| Author |
Message |
    
Sean H
| | Posted on Tuesday, March 12, 2002 - 10:56 am: | 
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The tires on my truck are almost gone (Stock 265/75 MT's. I would like to go with 255/85 but don't have the $$$ for gears. I need a new top desperatly. Is there anyone out there runnig 255's or 285's with 3.56's? The truch gets used offroad a fair ammount. |
PerroneFord
| | Posted on Tuesday, March 12, 2002 - 01:30 pm: |
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I just rode in a truck last week with 255/85 BFG MTs and 3.54s. Seemed fine to me, but this is 90ft above sea level. What part of the country are you in?
-P |
Sean H
| | Posted on Tuesday, March 12, 2002 - 03:28 pm: |
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Colorado,
I'm at 9100 feet. |
dmenasco
| | Posted on Tuesday, March 12, 2002 - 05:37 pm: |
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I run 255/85 BFG's here in Oregon with stock R&P's and mine runs great with them. I go out every now and then plus use my D90 as a daily driver.
Although, I must confess that I would prefer to have 4.11's. They complement the 255/85's very well.
By the way Perrone, what does elevation have to do with tire size and R&P ratios?
DJ |
Milan
| | Posted on Tuesday, March 12, 2002 - 07:39 pm: |
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Your engine looses power with thinner air. No need to exaggerate the "power loss" by adding taller tires. Although, I don't think the 255/85 is that much bigger than 265/75. |
PerroneFord
| | Posted on Tuesday, March 12, 2002 - 10:06 pm: |
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Yea, like Milan said. The elevation robs power because there is less oxygen in the air.
-P |
Sean H
| | Posted on Tuesday, March 12, 2002 - 10:18 pm: |
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I'm more interested in how it will effect my offroad performance. |
Troy
| | Posted on Wednesday, March 13, 2002 - 10:12 am: |
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Expect the same the performance as you would get with 33s. The 255/85r16 is ~33" tall mounted (depending on brand). I run those as my road tire, though I have 4.10s. They do ok running around town (I live in Boulder county), but at 9k-10k feet when I go skiing, it's bad .... and that's with 4.10s. Power loss is much more obvious; all climbing is done in 4th, sometimes 3rd gear. If you don't have far to drive and don't mind a top speed of 60 mph up hill, then go for it.
Off-road you can squeek by. I ran 35s with 3.54s for 3 months before I installed my gears. Steep downhills and large rocky ledges were the worst.
Troy |
dmenasco
| | Posted on Wednesday, March 13, 2002 - 12:18 pm: |
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Well, I understand that the air is thinner as you increase in elevation and that this is directly proportional to you engine's performance, but isn't it the EFI/CPU's job to compensate for this, thus alleviating the power loss?
I've never had the opportunity to push any elevation past about 6K ft. and at that elevation I don't recall experiencing any drift in power. However, it could be that I'm just used to the 3.9-L being "logy" if you will!
DJ |
alhang
| | Posted on Wednesday, March 13, 2002 - 04:52 pm: |
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dj, the efi does compensate a little but you can't compensate for what isn't there. in this case oxygen. basically the computer will probably thin the fuel to prevent a rich mixture. |
Craig M. Highland (Shortbus)
| | Posted on Wednesday, March 13, 2002 - 08:07 pm: |
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3% for every 1000 ft in elavation-
Craig- |
dmenasco
| | Posted on Thursday, March 14, 2002 - 01:26 pm: |
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So, just to see if I'm on the same wavelength as you guys, are you saying that there is a point in elevation at which the air is too thin for the EFI to collect and thus the volume required for the proper mixture would be too great for the engine to handle? Hence, the EFI wouldn't be able to keep the proper mixture and as a result you lose power? |
JEEPETR
| | Posted on Thursday, March 14, 2002 - 02:06 pm: |
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dmenasco,
The EFI's ECU will correct for altitude as high as you can drive the truck. The thing is, combustion relies on air and fuel combined, at a ratio of approx. 15 parts air, to one part fuel. As the Air thins out with altitude, the ECU reduces the amount of fuel injected to maintain this ratio of air to fuel, but it cannot magically increase the amount of air available. The fact that it compensates allows the truck to maintain smooth operation, idle, etc, which would otherwise be lost.
Anyway you slice it, if you increase the altitude, the Power available will decrease.
~Scott T.
'95 D-90 (JEEPETR)
'96 Discovery (5-Speed) |
dmenasco
| | Posted on Thursday, March 14, 2002 - 02:19 pm: |
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Scott
Ah HA! I did not know this. As you can tell my EFI prowess is less then optium. I've always thought that both the air and fuel could be adjusted and not just the fuel to provide the proper mixture.
I guess I never really have had to deal with this because I live at ~500-ft above sea-level! But thanks for all the information.
DJ |
Johnathan Tisdale
| | Posted on Thursday, March 14, 2002 - 02:49 pm: |
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You can compensate for induction(ie free flow air filters or a supercharger) but not density as Scott mentioned.
Johnathan |
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