gearing for 18" rim

[Singletrack_mind]

I'm thinking about building up a set of dirt rims for my GS800 so I can quickly swap from street-appropriate tires on the OEM wheels to full knobbies on an 18" rear and a 21" but narrower front. My question here is about gearing. The 18" rim with a knobby on it will be a bit bigger in diameter than the OEM setup, so I would need to lower the gear ratio via a larger rear sprocket or smaller countershaft sprocket just to maintain status quo. Ideally though, I'd want even lower gearing for the dirt so I could use 2nd gear more of the time; the stock gearing is a bit too tall in the dirt even with a 17" wheel.

I want to do this without exceeding the adjustment range on the chain tensioners so that swapping wheels is hopefully just a case of swapping wheels that have different sprockets. If I need to though, I could swap countershaft sprockets at the same time.

The stock gearing is 16/42. I'm wondering about leaving it that way for the street, and going to something like 15/43 for the dirt.

Has anyone else done this? Can you comment on what gearing choices you made?

Thanks!