I found a Garrett GT 30 for sale. The compressor housing .60 57mm front wheel, in the exhaust part the housing is .48 with the exhaust wheel stage 3. But I want to know what turbo is this? Is it a T34 or is it even bigger?

 

The GT-series of garret-turbochargers is a later construction than the T-series. These turbos is traditionally far more efficient, concerning spool-up and maximum throughput, than a T-series turbo of the same size.

This turbo is a few notches worse than, as an example, the t34 found as original equipment on the 4x4 Cosworths, equipping these.