It might be better to keep the internal speed computations as-is (measured in 60 per second), but have anything that changes x/y/z (e.g. "x += xspeed") take the delta into account instead ("x += xspeed*delta"). Changing the internal speed computations won't feel right until you change every acceleration, friction, maximal speed, etc... if you just change the "make speed affect coordinates" bit there's a lot less things that needs changing.