I went out and bought some Airsoft pellets today, gonna install them tonight. They come in at least two different weights (per pellet), I got the heaviest ones they had at Walmart, which is .20 grams per. That's nearly double the weight of the standard Airsoft pellet at .12 grams each, which means less is more- less pellets in each tire. They don't list the weight of the package, so I had to go to the kitchen section and find a scale. Cost was around $5 per 2000 pellet/ 12 ounce package. (The math works out to over 14 ounces for 400 grams, but whatever.

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I found a site for a commercial product that's similar, they have a chart for determining how much weight to add- my tires call for between 8 and 10 ounces per tire (35/12.5-16). They say you're better off with more than less. I'd have to agree, as I tear off chunks of my tires on trails and that can't be easy to balance.
I went with the pellets rather than powder, bb's or golf balls for several reasons. Powders seem to need special valve stems with filters. BB's rust eventually, unless you air up with nitrogen or something, and I've heard they can damage steel wheels and cause them to rust. Golf balls seem too large to evenly distribute the weight- what if you only need half a golf ball? What if you need two- where does the other one go? With hundreds of pellets, they can evenly distribute themselves around the tire. I can't imagine they'd come out of the tire if I blow a bead, as they'll all be in the bottom of the tire-nearly 10 inches away from the bead- and I run fairly low pressure on trails, but I guess anything's possible.
I'll be running some highway miles tomorrow, I'll try to post up my impressions. I have some vibrations that I think are from my tires being out of balance and probably out of round by now (that's what happens if you leave them out of balance too long), so it'll be a good test.
Here's some information I found-
http://4x4trailhunters.com/article.php?id=18 and the Web site with the chart for determining how much weight to add is linked there as well, and here:
http://www.innovativebalancing.com/BigTirechart.htm
-Spike