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...so the saga continues on our late night scientific testing to put various myths to bed. Tonight's was how tire pressure affects MPG.

Using the same method as last time, cruise set to 70MPH, AC on though not pumping hard with an ambient of only 74 degrees. We first did a test run to warm the tires up and then set them to exactly 40PSI with my nifty liquid filled racing pressure gauge, and the subsequent run then turned in 24.5MPG. The next one with tires at 35PSI gave 23.7MPG. And lastly 30PSI returned 22.9MPG.

That's exactly .8 MPG per 5PSI.

Astute observers will note that the MPGs above don't mesh well with my previous test at 70MPH on the test course. Why the difference? Only half a tank of 87 octane instead of a full tank? (weight), maybe ambient temp being 74 instead of 60? Winds? Hard to tell...
 

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Wow, you really got into it.............

I have read somewhere that tire pressure can affect mpg by as much as 10% - it would be consistent with your results. As to not matching previous results – atmospheric pressure, air humidity, vehicle weight, wind, ambient temperature – they all affect the mph……..so it is almost impossible to find consistent condition for so many test runs over such a long period of time………

WELL DONE!!!
 

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Mr. 3 K, 3/3/5. 5K,10/5/7
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Rule of thumb: ONE TIRE ! under inflated by 6psi reduces MPG by 3% GRIP :D REMEMBER THIS IS ONLY A GENERAL RULE OF THUMB! As they say "Individual results may vary". This is not my info but info I got off another form. They seemed knowledgeable but who knows. It all sound like it doesn't mean the end of the world. It would probably be nice to have mpg, stopping distance, time thru the salmon, tread wear etc. all vs tire pressure. GRIP :D
 

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Mr. 3 K, 3/3/5. 5K,10/5/7
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Man, you "made my weekend"! 54 pages! Anyway, i see it wil be interesting reading.

By the way - after switching to Yokohama Geolandars 265/60 -18 I had to reduce pressure to 32psi. End result - the fuel economy suffered. I would estimate it to be 1 - 2 mpg. Probably closer to 2 mpg. I would not care about it if I received other benefits: beter traction, cornering, comfort. None ot these happened (sorry, cornering is better).
 
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