Just went through this myself in a most painful manner. I bought a b6000DT that failed every test vic laid out in his how to buy a tractor post. The long and short is that I had to replace the front tires, and found to my shock that one of mine was 5-12 and one 6-12. Explained the busted gear in the front end on the 5-12 side at least...
Relevant here though, neither was a 4-9, like tractordata said, and like I dutifully believed my now beloved b6000 should be rolling around on. Spent hours searching for 4-9 ags. Even found a site that purportedly had some. They wouldn't call back or answer their phone though. Thankfully. Found lots of 4-9 tri ribs though, fyi...
I got really confused when I kept trying to buy the front tire by part no. fron tractorsmart and messicks. My manual showed 93016-01520. The new part no. is 70000-00026 but lists it as 6x12 not 4-9.
I was about to post a thread about the issue, but then I took a calculated risk and looked at the service manual. It's the third page after the table of contents, attached here for your convenience. I suppose I should have posted this anyway.
Long and short is that the b6000dt's are geared for a 6x12 front and a 7x16 rear. The 4-9s were stock on the b6000e. I'm assuming that most b6000s we see today are b6000et's, which rolled with 20x8-10s on the front, since most e's I've seen pictures of are rolling turfs. Assumptions make an ass out of u and mption though...
If you're thinking of modifying your tire size, this thread is worth looking at.
http://www.orangetractortalks.com/forums/showthread.php?t=853&highlight=b6000+tire&page=2
I'll try to remember to hit another thread of a guy in the UK who put some bigger titans on his 7001 tomorrow. His math was nice and clear and the tires looked great.