OK, in front of a computer.
This is really a four part answer.
1) 33 12.5's are a poor size regardless of the car on most surfaces.
This is due to the height/width ratio. (Thats is, they're short and wide)
On road, wide tyres have high rolling resistance, so whilst they might have high dry road grip, they are doing this with a high ratio of rolling resistance (and aerodynamic drag) for that amount of grip, because they're short relative to that width.
Off road, for a given amount of forward grip, 33 12.5's are inefficient because generally you have to deform the surface, (that's surfaces with yield) or remove dry or slick material off the surface (hard base/loose top covering) in order to generate traction. A tyre thats wide compared to its height has to push this material out of the way, compress it, or clear it out of the tread in order to generate traction. Think about that - A wide tyre has to work harder than a narrower tyre to achieve the same grip - more material needs to be shifted before you can generate traction. Ultimately that means more torque (at low speed) or more horsepower (at high speed) is required to achieve the same outcome as a narrower tyre.
Now think about steering response for a tyre in a surface with yield - it's partially operating as a rudder - presenting a turned face to the ground to achieve control. a 33 12.5 has the "squarest" footprint (especially as a radial) so turning the steering does almost nothing to direct the footprint. This along with a range of factors leads to 33 12.5's having peculiarly poor steering response on slippery surfaces.
2) 33 12.5's are an especially bad choice for a sierra.
There's a few reasons here. Firstly, they're a pain to fit, because the width is difficult to deal with. It's hard to run them on rims with appropriate offset (which I consider around 0 on an 8" rim) because of spring and inner guard clearance. So, the common "solution" is to run them on heavily offset rims. This significantly increases scrub radius, which along with a host of handling effects, also flogs wheel bearings, increases the guard cutting required around the headlight bucket and firewall, and, crucially, makes the car wide. This looks like a win (more stability) but is a loss overall, because a sierra that's as wide as a bigger car falls into their holes offroad, line choice is reduced, and because you're now sitting in the bottom of everyone else's ruts, meaning your diff clearance now becomes a bigger issue.
Next is ground pressure. A Sierra is light, maybe 1/3 of the weight of the typical car running 33 12.5's. Traction is a function of ground pressure on all surfaces except dry sand. Reduce ground pressure and you've actually reduced traction. This is easy to prove - walk on wet grass with dress shoes and then footy boots and see which one gives the best grip - the overall size and shape of the footprint is exactly the same, but one offers high ground pressure the other low. So, all things being equal, if you go from a 215 75 15 to a 33 12.5, to maintain the same ground pressure you'd need about 1/2 the tread. this isn't feasible for a number of reasons on a road driven car. (about the highest void tyre available commercially is a Silverstone Extreme)
Obviously, all the other issues outlined in point 1 still apply.
3) But why do you run 13.5's on Piggles?
Well, It wasn't always this way. I ran 9/34 swampers, and Q78 swampers (35.5X10) but some local club members were having very good results with the 35 13.5 Krawlers. There are a few reasons. They have an EXTREMELY flexible sidewall, so at 4-5 psi a sierra can "work" the sidewall very effectively. This is unusual. They are made form an excellent compound which works very well in Summer in victoria, and the width, when used with sensibly offset rims (0 or -13 8" rims) keeps the inside edges of the tyres close together - actually about as close together as on a NT Sierra. This allows maximum line choice. However, on slick surfaces they SUCK. Even at 4 psi in my heavy (and long) sierra, I have inferior steering control to a narrower tyre, and very little or no extra grip compared to a 9 or 10" wide tyre. But what I do have is HUGE rolling resistance compared to narrower tyres. I was staggered that in a 12-18" deep mud hole the car felt like it was being strangled compared to narrower tyres - the power just disappeared through drag.
4) Why do 33 12.5's exist at all then if they're so terrible?
The 1970's man, the 1970's. And Southern California. And jeeps.
So there's a couple of things that led the rise of the "floatation" sizes. Through the 60's and early 70's the huge growth of recreational 4WDing was driven out of California. For the dry sand and rock of california, and on small jeeps (remember this is pre gearing and tall lifts) the only way to gain traction was to use wide 15" rims from muscle cars with the largest available highway tyres (as in road car tyres) these were circa 28-29" tall. Incrementally, sizes crept up and manufacturers like Armstrong started to produce offroad specific tyres that mimicked the wide muscle car bias ply tyres already in use, but in more aggressive patterns. The visual appeal of these tyres was picked up by manufacturers, so they were used increasingly by manufacturers through the 1970's. In rock and on soft sand these wide tyres were fine, and led to high take up of the 33 12.5 radials when these came on stream in the late 70's/early 1980's. Never underestimate the effect of fashion. It's why huge rims are cool on road cars (there's little functional reason) and it's why 12.5" tyres are considered cool. The P metric sizes, and the narrower tall fitments came much later for the recreational market, even though in commercial and military use, they never went away.
However, manufacturers and the aftermarket, whilst not necessarily turning away from the 12.5 width, are turning towards tyres with higher height/width ratio. Consider a 37 12.5 isn't a short/wide tyre any more, to say nothing of a 39 12.5 or 42 13.5 - they're positively skinny compared to their height. and for our lighter, smaller cars, the sizes outlined by J--A--C--K are all easier to fit, easier on the car, and offer improved performance across a range of terrain.
Which brings me to my dream Sierra tyre. It's a BF Goodrich Baja T/A KR2, 35 10.5. It's a specialised class 10 offroad race car tyre, and it's featherlight, very soft, and tall and narrow. I can't imagine a better sierra tyre, perhaps with some light grooving to open up the lugs for mud.
