[2500hdon37s]

Same reason anybody changes the gear ratio in any vehicle. Performance reasons.

Gearing is nothing more than simple mechanical advantage. In my truck I went with a lower (numerically higher) gear ratio to make it easier for the engine to turn the larger and heavier tires. This returned it to a factory like final drive ratio. On a motorcycle it is common to swap to higher gear ratios to lower your engines RPM at highway speeds in your top gear to achieve better fuel mileage for long distance riding. I am lowering the gear ratio of my bike to get better acceleration and raise the RPM in my final gear so that the gear is actually useful and not lugging the engine at the highway speeds I run.

So if you're happy with the current gear ratio, there is no reason to change. I think depending on your riding style and what you expect out of the bike determines what gear ratio you should go with.

 

[2500hdon37s]

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.

larger 18in rear wheel wont have much affect on the final drive ratio as long as the new tire is the same diameter as the old one. You probably went with a smaller aspect ratio I'm assuming?