Bikes on roof & Fuel Efficiency

I am planning a camping trip down to France this year and plan to take the bikes.

I currently get around 50mpg with the car as is (diesel vectra) and was wondering what hit I would take on the efficiency with two bikes on the roof?

Alternatively what would the impact be if they were on the back of the car? I already have the roof racks so would be inclined to use them but a boot mounted carrier may pay for itself after the 2,000 mile trip.

Not sure if they would fit in the car with all the camping equipment although if they do, that is certainly the preferred option.

Thanks

I’ve just been looking at this too and haven’t been able to find a proper roof vs rear mount test, just anecdotes on forums like this.

http://ecomodder.com/forum/showthread.php/roof-rack-test-quantifying-aero-penalty-96.html

given all the fuel efficiency car mods are about streamlining the rear of the car, I’m not convinced bikes and rack on the back will necesarly be much better than the roof.

I reckon it knocks about 5mpg off on my diesel Focus, driven the same… Though I tend to drive a little more relaxed and so the end result is not so very different.

Yeah, I reckon between 5 and 6 mpg less with 2 bikes on the roof of my Ibiza Tdi.

I have a towball rack, I go from 52ish mpg down to about 45 ish on my 2.0 TDI Passat saloon. I think if it were an estate there’d be no difference. However, I think Thule do a towbar thing where you remove the towball itself and fit a special rack which sits much lower down – this would probably save a load of fuel since on mine half the bike sits up high in the air stream.

I have only used a roof mounted rack with a caravan, and it made precisely no difference to fuel 🙂

I took 4 bikes to Scotland on the roof of an Audi A4 which normally returned over 50 mpg and got low 30’s.

One bike on the back of my car on a high level rack reduces it from 36 to 29!

Usually get about 40-45 on mine without bikes, about 35 with 2 bikes on roof. And that’s with driving a bit slower. (Passat TDi 170)

I went to Italy and back with bikes on a towball rack on an estate and had no discenable difference in consumption. I made sure that the profile was not altered with the bikes sticking out of the side, but other than that the biking was good and the food excellent.

I get 1.6mpg less with two bike carriers & 1 bike on the roof, driving at a steady 70 – BMW 330i petrol ’52. 30mpg down to 28.4

(Yes the original is higher than std spec, car’s been slighty modified at the filter.)

Hmm so between 10-20% reduction on average… so about £35-70 extra fuel cost over the course of the trip. Not too bad I suppose (could be a lot worse!)

Cheers

Big petrol engines driven relatively gently are quite inefficient due to higher pumping losses caused by the throttle valve.. so you lose aerodynamic efficiency with the bikes on top but gain engine efficiency by opening the throttle more.

EDIT: wait – that is a windup, surely. BMW driven at 70mph? Perhaps you mean through town.

I drove to the south of France last summer with 3 bikes on the roof, and a slim roof box. Overall I think mpg was down by about 10, that is on a diesel Passat estate. The car was well loaded too, with 2 adults, 2 kids and stuff jammed in everywhere.

With bikes on the roof you can’t use the auto toll road payment thing (sorry I have forgotten what they are called!) If you are planning to travel during French holidays it can save time queueing at the toll booths. We did most of our traveling overnight so wasn’t really a problem.

molgrips – Member
Big petrol engines driven relatively gently are quite inefficient due to higher pumping losses caused by the throttle valve.. so you lose aerodynamic efficiency with the bikes on top but gain engine efficiency by opening the throttle more.

EDIT: wait – that is a windup, surely. BMW driven at 70mph? Perhaps you mean through town.

In first gear yes, to promote loud exhaust output and attract attention, why what’s the problem?

I like the suggestion that the bikes will make no difference to fuel consumption.

The car companies spend 16 trillion pounds To sticks cars in wind tunnels to help make them efficient and sticking a couple of iron wrought gates on the roof doesn’t have any effect?

went to scotland at the weekend 4 people,2 roof bikes,and luggage got 42mpg out of 2.0 passat diesel, normally returns 50mpg with my wife driving to work and back on her own, probably not a lot of difference between her normal driving and my driving with 2 bikes on.

What about trailers specific for bikes, how do those compare?

3 bikes on a A3 2.0TDI knocks the mpg down to about 35 from 55. 1 bikes knocks it down about 10mpg. Just the aero bars little difference but with the bike carriers but know bike about 3mpg.

rear carrier with 2 bikes on my 2.0 diesel picasso drops the fuel consumption by about 5mpg on a 400 mile steady run on french motorways

BUT on french tolls I have seen a car hit the low level height bollards at about 30mph with bikes on the roof there were tears there I cringed

I wouldn’t tow a trailer if there are other options. Limited speed, difficult manoeuvring and parking and risk of it being stolen.

Just having the Thule bars and bike roof racks on my Vectra knocked it from 33mpg to 30mpg – never checked other than that, as I’d company fuel.

risk of it being stolen.

😮

My car usually gets around 60-62mpg, but with 2 bikes on the roof driving at 65-70mph it goes down to around 53mpg so does make a considerable hit.

I can’t believe anyone would think a locked bike roof rack would be enough to stop a thief.

You can tell that was a STWers bike, it was stuck in the granny ring.

Fuel economy definitely takes a hit. I’ve seen me get as low as 24mpg with a roof rack and two bikes on the trip to Glentress.

Acceleration above 80mph takes a big hit too.

I expect the size of engine and fuel type will make difference to the % knockdown in mpg. More powerful engine wont struggle so much to do the same speeds? You also have to take into account that when carrying multiple bikes, the car is most likely fully loaded too, so that would in itself knock the mpg down somewhat.
We took 3 bikes on roof to Spain last year and noticed about 4-5mpg less. 2.0 ltr petrol Honda FRV

I’ve just gone from roof to rear carrier (on an estate) and the loss is pretty much as bad, 10mpg give or take.

Huge windage on a bike and a roofrack.

If you can get away with bikes in back and possibly a top box, you’ll be better off.

A friend got better mileage by putting his bikes on a rear-mounted carrier and taking the wheels off.

From past (dinghy and yacht racing) experience, you would be surprised at the windage created by a small diameter object – added up, the cross section can be huge, if you’re talking something like a 10m long piece of 6mm spectra.

I expect the size of engine and fuel type will make difference to the % knockdown in mpg. More powerful engine wont struggle so much to do the same speeds? You also have to take into account that when carrying multiple bikes, the car is most likely fully loaded too, so that would in itself knock the mpg down somewhat.
We took 3 bikes on roof to Spain last year and noticed about 4-5mpg less. 2.0 ltr petrol Honda FRV

This. I had a conversation recently with a mechanic which basically ended up stating two things:

a) Its all about usuable torque (Nm), not BHP and

b) Often there is a very wide gap between what manufacturers do to bleed from the MPG figures (rolling roads, optimum fuel ratio, optimum atmospheric conditions etc) and what you can achieve on the road. ie, sticking anything outside the car that effects the aerodynamics, or interuppting the airflow even with windows down damages the mpg considerably.

Vis a Vis there is les % difference between particulaly older sports cars and newer models designed with MPG in mind – my beemer for example, wasn’t designed to be the most efficient car ever, hence a low % reduction. Stick a bike on a Honda Insight though and watch the efficiency plummit.

2 comparable journeys in my A4 2.0tdi.

Alps and back for snowboarding, 3 people plus kit, everything inside – 62mpg average for the whole trip (there and back).

Alps and back for biking, 3 people plus kit, 3 bikes on towbar rack – 45mpg average for the whole trip (there and back).

Both trips were done with the same driving style – indicated 75mph (satnav says 70mph) on cruise control all the way on the peage autoroutes.

I dread to think how much worse it would be if they were on top of the car.

Dave

risk of it being stolen.

The lad who parked up at the Trafford Centre and lost both bikes off the roof…..he thought they’d be safe with the Thule locks….

Thing is if you really want the bikes you can rip the bars off the roof as well by leverage/screwdriver…wouldn’t leave a nice sight but it renders the d locks etc pointless.. then straight into the back of a transit.

Mine averages around 32mpg without the bikes on the roof, i lose about 2mpg with a couple of bikes on the roof. Turbo petrol vectra..

If you end up travelling with the bikes on it makes a big difference to cruise at a lower speed… to time and mpg!

The locks on the carriers really are rubbish. Even being careful not to damage anything they can be forced open in seconds. As the video above shows if you don’t care about the car or bike you can get the bike off even quicker! When I have bikes on the car I always carry a long cable lock with me so that I can lock them if I stop anywhere, it is really not worth the risk. I often use an audible cable alarm as well. Even when locked up I try to keep the bikes in sight.

A friend had a bike stolen off a rear mounted carrier, he reversed right back to a wall so the bike couldn’t be lifted off the carriers prongs. Unfortunately some one just destroyed the carrier to take the bike off 🙁

I lose about 2mpg (38-40 vs 36-38)with the 3bikes on the rear towbar carrier at motorway speeds. I like rear carriers since you don’t have to be worried about overhead barriers, you can see the bikes (no sunroof) if they become wobbly and they are easy to put on/off.

I do however carry a very big chain if I leave the bikes on the car unattended even for a moment.

Hmm. I’m lazy enough to leave my Saris Bones on most of the time when not carrying bikes. Am wondering how it is impacting economy.

Not read all the posts but for what its worth at Motorway speeds your looking at a 25% reduction in efficiency.

That’s down from about 52 – 53 to 38 – 40.
(VW Passsat saloon with normally two bikes roof mounted)

Its an exponential relationship (I think) so slower speeds will make a real difference!