over night gas mileage went from great to terrible why

Check air filter and clean throttle body replace fuel filter if have not done so in a while add Lucas Fuel Injector Cleaner to gas tank.

When was the last time your engine was serviced? Have you kept up on your maintenance according to the schedule in the owner's manual?

While firebird's maintenance tips are all important, none of those items cause significant excess fuel consumption, and any performance issues they cause happen gradually, not overnight. Have the trouble codes read. Something has failed or malfunctioned suddenly to cause a sudden increase in fuel consumption. There will be information stored on your car's computer which will tell you where the problem lies.