Its the first time I hear of a speed sensor on a quad unless you're running an aftermarket odometer. If that were the case it would calculate your bike speed using a magnetic signal from one of your wheels. If you bought the bike new, it probably didn't come with one unless you installed it yourself. So if that is the one you are refering to the answer is yes, the bike will run w/o it.
There is a speed sensor on the 660. It's in the trans, right side, on the same shaft as the front sprocket. I don't know what it does but may try unpluging it one day.
You may try unpluging it and see if it runs the same. There was a discussion in the 700 section, don't know if they found an answer to what it does or not.
For some reason when you stated speed sensor I automatically thought odometer. My mistake. The Raptor does come with a stock speed sensor which relays the bikes speed to the CDI. It's a sensor that works off a magnetic pulse from a a gear attached to the gears in your tranny. The faster the gear turns, the faster the magnetic pulses (rpms) the sensor detects. I believe it will temporarily shut power off to the CDI if the sensor detects constant high engine speed and serves a safety feature designed to prevent engine damage. When the bike's speed decreases, the CDI begins to operate again. I've read somewhere in this forum that someone has tried messing withe the wires in order prevent the sensor from limiting the bike's speed in reverse.
Yeah, there's been a pretty heated debate about this on the 700 forms, here's the lowdown on the 700 (I can't speak for the 660) The speed sensor is located ON the output shaft of the tranny, and so if your gearing is stock, you can expect to hit mid 70s before the computer retards timing to prevent you from going faster. If you change gearing, then of course you can hit higher speeds before hitting the limit. It doesn't care what gear you are in, but the REV limiter will stop you in every gear except 4th and 5th, long before you hit the speed limiter. In 4th, it's so close to the REV limiter that most people couldn't tell the difference, which is why it gets mis-named the 5th gear speed limiter, because the effects are only felt in 5th gear. Unplugging it on the 700 will allow you to run faster than the mid 70's, however after running for some time (about 8 hours according to some) the check engine light will come on, and the limitation will be effected no matter what speed you are going. However, it should be pointed out that SEVERAL people have NEVER had the light come on, leading me to believe that some computers came from the factory set to ignore that particular input. To reset the check engine line, one must simply plug the sensor back in, and roll forward a few feet and it will turn off. There has been only one person to definately beat the sensor, and he did it by connecting an interrupt switch to the clutch, so that the sensor is DISCONNECTED when the clutch is let out, but then restored (connected) when the clutch is pulled in. In this way, you can only ride for a few minutes, before pulling in the clutch to shift or stop, and in that state, the sensor sends a signal to the computer re-setting the 8 hour timer, preventing the light from comming on. However, I don't believe his solution was waterproof. You can see the details of this by searching for speed sensor on the 700 forums.