The model of the Speedo DRD is Y1 for the WR250R. You should be able to get the speed to read correctly, but the odometer will read slow. The two do not seem to be synced up for some reason.
I'm currently running 13/48 gearing, and I think it's about perfect for most of the riding I do with the WR, which is mountain and forest fire roads. I'm running a 120/90-18 back tire, which is a little taller than stock. The only time I really wish I had taller gearing is when I'm in sixth gear going down the highway. I'm pretty much stuck to going 60 MPH unless I want to rev the engine higher than I think it should be for cruising. I'll probably go with a 47 tooth rear when it's time to replace them, as I remember that setup working well before.
Lee