42 mpg at 75mph with roofbox - Should i be getting more?

[Jockdooshbag]

I saw another thread about someone getting 55mpg and it got me wondering about my car. My car only has just under 1000 miles on the clock and i did a longish run at the weekend with the roofbox on. Sat at an indicated 75mph and the trip computer indicated an average of 42mpg after 240 miles. How much do you think the roof box affected this figure? Havent got figres for this car yet without roof box as its new. My BMW was amazing on the motorway it never showed less than 50mpg at indicated 80mph but the golf i had before it was never this good. It only got around 45mpg at 80mph. The golf was a PD 170 with dsg. My skoda is the newer version i.e CR 170 TDI DSG. I didnt expect to match the BMW but was hoping it would get closer than the golf. In fairness i never had a roof box on the BMW so dont know what i would have got in that car. Would be interersted if anyone else has a car similar to mine with a roof box tio see what they get. Also do you think the lack of miles on the car would affect it?

[blueheart]

Your engine will still be tight anyway so 42 mpg isn't bad. The readouts generally aren't that accurate unless it's been calibrated. I too have a CR vRS DSG and my son in law has an estate with the same engine gearbox combination, consumption read out is often 8 to 10 mpg better in his estate but when we measure actual fuel used, they are very close.

Last year I went up to Scotland and back in my remapped manual PD vRS with roofbox fitted and averaged pretty much the same as you, 42 mpg.

[octimark]

DSG does affect the fuel consumption but I dont know by what %

The roof top box would also have an affect but I dont know if anyone has any actually figures.

I would have estimated maybe a drop of myabe 10-15% which puts you on around 45mpg at 10% less than 50mpg and 42.5mpg at 15% less.

[Golf-Fiend]

Speed kills economy. Simple. The most efficient speed to drive at is around 45-50mph where you can get into a reasonable gear ratio:engine speed and the wind resistance is not that much of a factor. Rising above 45-50mph will start to kill economy of around 5% for every 5mph increase in speed. At lower speeds the roofbox will thus have less of an effect on economy than at higher speeds. IMO 75mph is too fast with a roofbox if you want economy (and safety)

I got a measured 50+mpg on a camping trip from Bath to Scotland - (very) heavily loaded car with a roofbox. See my thread here: http://www.briskoda.net/forums/topic/169908-amazed-at-fuel-economy-actual-figures/ I was travelling mostly at an idicated 70mph but in actual speed at 65mph or a smidge below or above 2000rpm.

[Jockdooshbag]

Speed kills economy. Simple. The most efficient speed to drive at is around 45-50mph where you can get into a reasonable gear ratio:engine speed and the wind resistance is not that much of a factor. Rising above 45-50mph will start to kill economy of around 5% for every 5mph increase in speed. At lower speeds the roofbox will thus have less of an effect on economy than at higher speeds. IMO 75mph is too fast with a roofbox if you want economy (and safety)

I got a measured 50+mpg on a camping trip from Bath to Scotland - (very) heavily loaded car with a roofbox. See my thread here: http://www.briskoda....actual-figures/ I was travelling mostly at an idicated 70mph but in actual speed at 65mph or a smidge below or above 2000rpm.

Without roof box i normaly stick cruise control on at indicated 79mph. Dont know why just habit now i guess. Can cruise through all speed cameras at this speed with no danger of setting them off. In my roof box instructions there was a warning about going above 80mph hence why erring on the side of caution i stepped my speed back to 75mph. The instructions are almost certainly overkill as i doubt the roof box would fly off any where nerar 80mph in fact it would probably never fly off at the vehicles top speed. In fairness i couldnt stand going any slower than 75mph as you start getting caught up with the idiots who cant maintain a constant speed so you are forever re-passing cars who overtook you 2 minutes ago even though your speed doesnt change. Dont think 79mph is dangerous by any stretch of the imagination. Keeping safe distance and paying attention to the road ahead make driving at much higher speeds perfectly safe. Im not driving purely to get the best economy i can but it is satisfying getting good mpg. There must be a critical speed where mpg falls of the cliff so to speak due to aerodynamics when a roof box is fitted. I have driven a mitsubishi shogun to the french alps and the difference between sitting at 80mph and 75mph was massive. At 80mph the trip computer stated about 24mpg but at 75mph i would get 30mpg. Thats a lot of mpg's for only 5mph. Being poor aerodynamically the shogun may highlight characteristics that would be similar for my car with the roofbox fitted. As long as 42mpg is inline with what other are getting i'll be reasonably happy but there seems a lot of talk on hear about people getting 55 - 60mpg in certain conditions which makes me raise en eyebrow everytime i read it. Dont think ive seen a 55mpg average in any diesel car ive owned, even driving at 80mph. Saying that most of my diesels have had 170bhp as a minimum.

[carabus901]

I notice about a 10-15% drop in indicated fuel economy when driving with the old roof box fitted. Of course that is speed dependant but I generally won't do over 75mph with it fitted, mainly the missus nagging me about safety

[booke23]

With my full width, pretty big roofbox on (Kamei Husky XXL), I get about 42 MPG on the motorway at 75 mph. Without it I would get about 48 MPG at the same speed (calculated, not trip computer).

So about 15% penalty with the roofbox on. I suspect a cruise speed of 65 mph would make a big difference to the fuel consumption penalty.

[Rab-k]

I clocked up 2500 miles last month on a return trip from just north of Glasgow to Ljubljana / Trieste with a roof box. Autobahns in Germany saw us do a steady 80mph, with 70mph on similar roads in the UK, Netherlands, Austria, Slovenia and Italy. Average, according to MaxiDot, was 47mpg.

[jrw]

Roof box kills consumption.

I get about 42-45mpg with it on and without it cruising at 75mph, up to 55mpg.

[kevberlin]

My vrs dsg estate took at least 8 k to loosen up. Mpg improved significantly over time. A gentle 480 mile trip back from Spa Francorchamps got me home with av 58 to the gallon. Amazing.

I would have thought the roof box would have a significant adverse effect on fuel economy.

Enjoy your car. It is ace!

[silver1011]

It's easy to understand how a roof back can significantly increase your fuel consumption...

[Rab-k]

We traded our "house brick" for a narrow-guage version, more of a "lego brick"...

[Jockdooshbag]

It would be easy enough to see if there is a threshold speed where the economy changes massively. Just put the fuel computer on instant consumption rather than average, and you'll be able to see roughly what the difference is as your speed changes.

Mike

Never thought of that. Will try that next time. I would rather sit at 80mph than 75mph if fuel penalty wasnt the noticible. There is something phsycological (spelt wrong probably) about driving a few mph more. I like to drive at the limit of what is permissible under law rather than wasting any mph's. Ive never had hassle with cops when cruising at an indicated 79 so generaly hate doing less than this on motorways.

With our recent arrival in the family i doubt i will be back in France any time soon but i will definately go back when she is older and obvioulsy you can drive 10mph faster over there but i wonder how big a difference there would be at 90mph compared to 80mph. I suspect the mpg will suffer significantly at these speeds. Law of diminishing returns and all that. I would hate to drive at 80mph over there it would feel like i was going backwards.

[silver1011]

Even without the roof box the drop in MPG between 60mph and 80mph is quite noticable.

[M12MKD]

Seriously starting to think either I'm unable to adapt to the DSG box or there is a problem with my car as I never get more than low 40's

Should have bought the petrol :-(

[silver1011]

If you are getting low 40's in a diesel vRS then you can expect low 30's in a petrol vRS!

Do you drive it quite hard?

Edited 5 September, 2012 by silver1011

[Jockdooshbag]

Seriously starting to think either I'm unable to adapt to the DSG box or there is a problem with my car as I never get more than low 40's

Should have bought the petrol :-(

My overall average in my BMW 32od auto over 2.5 years was 40.3 mpg and its official mpg was higher than the VRS so i wouldnt worry about getting low 40's as an average. What do you get on the motorway?