Is there a list on-line of car fuel efficiency (MPG)?
Be aware that fuel economy isn’t the be all & end all of low ownership costs, particularly if you’re buying a new car on finance, in which case you’ll bleed money as the interest builds up while the car depreciates. Check out the links others have provided, but if your current car runs OK, you’re not going to save money by spending £10,000 on a new one that does 20mpg more, unless you’re a taxi driver doing 50-60,000 miles a year. If you fancy a new motor, fine, but do the sums first. The difference between a 30mpg car and a 50mpg one over 10,000 miles is about £600 a year, less than the interest on HP, and far less than depreciation on a new car.