It may seem confusing to determine which is the best fuel efficient vehicle either one that runs a diesel or petrol engine. It boils down to the type of vehicle driven, the kilometres driven and the load carried. It also depends on daily driving, how many kilometres put on the vehicle each year, plus the cost of the fuel. The cost of diesel and petrol goes up and down each year. Once upon a time diesel was a lot less expensive per litre than petrol, now it can be the opposite way. Generally, the owner of a diesel-fuelled vehicle will see the savings over that of petrol if driven for at least 2 years. Otherwise, if a vehicle is bought and traded within the 2-year period, the cost of the petrol vehicle will usually be less.
Maintenance costs depend on the make and model of the vehicle. Diesel engines generally cost more to maintain over petrol engines. Smaller diesel engines will cost more over all because there is no need to haul heavy loads, therefore any need to have a small vehicle with a diesel engine, unless you plan to keep it for years or it is a very recent model. A larger truck, especially ones that will haul heavier loads, the diesel engines will be cheaper to maintain in the end. A larger truck that runs a petrol engine will have more issues and therefore will cost more to maintain.
Diesel engines can be a higher in cost with the licenses and registrations too in some countries. Petrol vehicles are cheaper to maintain, with both paperwork and automotive maintenance, unless a person puts many kilometres on the vehicle. Petrol engines, even if well maintained, will have more issues the more kilometres they have on them. Maintenance, service, and repairs are less expensive on a petrol engine. The only way it works out in this instance with a diesel engine vehicle, even a small one, is if the owner drives it for long distances every day. However, very modern diesel engines had bridged this gap and are now very competitive for the average car driver.
To answer which vehicle has the better running and maintenance cost, depends on the vehicle make, the amount of kilometres driven and the load of the vehicle. Keeping a vehicle for a while in addition to driving it longer distances will make a diesel engine the better choice. Otherwise, shorter trips and the need to trade more often, a petrol will be the better choice cost wise unless it is a very recent diesel engine. Also factor in the cost of the fuel when determining the overall maintenance and running, because that fluctuates all the time. Some periods petrol is the better choice, others it is diesel.