dualsport conversions

The XR600 is fine on the road, it goes over 80 mph and is pretty comfy. It is a bit of a bear off road mostly because of it's weight. Depends on where you live really. Out west the big XR is great fun and a pleasure to ride under most all circumstances. However, in the tight East trails it can be a bugger to move around. It is a great bike and a lot of fun no matter where you are, just a little big for my taste on the East. I personally don't feel comfortable riding a 400 on the street, mostly because I like a lot of street power for hairy situations.

It all depends on how comfortable you want to be on that 20 mile ride. If it's a nice twisty road, even a 250 will be enough. If it's a flat, straight thing -- then go big.

Even an XR250 with proper gearing will happily buzz along at 60+ mph for 20 miles. Unfortunately, the comfort factor for the rider is pretty much gone after about 15 minutes on the highway. Something about the engine vibration and the pavement makes your rear fall asleep. It's uncomfortable!

A DR350 or an XR400 will have about the same effect, but would definitely pull taller gearing if required.

The XR6 will probably be the most comfortable on the road, but as stated before, may be a handful on the trails.

Unfortunately, finding a happy commuter bike that is also a good, aggressive Dual Sporter is not easy.

You'll have to decide what's more important, fun on the trails or fun on the blacktop!

Ditto the above. While I'm comfortable at freeway speeds on my XR4, I wouldn't want to do it a lot. My XR4s are set up for "95% dirt/5% street". We ride at 7000-10,000' and the XR4s run very strong (after proper jetting).

Yap...

All of the above. My XR is 100% dirt with a license plate and brake light. When I ride it on pavement I am always happy to get back on the dirt.
Depending on gearing it will do 70 to 75 easy, I run a 14 tooth front sprocket and 55 to 60 is comfortable but like I said, the comfort goes away fast:D