Posted Tuesday, January 20/09 in Mods & Tests

How important is tire pressure to rolling resistance and fuel economy? Apparently important enough that it became an issue in the 2008 US presidential election: one party mocked the other for suggesting that Americans could displace a significant amount of oil demand by simply maintaining their tires at the correct pressure.

Of course anyone with a shred of interest in efficiency already knows that tire pressure is one of the basic first steps.

But it's one thing to read stats, and another to see the relationship demonstrated in real life. So last summer I set out to do a snapshot test of how far (or not) my car rolls down the same course at varying tire pressure levels.

Overview:

• Some tire pressure/efficiency stats & info
  
• Test course & methodology
  
• Results: 1998 Pontiac Firefly (Geo Metro)
  
• Results: 1999 Toyota Camry
  
• Observations
  
• Resources

Some tire pressure & efficiency info collected from the web...

Before heading to the test road, here's some tire pressure trivia & facts to chew on:

• Regarding that recent US presidential election: one side "suggested that if all Americans inflated their tires properly and took their cars for regular tune-ups, they could save as much oil as new offshore drilling would produce." (Source: Time)
• Tire pressure changes an average of 1 PSI (pounds per square inch) for every 10 degree F change in air temperature. Tires also deflate naturally over time, as much as 1.5 psi per month. Fuel efficiency is reduced by 1% for every 3 PSI under inflation. (Source: checkmytires.org)
• In 1995 the US Energy Department said that under-inflated tires waste an estimated 4 million gallons of gas daily in America. (Source: pumpmeup.org)
• A vehicle with a recommended pressure of 35 psi whose tires are at 28 psi will have increased its rolling resistance by 12.5%. (Source: US Department of Energy)
• The effect of ambient temperature on tyre rolling resistance has been studied by Firestone Tire and Rubber Co. The results indicated that the initial rolling resistance at -20 C might be more than twice as large as at 40 C. (Source: University of Helsinki, Department of Mechanical Engineering)
• Based on a 2001 study of 11,530 vehicles in 24 Geographic Areas across the US:
  • Percentage of vehicles that have at least one tire under inflated by 6 psi or more:
    Passenger Cars: 40%
    Trucks, SUVs, and Vans: 45%
  • How often do car drivers check their tire pressure?
    Weekly - 9%
    Monthly - 21%
    When They Seem Low - 26%
    When Serviced - 30%
    Before a Long Trip - 1%
    Other - 6%
    Does Not Check at All - 7%

  (Source: National Highway Traffic Safety Administration)

Test course & methodology...

Test road:

The test course was a paved (asphalt) slope: 7 feet / 2.1 meters drop over 303 feet / 92.4 meters onto a level run out.

Methodology:

1. tires were pumped up to maximum pressure, drove to test route (< 5km), pressure adjusted
   
2. car was driven up the hill, turned around and stopped at a marked point
   
3. engine off, transmission in neutral, brakes were released
   
4. car rolled down hill onto a flat run-out
   
5. road was marked with chalk on the pavement where the car stopped
   
6. pressure was adjusted (dropped), or run was repeated in the case of the Camry
   
7. rinse & repeat

After all runs were completed, distance to each "stop" mark was measured with a bicycle wheel (counted revolutions from the starting point, then converted circumference to total distance).

Pressure readings were taken with an Accutire digital gauge, 5-99 PSI rated, with a manufacturer accuracy claim of +/- 1% + 0.5

Results, 20-60 psi: 1998 Pontiac Firefly (Geo Metro)...

Tires: 155/80/R13 Goodyear Invicta, rated 44 PSI max sidewall.

Placard info: vehicle placard recommends 32 PSI front/rear @ max load.

Weather conditions: temperature 19C / 66F; wind 10 km/h / 6 mph SSW (test roadway runs SW/NE)

Raw results for PSI/feet travelled

20 / 479.3
25 / 524.8
30 / 621.0
35 / 621.0
40 / 639.6
45 / 687.5
50 / 702.0
55 / 699.3
60 / 702.0

Click to view results to scale: 800 pixel image width / 1200 pixel image width

Note: the top speed seen was 18.5 km/h / 11.5 mpg @ 60 psi (the only time I checked)

Results: 1999 Toyota Camry...

Tire info: 205/60/16 BF Goodrich Traction T/A, rated 44 PSI max sidewall.

Placard info: vehicle placard recommends 32 PSI front/rear @ max load.

Weather: temperature 20C / 68F; wind 4 kts N wind (NW sheltered roadway ran SW/NE)

Raw results for PSI/feet travelled (1)/ feet travelled (2) / average

20 / 568 / 570 / 569
30 / 662 / 672 / 667
40 / 677 / 679 / 678
50 / 667 / 679 / 673
60 / 692 / 681 / 686

Observations...

• First, there obviously aren't enough data points to call these results representative. They're not rigorous tests (thus the "snapshot" title).
• I find it interesting that even with such a simple test there can be so much variation between runs with the same pressure (in the Toyota). Possibly the brakes weren't fully releasing, or maybe the breeze puffed harder on those runs. The "duplicate" distance in the Firefly at 30 and 35 psi was another oddity. Just goes to show how difficult it is to get good data outside of a lab.
• If a little more pressure is good, then is a lot more better? It's interesting to see that beyond a certain point, increasing pressure in both the Firefly and the Camry didn't seem to return a significant corresponding increase in coasting distance. The results suggest that the magnitude of the impact on rolling resistance is more pronounced (negatively) when pressure is low than when it's increased above recommended levels.
• The ideal way to have done this test would have been a simple coastdown from a constant speed, rather than coasting from rest, down a hill, and rolling to a stop again. However the challenge in testing from a constant speed is that most cruise controls won't engage below 50 km/h (30 mph). At that speed, a longer "roll-out" road is needed. Also, requiring the driver to shift to neutral at *precisely* the same point on each run for consistency is surprisingly difficult.

Resources ...

• Discuss this at EcoModder: Experiment: coast down distances @ various tire pressures - EcoModder.com