I think there should be additional settings for speed on highway, usualy i drive speedlimit on city and on highway i go higher than speedlimit. Would be nice to get more accurate results when planing this way also.
You can already do this with the reference speed setting. For example if you say 106%, it will only have a small effect on city (50 -> 53), but a bigger impact on highways (120-> 127). As the consumption on lower speeds is usually quite low anyways and on longer trips you are normally not that often in cities, you can increase that value as much as you want and it should still be accurate enough.