Here in the USA and Canada, if there is divided (interstate or autobahn) 
type highways (70 mph), with some cities (55 mph limit), and running 5- 
10 mph above the limit say 60% of the time, including stops for fuel, 
restrooms, and meals (quick lunch) (45 minute dinner), I use 55 mph to 
calculate the days driving. We try for 500 -600 miles/day, but 700 miles 
not unusual, and have done 2000 miles in 3 days more than a few times. 
This is with the wife and I changing drivers at most stops. As far a 
mileage goes, it depends on the vehicle. Our Cougar that has an 
instantaneous and trip computer has a fairly flat MPG curve from 55- 70 
mph with and average of 30 mpg there, go above or below and it drops 
off, say 5 mpg for 15 mph faster or slower. The cougar has a good 
aerodynamic shape which helps keep the speeds economical. And as I said 
earlier, it has enough power to stay in overdrive. The lower speeds it 
is more in direct drive and the MPG drops off. Not what the typical 
driver would expect. I am not a light footed driver!

Get one of Vitaliy's scanners, one of the software items is a simulated 
dashboard, and you can get instantaneous and historical over several 
periods of time MPG. :)

On 7/17/2010 3:14 PM, Tamas Rudnai wrote:
> Just made a test with my new car on motorway, and it makes around 47mpg
> (imperial) by doing 70mph using the aircon. As I am just about travelling
> throughout Europe I have calculated that it is about to spend 450 pounds in
> a 2186 miles journey, including EuroTunnel tickets and motorway fees in
> Austria and Hungary.  Quite steep compared to the flight ticket...
>
> Anyway, I also started to be curious how much time can I save by driving
> fast. So I took the speed limit in each countries, calculated the distance
> in each of them and played "what if". What if going on limit, or driving
> slower or higher. In a 3 hours journey from Calais to Luxembourg (which is
> my first stop) I can only save 30 minutes if I constantly speeding by 20
> km/h (compared to driving on the limit). Then from there to Budapest, if I
> go really fast on the German motorways (around 200 km/h), then I can save
> about 2 hours through the 650 km distance have to make there. That is
> something significant, however, even the fact that there is no speed limit
> in Germany, I have some doubt if it is possible to maintain that high speed
> (someone is always in front of you or there is a construction, or there are
> speed limits on bends and bridges etc... so shall we say 1 hour saving would
> be realistic?
>
> Now here is the hard part: How much extra fuel my car need for the higher
> speed? I could not find any information to that. Is there any good page or
> statistics for this kind of information?
>
> Tamas
>    
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