That's a relatively small mileage, worrying about relative fuel consumtion at that level is just spoiling your fun. Depreciation & maintenance are greater factors, and insurance is at least equal
You are doing 24 miles a day (at say 24mpg), call it 24 x 7 days a week = less than 9k miles per annum. If you get a car that does 10mpg more it will save you about £535 a year (at £5/gallon = £1.10/litre)
So if your car needs a decent set of tyres you can practically wipe that saving out. Change vehicles "for the sake of economy" and you can multiply that by "x" amount.
People get so hung up on fuel economy (not alone, the govt freak out about it too) when there are so many other factors. Sure it's getting worse but keep it in perspective.