But the best answer we've had so far is from Bruce Steo, who says lightning actually goes at variable speeds, because it is not just light, but electron plasma moving through charged air. Its can depend on air conditions, but the typical lightning bolt moves at 224,000 mph -- or about 3,700 miles per second.
However, the light you see from the lightning obviously travels at the speed of light, which is roughly 670 million mph, or 186,000 miles per second.
Thunder travels much slower -- at the speed of sound, which generally is about 760 mph.
Thus, you can figure how far you are from the lightning strike by timing the pause between seeing the flash and hearing the thunder.
Every 5 seconds equals one mile in distance. So 10 seconds between seeing the bolt and hearing the thunder means you're 2 miles away.