I've tried everything as an slippery indoor park wheel.
The speed vs. grip ratio always get's ya. They are either really grippy and not that fast or really fast and not that grippy.
Lucky for you the new Spitfire Formula Four 99's are exactly what you want.
Honestly they are the best indoor wheel I've ever used, the grip vs speed thing is right on!! They nailed it!
So far they just can't be beat as an indoor wheel.
So until Bones makes a slightly softer hybrid park wheel (it's coming), that still has speed and has alot more grip than the SPF's go w/ the Spitfire F4's! (this is coming from a lifer Bones guy too, so believe me when i say go w/ the F4's).
Bones SPF's are still the best outdoor/concrete park wheel you can get, they're the fastest and actually do grip on smooth but porous concrete surfaces. My personal fav wheel's though are a still good worn in STF's on everything. Street, indoor, concrete etc. Just my fav wheel. I got used to the slip indoors, you just learn to make it work for you after a little bit. Sweep your ramps and bowls!
The Rainskates 98a neon Yellow formula wheels are another good park wheel as well, the 95a Green ones are a little softer and have more grip, but is a little slower too. Just try the Formula Four 99's, they make all this nerd talk pointless, because they win over it all.