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Engine Mods do not hurt the gas milage.
I have done eveything except go inside the engine and I average 13 combined give a little each way. Now if I went around sticking My foot in it and running WOT then yes milage goes down more but, I do put My foot in it allot so that is included in average. I have not run it yet with supercharger as I'm making allot of outside and inside mods since I had it in My shop to add SC anyway. I will have it out and drive it some before I take it back to My shop to continue with more mods. I'm quite sure My MPG will go down at first until I get the WOW factor over with trying the SC out. I will be going to a 2.9 pulley and HP Tuning soon and do not know what it will do to milage but, honestly I do not care or I would not have bought the H2 to begin with and would not be adding all the performance mods. IN all honesty if You worry about milage a person does not need one. It is sort of like wanting to own a top end Jag or Ferarri and asking what is a tune up going to cost. Well they will run around $1000 give or take some. Oil changes on them are in the $200 to $300 range. So not trying to sound eletist which I'm not just a regular old country boy just trying to be practical about the topic. If You want a H2 just do it and do not care about MPG or repairs. If that is what will determine Your decision honestly do not buy a H2. You will be unhappy until You get out and drive it for a while and see how fun it is to drive.
Well just My experience to share so that is the best advice I can give is from practical eveyday use and adding the mods and watching what happened when mods were added.

I have probably got the second most modified engine next to Ken but, will catch him soon. We are planning some LS1 heads and cam that some F-bodies are running along with some other goodies. I have a LS1 expert in My shop so We are looking into the mods and planning when to do them. Not for racing but, for use when needed, well for some racing just a little. Will not allow the mighty H2 to be out run if I can help it at all. Plus, it is a WOW factor in shows and I found You get big points for having these mods to the H2 and get big points. Plus I just like fast off and on so the further engine mods will meet 2 goals. Fun and show points. It will be driven less but, I want to build a lifted rig to drive daily so it all gives Me an excuse to blow a bunch more money on my H2 addiction. You will become addicted if you get one.
TAZ