[blykins]

Yes, just count the number of revolutions it takes on the driveshaft to turn the wheel over one revolution.

If I were you, I'd sell your existing stuff and get the 600. I've heard of those transmissions lasting forever, and I've heard of them breaking easily. I wouldn't take a chance. They're rated for 400-425 lb-ft and if you ever hook up once, you could break some parts.