The difference in economy between a petrol Jazz and a diesel i40 is not going to be very much at all. And while an i40 is not an unreliable car, it is not going to be as reliable as a Jazz. Take that, add the fact that petrol is cheaper than diesel to buy at the pump, the fact that the standard servicing costs are going to be higher for the diesel, and the fact that if anything outwith standard servicing requirements goes wrong on the i40 it will most likely cost a lot more. and there isn't really a case for changing.

It is generally considered that you need to be doing well past 10k miles per year for diesel to become more cost effective than a reasonably efficient petrol alternative, and that is like for like (i.e, going from a petrol Jazz to a diesel car of the same class).

Even though my workhorse car is a diesel, I don't think there is any plausible case to be made for buying a 'modern' diesel car on grounds of economy. The fuel costs a bit more than petrol especially during the winter, but you would need to clock up huge mileages for that to become an important factor. Then allow for the cost of possible problems, and the zones where diesels are not welcome, IMHO it is best to stick with what you have. Changing cars always puts money in the pocket of the seller(s) anyhow, unless you switch to a golden oldie.

The depreciation already suffered is not relevant. What will happen in future is. The past cannot be changed.

Changing to diesel may save ~£700 pa. Assume 20k pa and fuel at £7 gallon.

• Petrol at (say) 40mpg = 500 gallons pa = £3500
• Diesel at (say) 50mpg = 400 gallons pa = £2800

Both the 3008 or i40 will have 150k+ on the clock in 5 years with limited trade in value. This may be a reasonable time from over which to make a judgement.

Fuel saving over 5 years will be £3500.

If Jazz is sold the cost to change will be £2k. If the 3008 is sold it will release £3k. Looking forward 5 years there may be little difference in value between the Jazz and 3008 with a 20k higher mileage.

The simple financial answer is sell the 3008, buy i40. Save £3500 in fuel costs + £3000 trade in over 5 years. But the answer is not always simple:

• keep the 3008 - be reminded of the fuel cost penalty every time you fill up
• driving pleasure/comfort - you will spend a lot of time in the car
• risk - can you anticipate events 5 years out (job, location, etc)
• i40 with 60k on the clock is a risk compared to known quantity 3008

In context - the total cost of running a car for 5 years is probably around £40k (£8k pa) including £15k fuel + £12k depreciation, + £3k insurance + £5k servicing and repairs. Saving £3-6k over a 5 year period is a little more than noise, but not compelling.

Thanks everyone for your input and thoughts, really appreciate it.

Taking everyone's views on board I am going to stick with. Whilst The £3-6K saving mentioned by Terry W isn't to be sniffed at it's a risk swapping either car especially when the cars we have are both reliable and both are above combined 40mpg.

Thanks again all!