[GB6049]

The 4*timing mod, bean box mod, and switching to a 38 tooth sprocket were some of the best and first things I did to my 06 Spirit. While the changes to the 18" rear wheel may have additional impacts in the ratio but, I have never had any problems riding 2 up in the mountains of Virginia/W. Virginia or solo through the Swiss/Italian/Austrian Alps on take off or up hills with the stock clutch springs. While I don't try to smoke the tires on take off, it is quick enough for a cruiser for those that are into that sort of thing. On the highway (autobahn) it cruises easily at 80mph and downshifting to 4th has enough pickup to pass when needed.

 

[GB6049]

Not a big selection for 18" tires to fit the 5.5 rim. Most are 50 or 55 in either 200 or 180mm. Running the Metz 200/50 with a overall diameter of 25.91" compared to the stock 160/80-15 with a diameter of 25.43", so you are right the differences are not much.