Hey - I'm about to go on a 3000mi trip with my Civic EX and was wondering how much of a hit I would take by bringing the mountain bikes along, just a standard bike rack where they would mount behind the trunk. It's a '08 Civic EX, the driving would be 60-75mph mainly...

 

I'm sure it would make a difference, probably no more than 5 mpg would be my guess, assuming the bikes don't weigh more than 150 lbs each.

 

We just returned from Arizona (to Michigan) having spent the winter there. We took 3 bikes on the back and it did affect the mileage, but not too much. Not nearly as much as high winds and mountains.