As a very general rule of thumb, diesels tend to be 10-15 mpg better than their petrol equivalents in real world driving. Diesels also have far better torque (mid range pull) than petrols. Diesel engines need regular use and occasional high speed runs or can become problematic, whereas petrols are better suited for those who do mostly around town / local driving. They are also quieter than their diesel equivalents, although some Diesel engines (like those made by BMW) you’d be hard pressed when inside the car to tell it’s a diesel as they are supremely quiet. Diesel is a bit more expensive at the fuel pumps and Motability seem to be putting quite big AP premiums on diesels now, so unless you do more than, say, 10,000 miles a year, a petrol car may well be cheaper to run. I tend to look at ‘whole life costs’ (AP plus actual fuel cost over 3 years based on realistic mpg). In the past, diesel has always won, but we are now retired and doing less mileage, so I think we might well revert to petrol next time.