For your Pred-90, you may want to give Billy Holt at WRH Racing a call. His site is: http://www.wrhracing.net/home
Here's the high level scoop:
- Replacing stock steel rims with light weight aluminum rims does help. Going smaller rims / rubber creates better low end power but over taxes the top end. Thus, lower top end ground speed.
- Many like the K&N, Uni and other foam filters. Some don't like K&N because it "might" let too much dust in. Pick the correct air filter that's best for your riding conditions.
- If you install a different air filter, you will have to rejet the carb. Most people upgrade the main jet. In the Pred-90, you have to drill the carb if installing a larger then 90 main jet.
- Some say the 16mm stock carb on the Pred-90 isn't the best for engine mods. Some can stablize the jets but others have a hard time when installing larger then 90 jet. Especially folks in extreme weather change climates (like myself). Something to do with low impulse pull. Many would recommend upgrading to a 20mm carb. I've been told many times that a 20mm carb on a modded Pred-90 engine handles climate and altitude changes much better. If you call Billy, you may want to ask him about this.
- Another option is to change the sprockets instead of the tires. On our Pred-90, we installed an 18T on the front and 39T on the rear - with a HD O-Ring chain and Spring loaded chain tensioner. For our technical trails that don't go above 24 mph, this was a great upgrade (for our riding conditions). Although, lighter weight rims would be much better.
Hope this helps...