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Old 03-13-2014, 11:27 AM   #1
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SMB Tire Pressure ???

I've read some of the other posts for tire pressure, in fact, I'm using someone's answer for my equation, but his weight was even front back.

Quadvan door sticker for my tires LT285/70R17 126S E/10 is:
Front 60psi Rear 80psi

Front:
2070/3070@60psi = 0.674267 x 60psi = 40.45 psi
(2070/tire scale weight, 3070 based on table and Quadvan 60psi front sticker)
but if I just go with the 80psi
2070/3750@80psi = 0.552 x 80psi = 44.16 psi
Rear:
2915/3750@80psi = .7773333 x 80psi = 62.19 psi
(2915/tire scale weight, 3750 based on table and Quadvan 80psi front sticker)

So.. Im thinking on running Front=50psi, Rear=70psi (10psi under what the door max sticker is). But if the Front wheel equation should be the 44.16 psi I would do Front=55psi and Rear=70psi. Recommendations are welcome.

Im just a little bummed that Im not suppose to exceed 112 mph. Ill have to slow down. (for the record the max I like is 67 on 65 speed limit)

Toyos Table
LT285/70R17 126S E/10 33X11.5R17
PSI Single Tire load limit
35 2105
40 2315
45 2510
50 2755 (C)
55 2890
60 3070
65 3195 (D)
70 3415
75 3525
80 3750 (E)
When an R or S appears in the service description, maximum speed capability is as follows: R=106 mph and S=112 mph.
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Old 03-13-2014, 11:57 PM   #2
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Re: SMB Tire Pressure ???

Seems resonable for your specs. I use a similar chart for my BFGs and the van rides much better at the recommend pressures based on weight.

This last Baja trip I used the digital thermometer on the tires a lot while at different pressures. In some case we did 20-30 miles on pavement with dirt pressures but kept the speeds down. Never exceeded 113F on the pavement. Had similar temps at 20 mph in the dirt.

Also interesting to note the 10F difference between the front and back tires based on pressure and weight.
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Old 03-14-2014, 12:38 AM   #3
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Re: SMB Tire Pressure ???

50/70 sounds very reasonable to me.

I run 50 front 45 rear since I have twice as many rear tires.
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Old 03-14-2014, 01:03 PM   #4
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Re: SMB Tire Pressure ???

You'd wanna go with the max pressure of the tire, not necessarily what's on the door. If that's 80 in front, then the 44 figure is it.

I think you're a tad high, unless you plan on adding more weight, but it is easier to dump some air than add it. So see how it feels. You need a NASCAR crew to do those lb adjustments to get not only the best performance but the best ride.

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Old 03-16-2014, 09:28 PM   #5
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Re: SMB Tire Pressure ???

You can't multiple pressures out. Use the Load tables. You also don't want to use the max cold pressure on the tire.
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Old 03-18-2014, 06:23 AM   #6
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Re: SMB Tire Pressure ???

Quote:
You can't multiple pressures out. Use the Load tables. You also don't want to use the max cold pressure on the tire.
You lost me Anzac, multiple pressures out?
--------------------------------------------------------------
So it seems I should be looking at it like this:

Front:
2070/3750@80psi = 0.552 x 80psi = 44.16 psi ~ 45
(2070/tire scale weight, 3070 based on table and Quadvan 60psi front sticker)

Rear:
2915/3750@80psi = .7773333 x 80psi = 62.19 psi ~ 63
(2915/tire scale weight, 3750 based on table and Quadvan 80psi front sticker)

Front 45, Rear 63, and then add some extra psi for safety?
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Old 03-18-2014, 10:42 AM   #7
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Re: SMB Tire Pressure ???

You cannot scale tire pressures that way using percentages of weight or multiplying. You need to use the tire sizes, and the load tables.
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Old 03-18-2014, 11:18 AM   #8
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Re: SMB Tire Pressure ???

Have you seen this thread?
Covers a bit:
viewtopic.php?f=39&t=11143
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Old 03-18-2014, 11:36 AM   #9
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Re: SMB Tire Pressure ???

Here's a load table I just compiled for 16" and 17" LT tires -- to help with my current new tire decision. My data was taken from the Tire and Rim Association Load Tables, and merged with some extra data from the Toyo Load Tables. This extra data includes the new "E" load-rated option for the LT285/75R16 size. These two tables otherwise match for 99% of the data. I also found the Firestone and Goodyear tables to be apparently identical (though I didn't import them into Excel and compare like I did with Toyo). So, it seems that the following table can be used for any brand of LT tires. The "%" column I have below compares the given tire overall diameter with that of my current tires -- LT285/75R16.

Code:
    Tire    |   W  A  R | diam(") % |   35 LR|   40     45     50 LR|   55     60     65 LR|   70     75     80 LR|   95 LR
---------------------------------------------------------------------------------------------------------------------------
LT215/70R16 | 215 70 16 | 27.9 -15% | 1340   | 1475   1600   1765 C |                      |                      |
LT225/70R16 | 225 70 16 | 28.4 -13% | 1435   | 1575   1710   1875 C |                      |                      |
LT235/70R16 | 235 70 16 | 29.0 -12% | 1525   | 1675   1820   1985 C | 2095   2230   2335 D |                      |
LT225/75R16 | 225 75 16 | 29.3 -11% | 1500   | 1650   1790   1940 C | 2060   2190   2335 D | 2440   2560   2680 E | 3000 F
LT285/60R16 | 285 60 16 | 29.5 -10% | 1750   | 1920   2085   2270 C |                      |                      |
LT245/70R16 | 245 70 16 | 29.5 -10% | 1620   | 1780   1935   2095 C | 2225   2365   2535 D |                      |
LT315/55R16 | 315 55 16 | 29.6 -10% | 1875   | 2020   2195   2405 D |                      |                      |
LT255/70R16 | 255 70 16 | 30.1  -8% | 1720   | 1890   2050   2205 C | 2360   2510   2680 D |                      |
LT215/85R16 | 215 85 16 | 30.4  -7% | 1495   | 1640   1785   1940 C | 2050   2180   2335 D | 2430   2550   2680 E | 3000 F
LT245/75R16 | 245 75 16 | 30.5  -7% | 1700   | 1865   2030   2205 C | 2335   2480   2623 D | 2765   2900   3042 E | 3415 F
LT265/70R16 | 265 70 16 | 30.6  -7% | 1820   | 2000   2170   2335 C | 2500   2655   2835 D |                      |
LT345/55R16 | 345 55 16 | 30.9  -6% | 2150   | 2325   2525   2755 D |                      |                      |
LT275/70R16 | 275 70 16 | 31.2  -5% | 1925   | 2110   2295   2470 C | 2640   2805   3000 D |                      |
LT265/75R16 | 265 75 16 | 31.6  -4% | 1910   | 2100   2280   2470 C | 2625   2790   3000 D | 3105   3260   3415 E |
LT235/85R16 | 235 85 16 | 31.7  -3% | 1700   | 1870   2030   2205 C | 2335   2485   2623 D | 2765   2905   3042 E | 3415 F
LT375/55R16 | 375 55 16 | 32.2  -2% | 2470   | 2630   2855   3085 D | 3285   3490   3750 E |                      |
LT305/70R16 | 305 70 16 | 32.8   0% | 2270   | 2465   2680   2910 D | 3085   3275   3525 E | 3670   3830   3970 F |
LT285/75R16 | 285 75 16 | 32.8   0% | 2130   | 2340   2540   2755 C | 2925   3110   3305 D | 3465   3635   3750 E |
LT255/85R16 | 255 85 16 | 33.1   1% | 1920   | 2110   2290   2470 C | 2635   2800   3000 D | 3120   3275   3415 E |
LT315/70R16 | 315 70 16 | 33.4   2% | 2405   | 2590   2815   3085 D |                      |                      |
LT295/75R16 | 295 75 16 | 33.4   2% | 2240   | 2460   2670   2910 C | 3070   3260   3415 D |                      |
LT315/75R16 | 315 75 16 | 34.6   5% | 2535 C | 2715   2950   3195 D |                      |                      |
LT375/65R16 | 375 65 16 | 35.2   7% | 2910   | 3165   3435   3750 D |                      |                      |
LT355/70R16 | 355 70 16 | 35.6   8% | 2910   | 3115   3380   3640 D |                      |                      |
LT365/70R16 | 365 70 16 | 36.1  10% | 3000   | 3250   3530   3860 D |                      |                      |
LT325/80R16 | 325 80 16 | 36.5  11% | 2755   | 2970   3225   3525 D |                      |                      |
LT395/70R16 | 395 70 16 | 37.8  15% | 3415 C |                      |                      |                      |
---------------------------------------------------------------------------------------------------------------------------
LT235/60R17 | 235 60 17 | 28.1 -14% | 1375   | 1510   1640   1765 C | 1890   2005   2150 D | 2235   2345   2470 E |
LT245/65R17 | 245 65 17 | 29.5 -10% | 1575   | 1730   1875   2040 C |                      |                      |
LT285/60R17 | 285 60 17 | 30.5  -7% | 1820   | 1995   2170   2335 C |                      |                      |
LT245/70R17 | 245 70 17 | 30.5  -7% | 1690   | 1855   2010   2205 C | 2315   2460   2600 D | 2740   2875   3000 E |
LT245/75R17 | 245 75 17 | 31.5  -4% | 1770   | 1945   2110   2270 C | 2430   2595   2755 D | 2900   3050   3195 E |
LT265/70R17 | 265 70 17 | 31.6  -4% | 1890   | 2075   2255   2470 C | 2595   2760   2910 D | 3005   3100   3195 E |
LT265/70R17 | 265 70 17 | 31.6  -4% | 1890   | 2075   2255   2470 C | 2595   2760   2910 D | 3005   3100   3195 E |
LT235/80R17 | 235 80 17 | 31.8  -3% | 1725   | 1895   2055   2270 C | 2405   2545   2680 D | 2815   2950   3085 E |
LT345/55R17 | 345 55 17 | 31.9  -3% | 2270   | 2410   2620   2835 D |                      |                      |
LT255/75R17 | 255 75 17 | 32.1  -2% | 1870   | 2055   2230   2405 C |                      |                      |
LT275/70R17 | 275 70 17 | 32.2  -2% | 2000   | 2195   2380   2600 C | 2760   2925   3085 D | 3120   3160   3195 E |
LT305/65R17 | 305 65 17 | 32.6  -1% | 2205   | 2385   2590   2835 D | 2955   3075   3195 E |                      |
LT285/70R17 | 285 70 17 | 32.7   0% | 2105   | 2315   2510   2755 C | 2890   3070   3195 D |                      |
LT305/70R17 | 305 70 17 | 33.8   3% | 2405   | 2605   2780   3000 D |                      |                      |
LT285/75R17 | 285 75 17 | 33.8   3% | 2210   | 2430   2640   2835 C | 2955   3075   3195 D |                      |
LT315/70R17 | 315 70 17 | 34.4   5% | 2535   | 2685   2915   3195 D |                      |                      |
LT315/70R17 | 315 70 17 | 34.4   5% | 2535   | 2685   2915   3195 D |                      |                      |
LT355/70R17 | 355 70 17 | 36.6  11% | 3000   | 3065   3130   3195 D |                      |                      |
So, using the table above, I see that for a 2800 lb loaded tire, I should be using at least the following PSI for the given tire size. The pressure values below are the absolute minimums, so realistically I'd be adding some margin of error:
  • LT285/75R16: 51 PSI[/*:m:2v6nfm5y]
  • LT295/75R16: 48 PSI[/*:m:2v6nfm5y]
  • LT305/70R16: 48 PSI[/*:m:2v6nfm5y]
  • LT315/70R16: 45 PSI[/*:m:2v6nfm5y]
  • LT315/75R16: 42 PSI[/*:m:2v6nfm5y]

-- Geoff
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Old 03-18-2014, 12:00 PM   #10
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Re: SMB Tire Pressure ???

Thanks geoffff.

While I agree with anzac that's it best and easy to use load tables, not every manufacturer publishes them. BFG doesn't for KOs. I used the manufacturer association ones from the octopup site geoffff linked. The relationship is not necessarily linear, but every other LT tire I looked at seems to be very, very close, usually within the margin of what I can measure with a consumer pressure gauge.

And even tire rack can be idiotic. Their first response to my query was that no one published load tables for modified vehicles. You can find more info on setting pressure based on load and weighing for the RV and big truck tires, but squat on LT tires. BFG had me call them; sorta got the impression they couldn't point me to any published load tables. Guy there said my pressures were fine for the weight given.
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