Tire Pressure (How low can I go?)

"Inherited" my father-in-law's old truck. He bought himself a new one and we got a great deal as long as we let him borrow it to haul
firewood. We bug him because this truck was a real work truck and now his new truck is too nice to get dirty. :)
Anyway, it's a 90 Chevy 1/2 ton 4x4 regular cab with a 350/5spd. Stock tires (according to the door sticker) would have been 225/75-16 at 45psi. On there right now are LT245/75-16's. According to the sidewall I can go up to 60 psi for max loading.
Now, my dilemna (other than not being able to spell dilemna) is what tire pressure should I be running? Half the time it's carrying freezers to the dump and helping people move, the other half the time it's going to be a second vehicle for the winter. (Winnipeg - lots of snow and -40.)
My father-in-law was running about 40 psi, but I've dropped it to 35 because it looks like the tires are wearing in the center (overinflation.)
So, when loaded I think 45-50psi is probably appropriate, but when Christmas shopping I'm thinking I could maybe even go as low as 25 psi for more traction on snowy/icy roads? I know no one can tell me exactly without seeing the truck, but looking for suggestions as to what a good pressure would be for an unloaded pickup. I know I can't ask GM or a tire company, because they'll all just tell me to run what the sticker says... and I'm not buying new tires to match the sticker. (the cool thing is I just picked up a shop manual on ebay and you can recalibrate the speedo to work with these size tires....) (I like the sticker on my Trans Am - 30 psi under normal conditions, 38psi under sustained (>100mph) driving.)
(and this thing is a REAL truck. No carpet, just rubber flooring. No A/C. Just a motor, a stick shift and a big hitch - perfect for towing my race car next year. The thing even has an oil cooler...)
So, anyone have suggestions as to what psi range I should be playing with? (I'm planning to keep dropping it until I feel it getting "squishy" and going about 5psi higher than that for the lowest to run.)
Ray
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I run my Jeeps on and off road in the Canadian winters and find a harder tire is better for most snow conditions. My tires are not wide enough to get good 'floatation' when aired way down so I keep them close to the stock maximum and put up with the bumpier ride.
If they get too soft, they will float up on top and I lose the steering control and traction really fast. I prefer to cut through.
Same for winter intersections. I have 'shift on the fly' 'part time' locked center 4x4 and much prefer 2 wd when making a left turn and sometimes even a right turn. Once around I then just shift back to 4x4.
Mike 86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00 88 Cherokee 235 BFG AT's Canadian Off Road Trips Photos: Non members can still view! Aug./05 http://www.imagestation.com/album/index.html?id !20343242 (More Off Road album links at bottom of the view page)
news wrote:

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My suggestion would be to get a hold of the load inflation tables for your tires (these are industry standard for a given size and type of tire). I pulled the following out of my copy of the Tire Guide:
For LT245/75R16
Pressure Load (PSI) (lbs) 35 1700 40 1865 45 2030 50 2205 (C) 55 2335 60 2480
The original tires on the truck would have been P series tires (not LT tires). For Light Truck usage, these are derated 10 to 15%. Here is the load inflation table for a P225/75-16
Rated 10% Derated Pressure Load Load (PSI) (lbs) (lbs) 20 1499 1349 23 1598 1438 26 1698 1528 29 1786 1607 32 1874 1687 35 1984 1786
GMs original pressure recommendation was 35 psi. This implies that they expected the tires to support a load of 1786 lb each. To maintain this same load carrying capacity with the LT245/75-16 Tires you will need to inflate them to around 38 psi. LT tires have heavier sidewalls than P series tires. This allows them to carry heavier loads at higher pressures. However, they also requires higher pressures to reduce flexing of the heavier sidewalls. Flexing the thicker sidewalls gnerates a lot of heat. For a loaded truck, I feel the 40 psi inflation pressure is a good number. The 35 psi inflation pressure is fine for an unloaded truck, unless you are doing a lot of high speed driving.
Ed

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C. E. White wrote:

Kewl. The GM sticker says 45psi for the tires as normal pressure, so I think they may have been LT's as standard. Irrelevant because the sticker also lists the GAWR and there's a weigh scale a couple of miles from my place so next time I go buy I might get an unloaded weight for this thing. I knew pressure and load were related, and I'm sure it wasn't linear, but didn't know how to calculate it.
Is there anywhere online to see this tire guide? I can't find it... (I found one for Michelin rig tires, but not passenger car tires.)
Thanks a bunch. Now I have a better handle on what pressures I should be running - the question remains how it'll handle in the snow at different pressures.... and I'm not in any rush to test that... winter's coming soon enough.
And hey, what about a P235/75-15? Assuming I can find 15" 6 lug rims, I've got 4 basically new tires from my Jimmy that I'd consider using - and they're closer to the stock height so my speedo would be less incorrect... but I don't want to start risking a blowout from overload the first time I carry a couple of boxes of kleenex in the back. ;)
Ray
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Remember that it is not as simple as weighing the truck and dividing by 4. The weight on the front and back will be different. This is just static load and when you accelerate or go around corners there will be weight transfer that will increase the a tires load. Stan
ray wrote:

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Stan Weiss wrote:

When I weighed my Trans Am (oink) the guy asked if I wanted the front axle and rear axle weighed separately - I just wanted a total, but for the truck I'll get that. If I was really anal I'd borrow the scales from the local sports car club and weigh it like I'm going to do my enduro car in the spring. :)
Ray
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Try http://www.trucktires.com/zip/Light_Truck_Tables.ZIP . It has some light truck tires, but not all. Also go to http://www.goodyear.com/truck/pdf/databook/loadInflation.pdf and http://www.michelintruck.com/michelintruck/pdf/LoadInflation.pdf .
I am surprised the truck came with LT tires. The information in the above reference does cover Firestone LT225/75R16 Tires. Here is the information for LT225/75R16 Tires from the above three sources:
Firestone Goodyear Michelin Pressure Load Load Load (PSI) (lbs) (lbs) (lbs) 35 1545 1500 1500 40 ----- 1650 1650 45 1750 1790 1790 50 1940 (C) 1940 (C) 1940 (C) 55 ----- 2060 60 2140 2190 65 2335 (D) 2335 (D) 70 ----- 2440 75 2510 2560 80 2680 (E) 2680 (E)
Note that at certain pressures, the Firestone rated loads are different than the industry standards - not sure why, maybe conversion and interpolation errors. Michelin and Goodyear are quoting the industry standards.
So, at 45 psi, the tires were rated to carry 1750 to 1790 lb each. The LT245/75R16 carry this load at around 37 psi.
The Goodyear Tables also include Load / Inflation Tables for P235/75R15 Tires (derated by 10% for use on Light Trucks)
Goodyear Pressure Load (PSI) (lbs) 26 1594 29 1684 32 1764 35 1844 38 1914 (extra load P series only) 41 1985 (extra load P series only)
Seems to me that if you inflate the P series tires to 35, you can maintain your load rating. Just be aware that the P series tire sidewalls are much less robust than LT sidewalls.
Ed

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C. E. White wrote:

thanks a bunch. I can't say for sure that the originals were P or LT, I am guessing based on the 45psi recommended pressure - most car tires I've seen are MAX 35 psi, recommended of 30ish.
I don't plan on going off roading with the 235's - it's just they're less than 2 years old and have less than 10k miles on them and it seems a shame to sell them for 1/2 price or not use them at all. The tires on the truck are in good shape, but are a little worn in the center. The ultimate test of if they're any good will be when the snow starts falling - and I know the 235's have pretty good winter traction.
Ray
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