Getting tires for the '97

[James Morse]

	Max load	Max psi	Load/psi	Sticker of oem tire assumed	Expected load
265/75R15	2469​	44​	56.1	30​	1,683.41
31x10.5-15	2270​	50​	45.4	37​	1,679.80

But it just depends what spec you use for the 265 I just picked a likely one.

 

[sgtsandman]

I like your way to looking at it. I'm not sure it gave me a definitive answer but between contact patch tests and the way it feels, and wear, when that happens. Right now my gut says 30 is too low and 37 is maybe what you want in a fully loaded maxed out truck but for everyday use I am tentatively sticking with the 32-34 psi range unless something pushes me to change that. I think at 35 it feels like it kind of wanders and at 32 more secure feel. For what that's worth. I could certainly be wrong.... but I think not horribly wrong.

THANKS.

The best I can say on the 37 psi is maybe.

With the OEM Goodyear tires and the truck loaded to the max, I pumped the rear tires up to 40 or so psi since that was the max they could hold. They seem to do ok. Not great but ok on the highway.

With the KO2s, I bump them to 50 when I'm loaded that much. 1,500# of retaining wall block is a lot of weight for a Ranger, even one with the 1,750# heavy duty springs. And don't forget, I still have the cap on the truck and all that junk under the platform at the same time. SO, when I say it's loaded down, it's loaded down! They do much better at highways speeds. With those springs, it doesn't look over loaded but you can tell it's carrying some weight.

So, you'll have to play it by ear and see what pressure is good for you.

The paying attention to how your truck drives is a good thing. If it feels like it is wandering at 35, then step it down. The psi is definitely too high. These tires will tell you when you are at a good psi range. The stock tires on my 2019 are only at 30 and tire wear on the factory tires has been fine for over 60,000 miles, minus whatever miles were put on the winter set. So, I am by no means telling you you can't go that low. The tires will tell you what they like as long as you pay attention.

 

[James Morse]

OK, thanks. I have the tool box with some stuff in it but altogether I doubt it totals 200 lbs. I carry a bunch of misc stuff behind the seats which probably isn't 50 lbs and that's stretching to the high side. For day trips off road all I'm adding is a cooler and a couple aluminum chairs, maybe a folding table, it still doesn't add up to a hill of beans compared to what you're talking about for loads. So really that all adds up to roughly carrying 4x 150 lb passengers (the "standard") and no payload. FWIW. So, usually, not really much weight added.
The other day I was running 31 psi all around and it drove real nice on dry or wet pavement, felt secure, didn't wander, steering felt good, neither heavy nor wandering, of course it'll probably never feel really heavy with power steering but that's the best I can describe it. And it was good off road. I didn't have any slipping or spinning or sliding.
Where I went I actually had been a good ways up in a Saab Aero (ok, dumb, I know, not much clearance but the front wheel drive gets you there). But it had huge gulleys down the middle of it. Since then it looks like they dumped a lot of dirt on it and it was pretty smooth, some stuff sticking up but a rock sticking up 3-4" doesn't really even come close to clearance. There can be other things like places gravelly and humped in the middle but usually you can find the good place. So that's all good and one would say "mild off-road" but here's the thing it was wet and it makes like a slurry or slime or film of fine mud on top and while I didn't do any tests like I used to do in snow ("how slippery is it?"), it seems that if you are on a steep grade, going up is easier/safer than going down because going up you can just let off the gas and you slow down immediately; going down, if you lose traction, logic tells me you are in for a slide and there's of course no guard rails and lot of places it drops off precipitously so a slide could easily mean you are walking home and probably have a wrecked truck at the least. So I call it then moderate off-road not because of road obstacles but simply the danger of it. I realize I'm wandering the thread here, but it has to do with, what's the grip of the tires on that kind of surface. They seemed good, tread is open enough to not load up with that silty kind of mud. Going down I just go real easy, feather the brakes, keep the speed slow. I probably should have been in 1st (auto trans), thought of that afterwards, might help a little. I did stay in 4x4 going down, not sure if that helps or not.
Certainly if I did ever really load up I'd add air. Sometimes off road maybe I want to air down. Anyway, 31psi might be a bit low if I were hauling stuff or if I were driving exclusively on the highway but I think it might be a good balance all around for what I do, although depending what I see in wear, contact patch, and feel (latter which seems really good right now), I'm not stuck on it. I don't think I'd go lower though unless it was airing down for off road.
I'm still wondering why is underinflated more dangerous than overinflated. I appreciate the help.

 

[ericbphoto]

Under inflated is more dangerous at high speeds. It allows more flex and less control. Also, as the tire flexes, the rubber and other materials inside it heat up. That softens the tire and makes it flex worse. Prolonged high speed use under inflated can cause the tire to fail catastrophically due to this.

At low speeds, no problem. Off-road, lowering the pressure softens the ride, helps the tires conform to the terrain better and increases the contact patch for better traction.

So, the answer is; “one pressure doesn’t fit all situations”. Personally, I can’t stand running my tires at 31psi off-road. It’s too bumpy and less traction.

 

[sgtsandman]

I'm still wondering why is underinflated more dangerous than overinflated. I appreciate the help.

The lower the inflation of the tire, the more heat is generated due to the flexing and straightening of the sidewalls. If the tire gets too hot, the rubber compounds break down and the layers will separate. Once things get bad enough, you can have a blow out. This is at road speeds and not off road where you are generally driving pretty slow and heat isn't a problem.

Too low of a pressure will cause accelerated tread wear on both shoulders of the tread, which I'm sure you already know. Over inflation causes accelerated tread wear in the middle of the tread because the tread is too round and not allowing the full tread of the tire to make proper contact.

An example of this is the Ford/Firestone fiasco that happened a good while back. There is plenty of blame to go around on that one but Ford and the vehicle owners take the majority of the brunt of it. The tire pressure on the Ford Explorers at that time was 30 psi. The tires selected for the vehicle marginally met the needs of the vehicle, especially at that psi. Whether Ford specified the wrong tire or Firestone supplied the wrong tire, I'm not clear on but I think the blame lays on Ford. The part where the vehicle owner takes the blame is that most do not regularly check their tire pressures. The tires were allowed to run long periods of time at too low a pressure and there was a rash of tire blow outs, and I believe some roll overs as well.

Another example, though somewhat unrelated because a good number of trailer tires are bias ply tires, is trailer tires. If the tire isn't inflated enough, the tires will get hot. As in too hot to touch hot. A properly inflated trailer tire will be warm but that is it and you can lay your had on the tire for long periods of time and not feel uncomfortable.

 

[James Morse]

Ahhh. I get it. The heat will increase the psi but what you're saying is it doesn't outweigh the basic problem you describe. Don't want blowout, that would be real bad. Good test, if it's too hot to hold your hand on it after highway driving, that's too low psi.
I remember track tires on the Fiesta you'd air them to be what you wanted hot not cold so maybe start cold 32 psi then check them after running and they'd be like 45 or something. Different situation kind of related.
The tire guy commented KO2 has quite flexible sidewall I guess he thought it was a good thing... I have no idea.
Thanks.

 

[ericbphoto]

The danger of the heat isn't pressure. The pressure increase from heat due to driving is negligible. The danger is breaking down the rubber and weakening the structural integrity of the tire.

 

[ericbphoto]

I remember track tires on the Fiesta you'd air them to be what you wanted hot not cold so maybe start cold 32 psi then check them after running and they'd be like 45 or something.

Might go up to 35. But that would be pretty extreme.

 

[James Morse]

You sure? Maybe my memory on it is shot, entirely possible.

 

[ericbphoto]

You sure? Maybe my memory on it is shot, entirely possible.

Here. I just asked Google for confirmation.

 

[James Morse]

Ok. For race tires it could increase say 8 so maybe if I had (Fiesta) set them at 32 and they were 40 hot that's possible. For sure I remember they were telling me you measure them cold and you also measure them hot (for track).
Interesting to get some quantifiable info around it.

 

[NMC_EXP]

What pressure should I run for road use? I think my original spec for oem 265/75R15's was 30 lbs (no door jamb sticker but pretty sure that's what it was). When I asked what they fill them they said 35 lbs so just wondering should I go with that or is 30 better?

I'm a tire geek. I was a truck stop tire repairman and pump jockey for five years. Then my career was in rubber technology which included some tire R&D.

When you change tire sizes and load rating optimum tire PSI is going to change. Ideally the PSI will result in the tire footprint being flat on the pavement from edge to edge. Wrong PSI = tire wear and poor handling.

PSI too high and the footprint will contact in tread center but not on the edges.

PSI too low and the foorprint will contact on the edges but not the center.

Here is a quick and dirty method to test tread footprint vs changing PSI. (1) Get a large (1" dia,) piece of construction or childs chalk. (2) Inflate cold to your best guess, probably start on the high side. (3) Find a parking lot with smooth, level pavement. (4) Chalk a 3" or so wide strip side to side across the tread. (5) Then drive straight forward 100 feet or so. Examine the remaining chalk on the tread.

If the chalk is gone in the center but not edges PSI is too high. Gone on the edges but not center it is too low.

If necessary, adjust the PSI accordingly and repeat until you have uniform contact all the way across the tread.

Not a perfect solution but better than a SWAG.

There is a test that involves driving several miles to heat the tires then using an IR thermometer to measure tread heat build-up and basing optimum PSI on that factor. Unfortunately I've lost the link to that article.

Jim

 

[NMC_EXP]

Many opioions re: optimium tire pressure. I have mine but who cares?

Whatever it is, check it and adjust it often.

Literally, tires are the #1 interraction between the car and the road. Ignore your tires and you are at risk of bad stuff.