Alright if i make 2 boxes 1.5 or 2.5 and tune boxes at 40hz is there a differneces between the 2?

 

absolutely.

 

Ok so what will i notice between the 2 then? i have no idea

 

This would be easier with pictures but..

The larger box will still play lower or have more authority down low with the larger box even though they are tuned to the same frequency. The larger one will likely play louder as well.

With vented boxes, larger is almost always better until the driver starts to overexcurt.