Yet another gearing question

Okay, I have a WR250X. I bought a Warp9 off-road wheel set-up. Currently I'm running a 112 link chain with a 13/47 set-up on both the 17" X wheels as well as a 13/47 set-up on the off-road wheels.

My question is, while keeping the front at a 13 tooth, how much smaller can I go on the rear sprocket when using the 17" X wheels and 112 link chain? I'm finding out the 13/47 set-up on the street isn't giving me enough top end.

Thanks for any help!

Put a 14 on the front, I run 13/49 (112 link) off road R wheels and 14/44 (110 link) on road with X wheels. At 14/44 stock speedo is almost bang on.