If you do not have to sell then dont,, I hardly use my V70r these days, that does about 22 mpg and been 75k miles.
To change it would cost me quite a few £k but what is the point, that car would drop in value too. It is only the difference in fuel, and not all the fuel, the difference in running the 129 SL320 to the SL500 is around £200 per year, or in 10 years £2k, so should I really go and change for a lesser car and spend £10k in the process