling resistance or low max pressure tires right now, it may be tempting to overinflate them in between now and when you can get better ones. But tire damage and blowouts are issues that must be taken very seriously, because when they occur, blowouts often result in fatalities.

There are a number of sources I can find online that claim that underinflation and overinflation both can result in a blowout. I think that type of advice is badly dumbed down. Here is the best information I currently have on this subject:

  1. The most common cause of a blowout is underinflation, typically at high speed, due to fatigue from flexing. Here is an article discussing the causes of tire failure, very good read.

  2. However, overinflation does make tires more susceptible to potholes. This was mentioned in the above article, but I like measurements. Here is a synopsis of a paper from BF Goodrich that includes such a measurement, see section on Dynamic Bruise Resistance. (Thanks to Barry for making this information accessible.)

  3. This reference teaches how to handle a blowout; increase throttle pressure initially and then gently back off and bring the car gradually and steadily to a lower speed before attempting to move out of traffic. This article also reiterates underinflation as the primary cause of blowouts. Another very good read.

  4. I’ve read anecdotal tales of pressure, even as low as placard pressure, inciting failure in tires previously damaged (i.e. cuts or gouges), I’m guessing this is probably true. However, I would guess that standard or underinflation would not alleviate the danger because of heating and fatigue issues; in this case you must have your tires inspected or replaced to be safe at highway speeds.

  5. At extremely high pressure, an overinflated tire may blow off the rim, or the rim may warp or fracture. However, the design margin is several times higher than the sidewall max spec - I’m quoting xcel of cleanmpg.com, as quoting various tire manufacturer engineers. http://www.cleanmpg.com/forums/showthread.php?t=6547&page=2

Second Disclaimer: I accept no liability, and your tire manufacturer will probably accept no liability, if you knowingly exceed the pressure specification on your tire.

I have 2 tires that spec 35 PSI as the max, I have overinflated to 44 PSI. Some hypermilers inflate their tires as high as 60 PSI, and report hundreds of thousands of miles without a blowout. I believe that the risk is not enormous, what I don’t know is how real it is, and whether it’s significant. Here are my thoughts at this time on mitigating overinflation risk (a lot of this applies to standard inflation as well):

  1. I know what to do in the event of a blowout. Again, here is that article.

  2. I inspect my tires for trouble, once in a while.

  3. I wouldn’t overinflate a tire that is old, worn, or damaged. I use my best judgment here. I would consult a professional if I thought it was damaged.

  4. I try to avoid potholes, curbs, big pavement seams, and when I have to I try to hit them gently.

  5. I don’t drive faster than the posted speed limit.