John, same tires all around, and I think they originated from the Pajero jeep kit. The front rims come from the Hummer and the rears don't I know what they're from. The inner rims are a bit narrower than the outers & are bolted together. The hollow tires are a very hard rubber & they grip really well, especially with each one packed solid with air rifle BB's, I think it's more than a lbs per tire, adds a lot of weight to the trk. The trk gets around great when it's empty.
I tried on Tower Hobbies last yr to find more of these tires, but they didn't have them... might be discontinued. But here's the narrow & wide rims
http://www3.towerhobbies.com/cgi-bin...?&I=LXGL03&P=7
http://www3.towerhobbies.com/cgi-bin...?&I=LXGL04&P=7
Wild Willy put me onto to this combo a decade ago and has a write up about them on his website.
http://www.customrcmodels.com/id306.htm
These tires are quite wide dualed up; if you were eyeballing them for your dump trk, they'll stick out 25mm each side (225mm vs 175mm stock). The box I got from Reg is 230mm width.
Cheers Mark.
When I get my garage/workshop done hopefully by summer, next winter my hobby should be in full swing again with regular updates of new projects & new activities.