D15, D16, D17 and Mini-Me swaps
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By chrono404
#45495 Depends on how you drive, and what transmission you have.
my D16z6 gets an average of Mixed Town/hiway of 30-35 or so and pure hiway of 39-41 with just 2.5 Exhaust and no real other performance Mods.

Why not go and find out your Mileage? Go to any gas station, fill up, then drive around, fill up again divide Mileage by how much gas you put in.
By MD-EG6CX
#45499 i have a cx i do the speedlimit all the time, so if itwas 42city/48hwy stock its probably 40city/42hwy now
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By Graham
#45547 Some mods actually improve you efficiency if anything - an engine that can breath easier and get rid of its exhaust gasses easier will return a better mpg than the standard engine that is hugely restricted from the factory

This is if you drive sensibly however...