Doing some rough calculations based on the current average prices for petrol and diesel, going from a 37mpg petrol to a 50mpg diesel over 11k miles will save you around £380 per year, circa £7.30 per week.

It is easy to forget how much other ’running costs’ there are than mpg. If something goes wrong on a diesel engine, it will cost significantly more to put right than a naturally aspirated petrol. Any fuel savings can and will be completely wiped out and then some!.

Given your budget is so low, and assuming your current car is reliable, I’d just keep that until such times as your budget allows for something much younger.

As has been mentioned, the 1.6 diesel in the Fiesta (as well as various other Fords, Citroens and Peugeots), has a somewhat chequered record on reliability. The engine in the Megane is fairly reliable, but all things considered, the rest of the car probably won’t be!.

I thought the fiesta was known to be reliable(oh well), Any suggestions for cars around the 3k mark.

I assume 37mpg is a real world figure for the OP's car in its current condition and with his driving style and usage. He needs to be careful comparing 37 mpg with published test data especially as he will be buying a relatively old and worn car. As already said, for meaningful advice he needs to reveal his current car.

At this money reliability and cost of repair are far more important that outright economy. I’d forget diesel here as they’re much more complex, less reliable and far more costly to repair. A good petrol supermini is the most economical way to travel for your budget and I’d shortlist the following. Ford Fiesta 1.25/1.4 Mazda 2 1.3 Suzuki Swift 1.2 Toyota Yaris 1.33 Honda Jazz 1.4 All offer reliability and 50mpg plus if driven with reasonable restraint and you’ll avoid the costly woe an older diesel can deliver.

Driving style makes a lot of difference. Years ago I bought a car from my best mate, he had usually got 25 mpg, on the same roads at the same times of day I got an average of at least 32 mpg.

How you accelerate, how you anticipate and how much you avoid having to brake affects your mpg: you don't take significantly longer to get from A to B and this style of driving is easier on the car.

Some older forum members may recall the Mobil Economy Runs, Here's a link to a Motor Sport magazine report which may be of interest

tinyurl.com/yxl7gx3p