Low Mpg after wheel alignment?

Doubtful an alignment would change MPG. Did they rotate the tires at the same time? If they did, and crossed the tires, then the direction of rotation changed on at least two tires, and this can definitely cause lower MPG. Another factor could be tire pressure, if they lowered it from where you had it set.

 

This would seem most likely. My dealer lowers the pressure to what's on the door post every time they rotate the tires.

 

Mind sharing the alignment numbers?
You may have high amount of toes in/out.
Tire pressure drop has more influence on fuel economy hit than excesssive toes I believe.

 

Depends on how badly the tires were worn and where the excessive wear was. After an alignment with excessive tire wear, it can be just like putting new tires on the car in that the tires have to re wear into the new alignment, and that takes time. 48 to 40 is pretty big drop, but again, all depends on how bad the tires were worn in the spots the mechanic pointed out. Are you thinking about a new set of tires, possibly? If you're already thinking about going back to the dealer, and possibly thinking about a new set of tires and possibly buying them from the dealer, you could ask your mechanic if new tires might help.

 

Rotating bald tires to rear and full tread to front puts larger diamer tires in front and pseudo mpg drop