As seen in volumetric flow rate, the volumetric efficiency is defined this way:

(1) 
Where:
Q_{m}  = Measured volumetric flow rate (m³/s) 
Q_{th}  = Theoretical volumetric flow rate (m³/s) 
The theoretical volumetric flow rate (Q_{th}) is more or less chosen arbitrarily, based on atmospheric air density. Because air is compressible, a volume does not limit the quantity of air that can be pushed in, especially when gas dynamics are considered.
So volumetric efficiency is not a true «performance» criteria for an engine because what counts is the air mass flow rate. With everything else equal, an engine with a large VE burns the same amount of fuel and gives the same power output than a larger engine with a smaller VE. But values do compare with similar engines.
Mathematically, only a complex gas dynamics analysis can give a good approximation of the volumetric efficiency, but here are some ballpark values:
intake & exhaust tuning  VE @ max power 
None (2stroke & Wankel)  55% 
None (4stroke)  75% 
Mild intake tuning (4stroke)  80% 
Mild intake & exhaust tuning (4stroke)  90% 
Tuned  95% 
Fully tuned  100% 
Best  110% 
forced induction  VE @ max power 
Street (10 psi)  135% 
Racing (20 psi)  165% 
Top fuel dragster (45 psi)  230% 
Tractor pulling − Pro stock (115 psi)  360% 
Tractor pulling − Super stock (200 psi)  485% 
