Obviously you shouldn't drive with no coolant, because in most cases obviously your car will overheat, blow a head gasket and all that fun stuff. I drive a 4cyl 2.5L Ford Ranger. One time I realized it had none in it, I planned on getting some but I forgot the next day. I drove it for about a week before remembering. My truck never even came close to over heating ... can anyone explain?
If you didn't have any coolant in the cooling system at all, your engine probably wouldn't last more than 15 minutes driving in a moderate climate before it seized. Just because you can't see any in the reservoir which only holds a small amount, it doesn't mean there isn't any in the engine, radiator, pipes etc. Obviously if you can't see any in the reservoir, you don't have a clue how much is really in there, so it is not a good idea to drive like that. Presumably you only had to add a small amount (a couple of pints maybe) to bring it back to normal. If this happens again, don't presume you can get away with it.
Just because you don't see any coolant in your overflow/reservoir, doesn't mean the system doesn't have enough fluid in it to keep your engine from over heating. There are other factors involved as well:
- Engineering of your particular vehicle may not require as much coolant as it normally would have ... ie: the vehicle engineers over engineered this one part of your vehicle. Feel lucky if this is the case.
- You may not have put much stress on the vehicle to cause it to overheat.
- Your climate may not be overly hot, which may not need as much coolant or coolant capacity.
I think the bigger thing to be thinking about here is why was your reservoir tank low in the first place? For the most part, the cooling system is a closed system, meaning it shouldn't be loosing/gaining coolant. If everything is as it should be, it should stay relatively the same (might be small fluctuations). If your reservoir has become empty, you need to figure out why it became empty so it doesn't happen again. Is the engine sucking in the coolant and burning it? Is it draining out on the ground? Did it overheat a little and come gushing out the overflow? Whatever the reason, I'd be on the lookout for it and get it fixed.