Lowered tire psi, got better MPG ??? - Fuelly Forums

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Old 10-16-2008, 08:26 PM   #1
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Lowered tire psi, got better MPG ???

Now this just blew me away, the entire summer I kept my tire pressure at 44. A storm was coming and I lowerd it to the factory 28 psi, on long crusies I really have to work to maintain 29 mpg on my on board computer. I lowered the tire pressure and jumped right to 34 mpg. After a trip to Phillie and back, with 2 hours of downtown driving, I came home with 32.1 mpg registered. This car is an oddity, it has factory low profile 18X10 tires, in the rain with max psi, the car will slide from side to side hydroplaning, in the snow I have to lower it to 25 psi, or it will just spin the tires forever.

The only thing I can think is the factory toe and camber adjustments must lend themselves to more friction with a harder tire. The lower pressure may throw off the friction point and thust lower rolling resistance. With the lower pressure I can feel the wheel is heavier in parking lots, it does stick to the road better in turns, the lower pressure makes the tires feel sticky.

But I got to tell you it was easy to keep it at 32-34 mpg at crusie and after city driving in Phillie brought me down to 28 mpg, I P&G'd it and it walked right back into the 30's. Wow!
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Old 10-17-2008, 05:33 AM   #2
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There are exceptions to the rule of thumb, although I would never have expected that one.

It's good that you lowered your pressure anyway...if the pressure is so high that your handling gets bad, then you really should back it off. There's no sense in saving a little bit of gas when you get in a big accident!

As a side note, I doubt it was hydroplaning with excessive pressure; more likely, your contact patch was too small and you just weren't getting enough traction from the rubber that was touching the road.
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Old 10-17-2008, 06:12 AM   #3
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Quote:
Originally Posted by theholycow View Post
As a side note, I doubt it was hydroplaning with excessive pressure; more likely, your contact patch was too small and you just weren't getting enough traction from the rubber that was touching the road.
i agree with this statement. hydroplaning means that it does matter how much deeper a puddle is, because the tire is not penetrating any further.

your theory on alignment could be true. also, in conjunction with your tires aging less tread(just from use) could be contributing to better mpg. i will admit that is a gradual thing you see over the coarse of a year or two, not over night but when you think about it with your theory of the sidewall being too hard then perhaps its quite realistic.
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Old 10-17-2008, 07:43 AM   #4
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Also it was raining, so that might have given a slight boost, but I wouldn't expect 4MPG. Let us know if it continues to be better on gas than the 44psi tires. Then maybe try more in the middle, like 35 and see what that does.
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Old 10-17-2008, 08:50 AM   #5
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Will do

With the jump in MPG, I'll watch it like a hawk and see if its a quirk or not, I'll keep you posted.
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Old 10-17-2008, 09:02 AM   #6
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Originally Posted by itjstagame View Post
Also it was raining, so that might have given a slight boost, but I wouldn't expect 4MPG. Let us know if it continues to be better on gas than the 44psi tires. Then maybe try more in the middle, like 35 and see what that does.
The rain should have caused greater rolling resistance therefore lowering FE.
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Old 10-17-2008, 09:05 AM   #7
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But then again when its raining its 100% humidity. Wouldn't that act as a water vapor injection at that point?
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Old 10-17-2008, 09:41 AM   #8
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Yesterday I did 70 mpg coming to work, and then going home in a downpour I got 49 mpg. Humidity does help, but any water on the road surface just sucks the gas like a horrid gas sucking thing.

On average, humid weather raises my mileage by 4 mpg, while rain lowers it by 8 mpg.
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Old 10-19-2008, 07:48 AM   #9
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Then came the cold.

When I did my mileage jump it was a 78 degree day with high humidity, it just crashed to 54 with low humidity. I got 28 on the last outing, same run. This run used to be 27 mph, so I got a 1 mpg bump from tire pressure. Time to break out the old WAI for winter. The one thing I noticed about the lower tire pressure is that the ability to recover mileage after 2 hours of traffic, it recovers faster. So far with the lower tire pressure, it still seems to get better MPG.
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Old 10-20-2008, 09:18 AM   #10
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When it rains, people generally drive slower and with more cation. This translates into less movement to the go pedel. This might explain the higher MPG with tire PSI lowered.
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