Modeling tip here. Spinning wheels in FlightGear use the "spin" animation, which is looking for an input using the units "rpm", or rotations per minute. I base the input on a property available in JSBSim called "fdm/jsbsim/gear/unit[n]/wheel-speed-fps". Be sure to change the "n" in the property name to match the number of your gear unit, as defined in the FDM configuration. For most of my models unit 0 is the nose gear, unit 1 is the left main, and unit 2 is the right main, but you should check the FDM config to make sure.
So, how many rpm's does a wheel of diameter "D" (in meters) spin at when moving at "wheel-speed-fps"? I use (5.82 / D). I'm going through all my models now and changing this because I had previously guessed that the spin animation units were degrees/sec, but I was wrong.
BTW, using separate speeds for each wheel allows them to spin realistically in a turn. Zoom in on one of the models in the hangar while it's in a tight turn on the ground, and you'll see the inside main wheel turning at a slower speed than the outside main wheel.