Here's what I did, think its been done before, but what can I say? I'm proud of my last trip to the hardware store. I have two dual cell racers (shens) that I widened the footprint of the rear wheels on. Instead of doubling up two stock wheels on each side of the rear axel-- I glued up one car with some aluminum collars I found, and the other with some threaded brass caps that matched the diameter of the stock wheels. I simply left the stock wheels alone and glued these 'extensions' onto them with super glue gel.
Here's where I did sometin nifty, I then got some surgical type tubing and cut new fatty tires from it and stretched over my new widetrack wheelsets. Then I got some 100% black silicone and coated those new 'tubing-tires' . By simply placing the fully charged car upside down, I applied the silicone as I drove the rear axel by the cars own power. With the silicone caked onto the 'tubing-tires' I then smoothed it all out with a piece of mylar as I spun the wheels ,once again, with the car's power. get the mylar wet first so the silicone doesn't stick to it though.
Anyway, my dual cell racers stick to the course now (silicone tires) and don't flip anymore on high speed turns due to the widened rear wheelset. Also because my batteries are internal, and the fact that I used metal hardware for the wheel extensions--these cars have a very low center of gravity. I'll try and attach a picture but don't laugh at my paint jobs if it shows up!

--and I know you guys are going to tell me, 'hay that's not a mitsubishi!' or 'hay that's not a toyota!' but who cares, those were the only stickers I had--dont have a clue what body style I have.
Cheers!