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Old 06-12-2008, 05:55 PM   #11
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I found this webpage that discusses tire pressures for RVs, etc. It has links to a large pdf file that has tables from Goodyear, Michelin, and someone else. I am in the boondocks, so my connection is too slow to see the file. It may have some decent info. My Ford TPMS started working again about 50 miles after I did the reset.

http://www.dmbruss.com/images/FullTimin ... ochure.pdf

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Old 06-12-2008, 06:30 PM   #12
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Quote:
Originally Posted by Steve_382
Ford dealer said the easiest solution was a piece of tape over the light.
The problem with this is that it also pretty much covers up the overdrive light as well.
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Old 05-11-2013, 04:54 PM   #13
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Re: Tire Pressure - Highway

Forgive me for tacking onto this ancient thread, but this is topic is a good match.......

For highway driving, I've been running about 50 PSI front, and 55 PSI rear. Not sure how I came across these numbers.

Anyway, I see from somewhere on the web that my tires (BFG AT KO LT285/75R16D) are rated as follows:

Code:
PSI	Tire	Axle
   	Load	Load
---------------------	
35 	2130	4260
40 	2350	4700
45 	2540	5080
50 	2755	5510
55 	2925	5850
60 	3110	6220
65 	3305	6610
My van weighs in as follows:

Code:
Tire 	Weight
---------------------	
FL   	2100
FR   	1950
RL   	2800
RR   	2650
---------------------	
TOTAL	9500

	
End  	Weight
---------------------	
Front	4050
Rear 	5450
---------------------	
TOTAL	9500
If I follow this guide, my tire pressures should be more like 35 PSI front, 50 PSI rear. That sure would give me a cushier ride! Would this be insane?

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Old 05-11-2013, 05:22 PM   #14
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Re: Tire Pressure - Highway

This is copied from old thread about tire pressure:

The best method to calculate tire pressure requires weight measurements for the front and rear of the van, even better is each individual tire weight measurement. Then calculate how much pressure you should be running. This from an earlier thread:

Old school answer:

The pressure rise from cold tire to hot tire should be 4-6psi. Less than 4psi rise means the cold pressure is too high, more pressure rise means the cold pressure is too low.

Best answer:

Get the actual weights on the tires and ask the manufacturer for the proper pressure for that weight.

My method:

Again, get the actual weight. Compare it to the max weight on the tire. Do some math.

Actual weight/Max weight X Max Pressure = Desired pressure

If you have 2500# on a tire rated at 3000#@80psi you get:

2500/3000 = .8333 .8333 x 80psi 66.66psi, which I round up to 70psi


Then I always round the pressure up. Much better to err on the side of too much pressure than on the side of too little pressure.

As a general rule, D and E rated tires carry the same load at the same pressure. The big difference is E rated tires can handle more pressure, hence more load.

My van weighs in at 9800#, fairly evenly split front/rear side/side. The tires are LT275/70-18E rated at 3640# @ 80psi. For 4 tires, that would add up to 14,560# at 80 psi. My math says I have 9800/14560 of full load. That is about 67% of the max tire ratings. 67% of 80psi is about 55psi. I run 65 psi. A good safety margin. I will never run a tire at its full capacity. Just too much at stake for me.

4 tires rated at 3000# would be too close for me. Going down the road with some lean to the road, the outside tires could be overloaded for many miles. I wouldn't do it. I always want at least a 10% safety factor, 20% is better. Hence, my 65psi in my tires.

I always round up with tire pressures since higher pressures (within the max tire ratings) will reduce tire temps and might tend to wear the center of the tread. Lower tire pressures will cause higher tire temps and can easily cause a tire to fail.

Mike
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Old 05-11-2013, 05:54 PM   #15
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Re: Tire Pressure - Highway

Thanks for the sanity check. Your linear approximation is close to the chart given by BFG (which is not quite a straight line), but ends up being bit more conservative. Using linear ratios like you suggest gives me 40 PSI front, 55 PSI rear.

Adding 20% would give 48 PSI front 64 PSI rear.

In any case, what I usually do is check the tires to see if they're heating up. Perhaps I should run my fronts a bit lower and my rears a bit higher than I do.

This is for my van already loaded up with stuff, by the way.

-- Geoff
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Old 05-12-2013, 10:13 AM   #16
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Re: Tire Pressure - Highway

It's definitely no good idea to add 20% security inflation. Your breaking distance will be much increased, transversal adherence reduced compared with right inflated tires and the tire life (untill thread depht will be less than legal) will decrease up to 30%! So you're very rich if you can do that !

Remember : load rating given by the manufacturer for a specific pressure is at permanent maximum speed (shown by speed index), a situation that you will probably never know. Moreover, load/pressure ratings published by manufacturers allready includes security margins.

Since I work many years in tire business, I prepared a paper about tires for this forum, which should be shortly published... Hope it will help to change some very common belief which are mostly wrong...
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Old 05-12-2013, 11:18 AM   #17
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Re: Tire Pressure - Highway

Yes, the overinflation can be a bad thing. For example, on my street car in autocross when I want the rear to slide (more oversteer) I just raise the pressure a bit, maybe not even 20%. I can feel the difference in the next run. You aren't pushing vans that hard, but the principle is the same, and in a sharp maneuver it might can unwanted effects. And I dunno what it gains you, besides maybe less rolling resistance and hence slightly better mileage. I'd prefer to err on the side of safety.

I saw this first hand on my van recently. Unbeknowst to me, my helpful mechanic shot my rears up to max pressure for a smog test (maybe required by law?). Normally I'm at about 50; this was 65. I was wondering why the rear end was all over the place and beating me to death. I even started inquiring about new springs...then I checked the pressure. Whoa! Duh! I'm not used to checking as often as I do with the street car, but now I will.

Looking forward to the article, thanks.
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Old 05-12-2013, 08:31 PM   #18
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Re: Tire Pressure - Highway

Interesting conversation here, I hate that I never saw this thread until now. I just wanted to add that since Ford vans have VERY unusual handling issues, especially the older vans, this can be manipulated with tire pressure very easily to improve handling. I have an 02 EB now which handles great compared to my previous 92. I do realize they changed the front suspension pretty drastically in the late 90's, but I'm no expert on what they did, I just know it worked. My wife wouldn't drive our 92. It was all over the road. Tons of play in the wheel and horrible floating around even after I had the steering box changed, new Bilsteins, The ambulance steering damper added, etc. I fixed all this stuff not once changing my tire pressure, which I've generally always run at pretty close to max cold pressure (on the sidewall). I dropped the pressure severely very shortly before I retired the van and the handling improved dramatically. IIRC, I went from 75 to 50-55 and it was MUCH better. What I learned (the hard way) is that adjusting down for lighter loads helped alot. Before that I always thought running at max cold pressure would give me the best MPG, but that's not always the most important thing.
I'm sure most of you 4x4 guys that adjust pressure often know all this already anyway.

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Old 05-22-2013, 09:01 AM   #19
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Re: Tire Pressure - Highway

Just thought I would throw in my recent experience also. Just had new tires put on, BFG AT/KOs. The dealer put 80psi in the rears and 60psi in the fronts. We could hardly keep it on the road. Felt like the back end was sliding around and the van just couldn't find a stable place on the road. Lowered the pressure to 55 all around, which is what I calculated based on my just under 2,000 pounds per wheel loading the van drove much better.
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Van Weight 8,100 pounds, added one rear leaf spring, BFG AT KO LT265/70R17 E Tire press 50psi.
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Old 05-22-2013, 10:25 AM   #20
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Re: Tire Pressure - Highway

Quote:
Originally Posted by Steve_382
Just thought I would throw in my recent experience also. Just had new tires put on, BFG AT/KOs. The dealer put 80psi in the rears and 60psi in the fronts. We could hardly keep it on the road. Felt like the back end was sliding around and the van just couldn't find a stable place on the road. Lowered the pressure to 55 all around, which is what I calculated based on my just under 2,000 pounds per wheel loading the van drove much better.
How did you "calculate" it? You can't just take pressures and scale them based on load. You have to use the Load Inflation Tables for the size tire you are running, matched back to the original load carrying capacity of the manufacturer's original sizes and inflation pressure.

Read more here:
viewtopic.php?f=3&t=1582&start=22

Just to be clear, the brand of tire doesn't matter. Just the original size, vehicle manufacturer's original pressures, and your new tire sizes being inflated to the same or slightly greater load carrying capacity.
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