MPG impact of bike on roof?

[Sensibleboy]

Does anybody know how having a bike on the roof will impact fuel consumption at motorway speeds? I.e. is it significant or just a couple of MPG?

I'm going up to Scotland in the summer which will involve about 200 miles of motorways. If I get an average of 50mpg at the moment and I keep it to a smooth 70 mph anybody know how much it's likely to increase fuel consumption?

I'm still taking it regardless of the answer it was just out of interest that's all.

[dub16v]

AE did a test four or five years ago with/without a roofbox (near filled to capacity) and the implication was 0.2mpg. However, most people (google it) seem to suggest a more realistic figure is around 10-15% with a single bike on the roof...better to mount it on the back if given the choice.

Here: http://www.autoexpress.co.uk/car-news/31861/mpg-my...

[RussH91]

Is it a chunky mountian bike or one of those posh aero road bikes?

[Ynox]

I notice it (2 bikes on roof mount carriers).

Not a great amount, but about 5ish mpg (usually get about 30 mpg with bikes off the roof). So between 15-20%.

[Skyedriver]

Going to have to start carrying a couple of bikes on the back of my car now young son can ride.
The back of my car gets scruffy, how do I stop this happening to the bike and most importantly, the chain/gears etc?

[Sensibleboy]

It's a mountain bike with 29" wheels. I'm not a fan of boot mounted racks. Can't access the boot, the bikes get dirtier and need to sort out additional rear lights.

[vikingaero]

Depends on the car. On Mrs V's Mitsubishi it knocks off 33%. On the diesel bus about 15%

[muckymotor]

Mine drops from 39mpg down to 34mpg on the motorway.

[996TT02]

You could always drive more slowly, if it matters. Wind resistance increases exponentially with speed.

I often carry 5 bikes on the BACK of my Landcruiser, but with the front wheels above the roofline, plus a cage type roofrack with 2 spare tyres (mine and a landrover's) and some luggage and it takes 20km/h off my top speed. If I back off and don't go for gold I can use much less throttle.

[Wild Rumpus]

I drove from Scotland to France with 4 bikes on the roof of my Yeti, it had a very noticeable affect on the high speed performance and on the economy, much more than a roof box. At normal motorway speeds it knocked around 10mpg off my consumption, from low 30s to low 20s! I now carry the bikes on a tow-bar mounted carrier - that has a negligible affect on fuel consumption.

[D1bram]

I have my roof rack and bike carriers on the car all the time at the moment, they drop MPG from high 50's down to low 50's. Really must remove them, but I'm having work done on the house and the garage is a midden of crap, so really don't want to add to it!

Bizarrely, adding my roadbike doesn't seem to affect it much further. Chunky MTB is going to be a different matter though.

[RobM77]

dub16v said:

AE did a test four or five years ago with/without a roofbox (near filled to capacity) and the implication was 0.2mpg. However, most people (google it) seem to suggest a more realistic figure is around 10-15% with a single bike on the roof...better to mount it on the back if given the choice.

Here: http://www.autoexpress.co.uk/car-news/31861/mpg-my...

On the motorway, I agree with the consensus of about 10-15%, which is exactly what I lose on the motorway having my kayak or windsurf board on the roof (w/s board around 10%, kayak nearer 15%). This does vary with speed though and obviously drag increases markedly the faster you go, which explains the very low AE results. I doubt it makes much difference through town at 30mph or 40mph, but once you're up to speed on the motorway the noise and mpg losses are obvious.

[BGarside]

With a high-mounted rear rack, attached to the tailgate with the bikes (3 of them) mounted laterally in the airflow over the car roof, I lost over 10mpg driving my old diesel Seat Leon, from 53mpg average to 42. Was quite surprised how big the drop was...

[RobM77]

BGarside said:

With a high-mounted rear rack, attached to the tailgate with the bikes (3 of them) mounted laterally in the airflow over the car roof, I lost over 10mpg driving my old diesel Seat Leon, from 53mpg average to 42. Was quite surprised how big the drop was...

From memory I'm fairly sure I lose less mpg with bikes on my tow bar carrier than with something on the roof, but it's still a fair hit - perhaps up to 10%.

[BGarside]

RobM77 said:

From memory I'm fairly sure I lose less mpg with bikes on my tow bar carrier than with something on the roof, but it's still a fair hit - perhaps up to 10%.

With the bikes on a towbar rack behind a hatchback, I noticed little or no effect on mpg, as the bikes are mostly in the turbulent wake behind the car and don't increase the size of the wake by much, if anything. Probably a different story with a saloon though as the bikes will inevitably protrude above the boot lid into the laminar airflow over the car's upper surface.

Roof racks though are much worse...

[Pablo16v]

My old Passat 2.0 TDI estate averaged 45ish MPG, without a bike on the roof but with the roof bars and bike rack in place, and with a bike on top the mpg would drop to around 40mpg. That was driving on motorways and dual carriageways between Aberdeen and Edinburgh.