Beyond Dismal MPG with cargo rack

What speeds, what distance, how much weight carried, shape of the load?

Yes that is probably the biggest mpg effect from roof junk that has been reported.

 

that was a short commute. i am sure it will build up. but, still 28.5 is still low...really low.

 

Want to know the answers to those questions, too.

If anything, this just shows how much Prius aerodynamics helps it's freeway fuel economy, though roof racks are notorious for killing fuel economy.

 

Aerodynamics does that to ya.

That and it looks bloody cold out there... cold air is way less forgiving of poor aerodynamics then hot air is.

 

wow! that is a huge diffrence, and in my mind should not be that bad.

were are you btw? snow?

 

The cargo on top is clearly not the only factor at play.

With my two kayaks on top (cockpits open), at 70 MPH in the summer MPG dropped to 36 MPG.

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The Prius relies heavily on aerodynamics for its high speed fuel efficiency. It is in fact the most aerodynamic car sold in the US right now; so it doesn't come as a surprise to me that when you disturb its shape the fuel economy suffers that badly. I'd consider a small aerodynamic trailer instead of a roof cargo rack. I think a trailer would tuck into the area of low pressure behind the car and would not impact fuel economy as much...

 

A moderate head wind, if you had one, would have contributed. At 65 MPH, the Prius MPG Simulator predicts a drop of from 51 MPH to 31 MPH with the addition of a 10 MPH head wind and a medium size aero cargo carrier. This assumes 70F, smooth dry pavement, steady speed, and level terrain. It does not factor in the inevitable added weight from the added cargo.

 

Not sure what you mean by "speed didn't matter after 30mph". Drag is proportional to the square of airspeed, so the effect would have been much more pronounced at higher speeds.

 

Exactly, and the proportional constant is much higher with that big ugly box on the roof. Judging from the photo, at least part of the trip was in the mountains with snow, which is also a factor.

Tom

 

We got 49.74 mpg on our 7,500 mile cross country trip with roof rack. Two adults, two teens, and all the camping grear. I tuned the areodynamics and we drove 60 mph. It can be done, but most don't make this choice.

PA P

 

Wow - I would imagine a hit in terms of mpg, but that is crazy!

... Brad

 

Hmm, what do you mean, you "tuned the areodynamics"? Can you explain further? Thanks.

 

Being an aviation-type guy, the first thing I noticed about your photo is that your pod is sort of shaped like a wing. The second thing I noticed is that your pod is probably at a positive "angle of attack" when it is getting driven through the air, to use that term loosely. The reason why I mention that is that the air, flowing over the roof the of the car, would probably be striking the flat surface area of the bottom of the pod, which would increase drag on the car. If it were me and I was using that pod, I would try to adjust (can you adjust the pod on the roof?) the pod so that the air would be hitting the pod squarely at the "leading edge" of the pod vs. striking the flat surface of the bottom of the pod, and that would involve lifting the aft portion of that pod relative to the front or perhaps sliding the pod toward the hood of the car so the downward slope of the Prius forward roofline would effectively "tilt" the front of the pod more down toward the ground. I haven't a clue exactly how air flows around the roof of a Prius nor if any of the above would actually help, but if I was concerned about mpg, I would give it a try.

ualdriver

 

I agree, try moving it forward a little to level it out. If the wind is effectively trying to lift the box off the roof, that's not going to help your mpgs at all.

I'm also puzzled by your 12 mpg loss with just the bars alone. I just put a set of Saris bars on and have driven around for 3 days at various speeds (no extended highway but several trips at 50-60 over some large (2 mi) bridges) with no effects at all on mpg.