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Old 06-01-2008, 03:52 PM   #1 (permalink)
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PSI question

I'm taking a 330 mile trip tomorrow and have a lot of stuff packed into the trunk and some in the back seat. I've heard you're supposed to inflate your rear tires more to accommodate for the added weight. I was already planning on pumping up my tires to 40 psi from 32 psi, so my question is how much more should I pump them up in the rear? Should I pump them above 40 psi, or would 40 psi all around be enough to take car of that extra weight?

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Old 06-01-2008, 03:55 PM   #2 (permalink)
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Do you know what the extra weight is?

The difference between extra weight - 2 bags and an umbrella & a trunk loaded with concrete blocks is significantly different. See if you can calculate the extra weight and I am sure one of the gurus here will give you a better answer.
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Old 06-01-2008, 06:33 PM   #3 (permalink)
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The total weight added is just shy of 200 lbs, the max sidewall PSI is 51, but it also says not to inflate beyond 40, so I'm a little confused as to which it is. I'm looking to inflate it to account for the added weight but also to evenly distribute it so as to not decrease handling by a big amount, while also gaining some MPG.
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Old 06-01-2008, 07:54 PM   #4 (permalink)
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If it were me, I'd go with max sidewall. I haven't seen any study that proves you would be at a greater risk running max sidewall pressure.
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Old 06-01-2008, 08:07 PM   #5 (permalink)
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I'm afraid of doing that really, for normal cruising I don't see much of a problem but on roads that are very windy, twisty and sharp your tires are all you have to keep you from losing traction/grip and crashing. Not to mention the random objects like rocks lying in the middle of the road around a sharp corner that are unavoidable.

45 is the highest I'd go personally but I'm starting out at 40. I just want to know if the rear tires need to be inflated more than 40 to account for the extra 200 lbs of weight in the trunk.
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Old 06-01-2008, 08:47 PM   #6 (permalink)
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I wouldn't lose sleep over adjusting pressure for 200 lbs extra. (EG: Do you normally inflate your tires more when you take rear seat passengers?)
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Old 06-01-2008, 09:02 PM   #7 (permalink)
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LOL good point, I guess I was just worried because the back suspension is pretty hunkered down with all that weight. The tires look fine, not like they're going flat or anything, just figured with all that extra weight might need to up the PSI.
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Old 06-05-2008, 11:54 AM   #8 (permalink)
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Quote:
Originally Posted by MetroMPG View Post
Do you normally inflate your tires more when you take rear seat passengers?
I guess that depends on the size of the passengers?
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Old 06-05-2008, 04:04 PM   #9 (permalink)
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I got a little scared so I took Highway 20 instead, I'm more familiar with it and my sister was in the car with me so I didn't want to freak her out. I averaged about 40 MPG over 8 gallons, not bad but I was hoping more considering I pumped up the tires 20 PSI above door specs. I'm going to do one or two tanks of city driving with my tires this high just to see if it helps city MPG any. If it's not worth it I'm going to go back down to 32ish, center tread is bulging out noticeably and that's going to wear it hella fast.
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Old 06-06-2008, 07:05 AM   #10 (permalink)
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Frankenstipe, what you see on the tire is not necessarily what is happening at the contact patch. Have you checked it? Lift the car, apply some water base paint on the tread so it's easy to clean, lower the car on a sheet of paper and lift again to look at the result and see if the whole width of the tread made contact.

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