Does anyone here know the exact difference between the GT2530 and the GT2540? I don't but I believe it is the size of the exhaust wheel or housing, not the compressor. I was under the impression that the first two digits refered to the compressor and the last two were the exh specs. Either way, this is an interesting question. I am going to do some research and find out. I know there are a lot of GTR's running the GT2540's so they can't be all that bad :-)