C5 sedan with roof box fuel economy at 100-110mph?

[GB]

Has anyone got any experience driving a C5 through Germany (hence the 100mph) with a roof box? Considering getting a Hapro Zenith roof box for my next cross-EU trip in June, but it depends on whether it's economical fuel wise.

What sort of fuel penalty to expect? Also, what's the wind noise like?

On my last trip the car was getting 44.5mpg, doing mostly 90-110mph.


PS
After a constant 2 hour 100mph run through Germany last week, the bottom engine cover front fasteners got ripped out and the cover almost came off. Got it fixed at the nearest service station (using proper bolts this time) for a total cost of two beers.

GB

[KP]

I would expect it to drop a fair amount as the roof box is a fair old bit of drag, even the sleeker shaped ones...

If you can mount it on the rounder shaped roof bars as i've seen them on a few cars now and they may not help much but every little helps.

Maybe consider getting a K&N panel replacement at the next service as this would help claw back a little by making it easier for the engine to breathe and not use the AC either.

if it were me i would chose to fill the car up more than have a roof box, i remember days back when my family used to drive to stranrear from fleetwood in an old VW polo estate type thing(3 door) and we would have everything packed in around us for the journey there and back as well as blankets as it would often get very cold in dec/jan on the journey up as there wasn't as much motorway back then and the roads were much slower/icier than these days.