The cost of fuel has fluctuated drastically over the last ten years. Prices can increase and decrease but quite a large amount literally over night. Diesel is often more expensive than petrol to buy, but even in the last few days I have seen many garages selling it at the same price and sometimes even selling diesel cheaper. Diesel cars are generally more to buy initially but do offer a better miles per gallon for the equivalent petrol car.
When it comes to fuel prices, you really do need to shop around to get the best deal. Some people will drive ten or so miles just to get fuel for their car that is a few pence per litre cheaper. If you are considering this, you need to work out how much it will cost you in fuel to get to a garage that is further away as often it is simply not a viable option.