Great buy on the lumber. I can never seem to find anything useful that is used and at a good price. I've been looking for a good tiller for about six months and the used ones just aren't that much cheaper than a new one from TSC.
My F13D came with ag tires and was rough on the yard and kind of bouncey on pavement also. I think my rear wheels are 16 inch, and I got a set of used pickup truck tires and mounted them on the 16 inch rims. I still have the ag tires on the front, but think I might change them to a turf type tire because the ride is still bouncey on hard ground. The rear tires are great though, and it's a cheap fix as opposed to buying tractor turf tires. Used tires were about $20 each, including mounting. I don't do a lot of tough things with the Yanmar, but the tires have stayed on the rims just fine and I've it this way for about three years.
The one thing you need to make sure of, is that the perimeter of the tires you get are the same as what you replace. This is important in 4 wheel drive in that the perimeters of the front tires and rear are relatively matched, through front/rear axle ratio to work together.
More Great info Thanks Ron. I never even considered using other tire. I just had to look & the rears are 8/16 s . so I might look into that . Thanks again , Bob