When I was researching making an MX bike street legal, I came across a lot of strong opinions on why you shouldn't do it, often from people who had never actually done it. We've all run down the street to see what it's like, but I would not give someone my opinion on the matter based on a few quick blasts down the street...
Below are my thoughts and experiences, and some suggestions, on riding a street legal MX'er.
First of all, it's FUN, and that is what this entire project has been about for me. I wanted a bike that would be fun on the trails and open fire roads, easy and cheap to maintain, and easy to handle/lightweight. An MX'er fits the bill perfectly. I also wanted something tough, that I didn't have to baby on jumps, or at all really.
I generally run between 30-40 mph around my subdivision, but I will open it up on trails.
Now for the reality... A CR250 on a highway (forget freeway) at 55-60 MPH, for more than 2-3 miles, would be scary. Riding under 50 or so for a few miles really isn't bad at all. I've ridden for an hour straight at 35 mph or under (cough), all street, and it was a a lot of fun.
Reality check #2, and this is for all of you thinking about dual sporting your bike without turn signals (like I did). People DO NOT understand hand signals. This shouldn't be suprising though. People can't even grasp that the big sign that says "Slower Traffic Keep Right" means that they, EVEN IF TURNING IN A FEW MILES, should NOT be in the left lane if there is traffic moving faster than them. Why I assumed that someone would understand that holding my left arm out straight, when slowing or sitting at a stop sign, means that "hey, I'm turning THIS way", is beyond me. I have yet to not be pulled out in front of at an intersection using hand signals.
Back to riding. With different gearing, and tires that are slightly more street oriented, an MX'er would be a bit more at home on the street, but I still wouldn't recommend one for extended street use. For a short trip, or at speeds below 50mph, they are a blast. Knobbies are great in the dirt, mud, and sand, but they like to "walk", and sometimes give the sensation that they are flat, on the street.
Even running at 30-35 mph, it quickly becomes obvious that an MX bike is tuned/made for accelleration, not stready speed runs. However, having almost "sport bike" like acceleration on the street and being able to hit trials, or an MX track on the same bike is really cool.
From my house to local trails that connect to various streets in my neighborhood, my setup works great for me. Even on days that I don't want to get my bike muddy, just riding around my neighborhood for an hour is a lot of fun. Not having to load my bike up in my truck, unload it, change into riding clothes, and repeat after the ride, is well worth having the bike street legal. I was actually kind of looking forward to being stopped, but none of the police officers I have seen ever gave me a second look. I thought I'd get stopped just for the sheer oddity, but I haven't been.
I can put on my riding clothes, hop on my bike, ride to the trails, ride to the gas station, then back to the trails. For now, I wear a backpack and I keep a cup (with measurement marks) in a ziplock bag, along with a quart of premix. I also keep my kickstand in the backpack. I can easily pull up to a pump, get out the oil and kickstand, fill up, add premix, and put everything back in my bag in no time. It's a LOT more fun than loading the bike up, or taking a gas can to the station and back. It's also much easier. I don't have to keep any extra gas in my garage either. If you do this, be sure not to fill your tank too much and give it a good shake before you go anywhere to mix the gas and oil.
Overall, for a trail connecting bike, or an exciting trip to the corner gas station, I would highly recommend you make your MX'er street legal. For extended riding over 50mph or anything at highway speeds, other than on a track or trail , get a purpose built Dual Sport, or at the very least, a larger displacement 4 stroke.