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Old 10-28-2008, 09:41 AM   #13 (permalink)
Jim Allen
5.4L Econo Box
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Join Date: Jul 2008
Location: NW Ohio
Posts: 36

Plugger - '05 Ford F-150HD 4x4 XL
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Interesting thread.

I think hyperinflated tires (my definition: tires being inflated past the mfr's maximum) are a big mistake. Sometimes people get myopic about fuel economy and forget that you have to steer, stop and corner. A hyperinflated, or overinflated for the load, tire reduces the tire footprint and changes the whole roadholding dynamic and the way the tires, brakes and suspension work together. In the long run the money you might be saving will just come out of your paycheck in another way, such as in a set of tires much sooner. Or chiropractic visits ( : < ). Or a sudden, devastating tire failure. MetroMPGs second test should help convince anyone that there's a plateau for any tire.

I experimented with my newer pickup and discovered, as MetroMPG did in his latest test, there's a "Three Bears" just right and any gains above that point are fractional. In my case, the truck manufacturer's loaded pressures of 50 psi were it. I didn't see any marked gain by going up another 10 psi to the tire sidewall maximum. It also turns out that the difference between the tire placard max and a slightly lower pressure that offered reasonable ride quality, performance and tire wear was very small. When I upgraded to larger, more farm field friendly tires (taller and slightly wider), I expected to lose a little mpg but I actually gained some, most likely from the "overdrive" effect of taller tires (truck has 4.10 towing gears) that cancelled out the losses due to rolling resistance and gave me a little extra. Surprisingly, the height gain didn't throw the aero off too much, at least at the moderate speeds I cruise at but I did lose noticeably at 70 mph and above. I might be able to get some of that back with a bed cover. Will be doing some coast-down tests next spring to check it all out.
Jim Allen
The Frugal Four Wheeler and Farmer

My ultimate goal is not necessarily the highest mpg but to make my trucks more efficient configured as I need them.

Old Reliable '86 Ford F-250HD 4x4, 6.9L diesel

Red '00 Honda Accord Coupe, 3.0L V6, automatic

The Plugger '05 Ford F-150HD 4x4, Regular Cab, 8-ft bed, 8,200# GVW, 5.4L V8, automatic, 4.10:1 ratios, 285/70R-17D tires

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