If you are doing mostly short runs (shopping and going to the station) petrol wins hands down.

I am presently doing about 15-18K per annum with a lot of long runs and with quite a lot over the other side of The Channel where diesel is about 20% cheaper so I have a diesel no contest.

Somewhere in the middle is a crossover point

A diesel engine will give considerable savings in fuel when operated at part load. Full load savings are minimal. So, gently driven, a diesel is a good bet if it's reliable.

As a work-hardened ex diesel engineer, I'll make the following generalisations - I haven't time to write a long post.

Only the French and the Germans make good small diesels. Avoid American (Ford, GM) and Japanese. Common rail systems are marginal in terms of pump follower survival (especially after a mis-fuelling) and DPFs are bad news. DPFs using additives should be avoided at all costs - maintenance nullifies the diesel advantages.

So, where do you end up? On a limited budget, get an old IDI such as a PSA XUD. On a bigger budget, the earlier (1.9) VAG PD engines are a good bet - excellent Bosch oil-lubricated fuel system, strong basic engine and plenty of pattern spares, diagnostic software etc. I would not recommend the 2.0 PD.

Go very carefully with any engine using Delphi fuel injection equipment. A quick Google for "Mercedes blue efficiency problems" will illustrate the point, as will the number of Ford injectors requiring "re-coding". The "EPIC" fuel pump was hardly a product of lasting value for most long term users.

I'll sit back and wait for the flack...

659.

I'm currently a company car user, and diesel makes the most sense in the current tax regime.

However, even if I was buying privately, I think I'd still go for a diesel as the engine characteristics suit my preferred driving style. I like the grunt, and I'd want a private car to have similar punch :)

Having had a petrol turbo (Volvo T4) for 3 years, the engine was a treat to use but struggled to return 30mpg even on a long run, and averaged around 25mpg.

My old Passat PD130 and current Mondeo IV TDCI deliver identical performance in the real world and 75% better economy.

Had no issues in 5+ years with the Passat, other than the B5.5 Achilles Heel of corroded top knuckle joint in the front suspension (£450 per side). The Mondeo has needed only a brake light bulb so far in 2+ years (£1.25).

Edited by craig-pd130 on 05/05/2010 at 10:17

Reliability - I think the evidence is there that CR diesels are a nightmare when they go wrong, and massively expensive, but if you're lucky they don't always go wrong. I've got almost 300,000km on an eight year old FIAT multijet - door hinges, and anything plastic in the cabin aooears to be the biggest source of unreliability in my case.

I think the big question mark is the driver on a limmited budget buying a 3-6 year old car with reasonably high mileage and an unknown history, but only doing a limited mileage a year. Outside of warranty, lots of (expensive) things to long, limited budget, and low miles makes the economy benefits minimal.

Petrol is going the way of diesel

Look at the new BMW petrol's, the 3.0ltr straight six is now use high pressure piezo injectors and fuel rails. Along with EGR valves.

High precision fuel injection, low co2, good fuel economy but drastically increased complexity and the formation of carbon deposits on the injectors with expensive repair bills.

Generally, town miles / short journeys < 10 miles or <10k PA absolute min, it has to be petrol.

However, buying 2nd hand is a different story. Often diesels are actually similarly priced. E.g i was looking at a BMW 330i for my 10k PA. A peach of an engine. But because my journeys are normally longer than 20 miles one way and the fact that the 335d's are also in budget, i'm going for a 335d.

Auto too, so no DMF or clutch to replace. Gambling on a 4k gearbox lasting 160k tho :) But, i wont keep it longer than about 60k as i dont keep my cars very long

Edited by OldSkoOL on 06/05/2010 at 17:11