I have a Carlton SP-2000 walk-behind and it will go places the bigger ones won't... but, with a 19", 18-tooth wheel, it takes awhile to do the big stumps. I've found it's easier to grind out the 30" and bigger ones by coming at it from three angles. Imagine a pie cut into three equal slices... you grind out a third, move around the stump and take out another third, then take out the last third.
With the walk behind grinders, it is hard to see what you're doing, so it helps to have someone there to help guide the cutter. My wife does this for me with hand signals, and it helps a lot.
I have seen much smaller units than the Carlton at the equipment rental places, with engines under 10 HP and small cutters. Mine has a 27 HP engine. I can't imagine those little ones will cut a 30" stump in much less than 3 or 4 hours. You get faster as you gain experience using them, but it is a time-consuming process and you have to be careful. If you hit something in the wood or ground that you weren't expecting to be there (bricks, concrete, steel, rocks, kids toys, gravel, glass, cans, bottles, license plates, etc.) it is not only hard on the grinder's teeth, it can be hard on your teeth, too, if it comes flying out of there. Underground sprinkler lines, buried telephone and cable wires, etc. often show up in there. I even found a pile of bones that the homeowner's dog was burying next to the tree. Owner said the dog had died 15 years back, and the tree's root system had enclosed the bone pile. For a minute there, I thought I had found Jimmy Hoffa.
