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The Z1 is going to deliver the best gas mileage, no doubt, but the problem is no stock ECU will properly run an automatic D15Z1 setup. All USDM Civic VX's were manual. It would run, but no other ECU will be able to control lean burn which is the only reason to get this engine.
So between the B7 and Z6 it's a matter of power. The Z6 has more power, but the B7 was meant to be more economical. What is interesting is that you got 46 MPG from your automatic Z6, so it seems to me you could use another Z6 and get more power and good fuel economy. If you were able to get great gas mileage before, why switch to a less powerful motor?
If it were me I would buy whatever I could find the cheapest, in the best condition. Used motors are sketchy sometimes, especially off craigslist.
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