I had a 2005 Mazda3 5 door with a 5 speed, 2.3 and was getting 34mpg on a regular basis. I now have a 2008 Mazda3 5 door with a 5 speed, 2.3 and rarely get 30mpg and usually less.

Any ideas why the two cars would be that much different in the mileage thay get?

Tom

 

Were both engines properly broken in? You can verify the oil viscosity though I doubt that has this large of an effect. It could also be a horrible alignment or brake drag. Though with that big of a difference i'd think it would be a very obvious problem you'd see.
Check that there's no restriction in the air intake - like mice. Or an exhaust restriction, not that there's much hanging down to get dented.
If none of that does it - what's the mileage on the car? You can try disconnecting the battery for a few seconds, that forces the computer to readjust the fuel trim values in case a sensor got messed up from sitting in the lot with too many short drives or something.