"Matt Whiting" wrote in message
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That is because you aren't correct. Tire pressure only has a significant
impact on very soft surfaces such as sand, where the extra surface area
helps with flotation. In most snow, it makes little difference.

Actually, we're both incorrect.

My statement was based on a theoretical understanding of friction in which
the friction depends on the force over an area. Since tire pressure
directly determines this, I assumed it had a direct effect on friction.

I found at least one reference that says that physicists ignore the area
over which the force is distributed, for the purpose of determining
friction. It did say that's actually an incorrect assumption, but that it's
"close enough" for most purposes.

I didn't bother to look further to see just how far off this "close enough"
assumption is. The reference didn't go into much detail on that regard.

Beyond that, the same reference also had a discussion of tires on snow,
oddly enough (I wasn't even looking for that specifically). They claim that
increased tire pressure actually *reduces* friction, because packed snow has
lower friction than unpacked snow, and higher tire pressures result in
greater packing of the snow.

So, tire pressure has a very significant effect on tire friction when
driving on snow. But it's opposite what would be the case on a solid
surface. So, chalk that point up for the anti-SUV crowd.

I still think it's a silly argument.

Pete