I had my front tires filled with a polyeurathane? foam at a forklift tire shop. It added a lot of weight to the tires and let me stop worrying about all the scrap metal in my yard. You can drive a nail into them with no effect. Only thing is the rims aren't likely to be re-usable when the tread wears out. The extra weight also helped control steering with a rear mount snowblower. The amount of foam depends on the air pressure rating so you don't lose all the cushioning effect of pneumatic tires. My fronts now weigh 62.5 pounds each. I didn't do the rears due to the size. Extra cost,weight if I ever have to remove it, and my back didn't like the thought of a harder ride, and I am quite happy with my boxblade as a counter-weight for the FEL.