can anyone tell me why my 91 4 runner is starting to get realy bad gas mileage?

 

my truck goes from an average of 19mpg in warmer weather to around 15mpg in the winter, I've heard they put some type of additive in the fuel in some areas and it causes worse gas mileage.. plus the fact that it idles higher and takes longer to warm up...

 

could be a number of things.......

whens the last time the spark plugs, wires, dist. cap, air filter, and fuel filter were replaced/cleaned ??

id start with any of that stuff first............

 

What do you call bad milage?