250-500cc Sprocket Size

Right on!
I'll check out my tire size this morning when I get moving.

While tire diameter plays a big role in gearing keep in mind that a knobby doesn't change diameter at high speeds like a trials tire will. The trials tire is basically flat across the contact patch and the sidewalls are not ridged, making them very similar to a drag slick. At speed the sidewall will narrow up and the contact patch will round out making it taller by quite a bit. If you don't leave about 5/8 to 3/4 inches it will rub your mudflap and wear a hole in it in about 10 seconds. Obviously if it will grow even 1/2" that is just on the radius so the diameter is increasing over an inch. If you plug that in to your math formula you will see that the circumference will change enough to equal a gear change. The advantage is it's smaller or lower geared when you take off and taller thus higher geared at speed when you need it.

Just one more plus of running
a trials tire -overdrive. Gotta love it.

Right now I'm running the following tires from previous owner:

Front - Bridgestone - M403 90/100/21
Rear - Dunlop MX31 - 110/90/18

I ride mostly South Jersey so the rear tire is fine. I'm switching the front to a Bridgestone M59. I read a bunch of opinions on ECEA site and they say the M59 does well in sand and rocks. I do 1 or 2 rock runs a season, maybe more if things go well.

You really need to try a Golden tyre 90/100/21 AA Fatty

I've got the 90/90/21 GT (non-fatty) and it's is absolutely brilliant. I can only imagine what the fatty is like in the right conditions.

I ride 99% desert and ended up with 14 X 47 and love the easy cruising. I don't do the rock crawling thing so I don't know how it would do. The motor sure chugs nicely though when needed. (11 wr300)

I run 14:48. Came with 13:48 way too short and tried 15:48, awesome on the road but too tall for the bush.
14:48 in the forestry and tight stuff is pretty good. A bitta clutch work in the tight stuff and falls short on the fast sections but on a 5 speed box its the best its gunna be.
Mate on a 2014 pumkin sixdays can pass me while ive got mine wide open then clicks another gear....( bastard!!)
But thats the only time he can get near me... hehehehe

I prefer 13:50 with a AT81 @ 4 1/2 psi with Tubliss for what I ride. I can do 13:48 with just a bit more clutch work but 14:anything would likely smoke the clutch in the tight stuff that I like to ride. I think the over rev is better on my 250 then the 300s I've ridden so maybe that's why I like the lower gear. The trials tire was better on rocks and logs but lacked in mud and sand and I could never quite get the hang of steering with the back wheel because it seemed like it went from hooked up to loose in an instant. If I did a lot of road riding the MT43 would be my #1 choice.