well not impossible for a V70 , so think what's different besides the obvious drag co-efficient which is a bit higher , but under say 60 mph wont have such a great effect ..
The mass of the car will be unfavourable if accellerated a lot but at constant cruising speed wont have much of an effect at all .
The killer for fuel economy on an XC90 is accelerating and going up hills .... you only have to watch instantaneous MPG to see this ... same with cruising at 60 mph on a flat road .. very good ...
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 My comments are only based on my opinions and vast experience .
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