I just put a Sentinel bike rack manufactured by SARIS on the back of my 2006 Prius with 2 bikes. I took the car on a 400 mile road trip. To my surprise, my gas mileage dropped from a normal 43 MPG down to 32 MPG which was more than I thought would have occured. The issue seems to be wind resistance and not weight. Besides gas mileage, the rack was easy to use and felt stable. I would not recommend putting more than 2 bikes on the back of the Prius (SARIS makes a 3 bike model....) due to weight on hatch.
That's interesting. Probably a combo of weight and drag. Do you know how much the mpg would drop with just the rack with no bikes on it?
Your FE/MPG reduction came mostly from the bikes sticking out to the sides. The bike rack is tucked up behind and while it would have some effect it would be small compared to the bikes. As a simple test have you ried driving around for while with just the bike rack mounted? The Prius' iconic shape is very aerodynamically slippery. Of all the cars being sold, only one, a MB has better numbers. Almost anything that changes to the shape of the car or hangs out into the breeze creates additional drag, which increases as a square function of the speed... but the power and fuel needed to overcome it increases as a cube function. Three very general rules for good FE/MPG are: Keep it (aerodynamically) clean. Keep it light. Keep the speed down. This is difficult to do when traveling with family and extra gear on vacation.
If the rack actually leans on the glass, that's good advice. But there are other types of racks available. The Bones leans further back, shifting almost all the weight back onto the vertical support legs. So, you could carry 3 then. .
Back from the 850 mile trip. Below is a picture of the setup. I have to agree that the mileage drop is related to the bikes sticking out the sides and the top of the car as well as the wind trail having to snake through all of the bike parts / frame. The bikes sit about 1 foot back from the car... so you can imagine all of the wind action swirling through them. I don't think the gas mileage drop is weight related as I have driven the car with various weight loadings before with no significant change in the mileage. On the way home, I did remove the water bottles from the bikes in an effort to reduce drag. I also filled the car with mid-grade 89 octane fuel instead of regular 87. Overall result: I got 36 MPG on the way home versus 32 on the way out. I wouldn't swear that my actions resulted in the better gas mileage; but at minimum I can say my actions did not hurt. In terms of my comment about not putting more than 2 bikes on the back. I am concerned about the overall load on the hatch and its support points (the hinges and the bottom of the hatch.) The hatch not only has to cope with the weight of the bikes, but also the dynamic loads from the wind and the bouncing around. I think those loads are higher than the static load from the bikes. Had a lot of fun... and it was great to take the bikes along!
