Fuel econ and wind resistance

[fruitcakesa]

So we packed up the 03 sedan for a Canada trip and put 2 bicycles on the bike rack which is trunk mounted and 2 passengers and minimal luggage otherwise.
The wheels of the bikes stick out about a foot on either side of the car and the handlebars are about even with the roofline.
I normally average 48 mpg's on the highway with no bikes etc.
So far on this trip we are getting about 40 mpg's running 70-75 mph on the open road.
Would you expect such a large drop in fuel economy with this setup?

 

[DieselBruce]

Of course, its not just the frontal area that's increased. The way the wind whips over the car is completely changed with the bikes up there. That can definitely give a big drop in fuel mileage.

 

[tditom]

Not real surprising if you are loaded up and you are travelling on a general incline.
Do you vent when you fill your tank? That is the most accurate way to measure fuel consumption from tank to tank.

 

[fruitcakesa]

Not real surprising if you are loaded up and you are travelling on a general incline.
Do you vent when you fill your tank? That is the most accurate way to measure fuel consumption from tank to tank.

Not a real big load in the car and the bikes are down low on the trunk.
Most of the drive is on on pretty flat and level terrain.
I do vent and have been tracking mileage since I bought it.

 

[Cogen Man]

A lot of variables to consider. The extra weight and some bike drag being the biggest. I'll assume you run 75 mph all the time on the open road even without the 2 bikes and luggage. Have you considered slowing down a little ? Or would this get you run over by other motorists ?

 

[chronoreverse]

Drag grows with speed quadratically. A small increase of resistance at 75mph results in a relatively large difference.

Only way to tell for sure though is to do a similar route without the bikes.

 

[yatzee]

that seems normal. I used to see a diff with the Golf when I only had cross bars on it

 

[puntmeister]

The extra weight of the bikes isn't much of an issue - but, bike wheels, with all their spokes, will create some serious air turbulence (drag) - especially at 70+ mph.

If truly concerned about the MPG, could take the wheels off the bike frames, and put them in the trunk....

 

[patbob]

The wheels of the bikes stick out about a foot on either side of the car and the handlebars are about even with the roofline. I normally average 48 mpg's on the highway with no bikes etc. So far on this trip we are getting about 40 mpg's running 70-75 mph on the open road. Would you expect such a large drop in fuel economy with this setup?

Absolutely. Just washing and waxing my old 1970's era Horizon before a trip was enough to get a measurable 1 MPG improvement, and that car wasn't anywhere near as fuel efficient. The more fuel efficient the vehicle is, the more of an effect a change like that will make.

 

[CopaMundial]

I normally average 48 mpg's on the highway with no bikes etc.
So far on this trip we are getting about 40 mpg's running 70-75 mph on the open road.
Would you expect such a large drop in fuel economy with this setup?

The answer is simple.
Take the bikes off of the rack and ride them. Your fuel economy will improve greatly. Unless, of course, you're trying to pull the car behind you

 

[fruitcakesa]

The answer is simple.
Take the bikes off of the rack and ride them. Your fuel economy will improve greatly. Unless, of course, you're trying to pull the car behind you

Great call, we do take them off for the occasional spin but its a long ride to North Ontario and the headwinds are wicked

 

[Diesl]

Yes .

 

[Jersey John]

I see a 3-4 mpg drop on my 01 jetta with just the bike on the hitch rack. Now load it up with 2-3 other people + gear and I would say 5-7 mpg drop if not a little more

 

[My_name_is_Rob]

The extra weight of the bikes isn't much of an issue - but, bike wheels, with all their spokes, will create some serious air turbulence (drag) - especially at 70+ mph.

If truly concerned about the MPG, could take the wheels off the bike frames, and put them in the trunk....

^this

I just did a 7000km/4 week trip, with my mk3, also with bikes on the back. I just removed the rear wheels from the bikes when we loaded them up, as I didn't like the over hang on either side. I also had a ton of extra weight, as we had all our camping gear and such along with us. I averaged about 48.5 MPG, which is more than normal, but the average speed limit was about 20km/h lower than normal as well.

 

[fruitcakesa]

update

On our return home we took a different route that had a higher percentage of secondary roads that kept the speed under 60 and got 44.7 mpgs but that also included over 200 miles of non-bike driving while up there.
Total mileage was north of 1500 with 3 fillups needed for the whole trip.

 

[Wankel7]

You could consider a hitch rack to keep the toys out of the slipstream.