Questions from above:
Q: How about more info how these bikes ride down the road?
A: On the highway you'll be looking for another gear, even if you have gone up a tooth on the countershaft. Most dirt bikes will vibrate quite a bit on pavement; it will be much worse if your wheels are out of round or out of balance. You can improve the balance of your wheels by wrapping some thick bits of solder on the spokes opposite the rim lock.
Q: How far will they go on a tank of gas?
A: Will vary dramatically based on the road/trail, how hard you are pushing it, your jetting and gearing. My '90 would go up to 100+ miles, but I think it had a little bigger tank than the new ones.
Q: How do you fill the bike up with gas when on the road?
A" I would carry little Rubbermaid plastic bottles that I got at Target. Hold about 9 ounces; almost enough for a full tank. I marked ounces on the side. Compact, no leaks, but put them in a zip-loc just in case.
Q: Do you mix the fuel/oil in the tank?
A: Pour a little oil in, add a little gas, etc... If you are cruising on the road, you'll probably not be pushing that hard, so if you accidently mix at 50:1 when you meant to do 45:1, you'll never notice the difference.
Q: How fast will they go?
A: Hard to say for sure, but I'll guess around 70-75 with stock gearing, absolutely wound out in top gear.
Q: How fast will they cruise comfortably?
A: Maybe 50 with stock gearing; 50-55 mph with one tooth bigger countershaft. Sustained 60+ feels like work. If you need or want to do lots of high speed road miles, you will be happier with a 400+ cc four stroke.