We've had a variety of extensions and simulators. The lug nut variety of simulator we had required a really thin flat washer between the simulator and the simulator lug nut cap keep it from flapping around. Otherwise it was a nice design but I had to search all over to find that special washer. The grip type simulators fell off after a couple hundred miles, so that was a waste of money. Currently I've sprayed the rims with grey metallic paint and it looks just as nice so we'll stay with that solution (at least until the can runs out).
We're well satisfied with Firestone transforce HT, and keep them aired up right at Discount Tire stores.
We have straight metal extensions for the inners and the U style for the outers, but the simulators usually have to get removed, and sometimes the tires too. The fronts have plastic straight extensions and they work just fine.
