18 inch tamers and ground clearance??

[kineo3]

My question is two fold. First how much actual ground clearance will I have with 18" tamers on 8" rim. I currently have 20" razr on 9" rim And there certainly isn't a heck of a lot of room. I also would like to know how these would match up with 21" razr fronts. I am using this set up to race mx. I am just trying to get out of buying new front tires. This set up would go on a 2000 400ex. If I do need new fronts should i get the 19" or 20"?

[GNCC#829]

If you are going 2 inches smaller for the rear tire, you should have one inch less ground clearance.