That's what I did yesterday, Jacked, up wheels off, brakes checked.

Full engine Obd test, tyre pressure, and fuel line check. Oil checked. All came back as Good.

Personally within a few miles of leaving the forecourt you could feel a change in performance. The torque felt less at lower rpm.

This is why I I think it a fuel quality constancy problem.

I am just putting this out in the public domain to find out what we as consumers can do, when face with these factors?

Huge drop in mpg - I would be looking at what might be causing it rather than blaming the diesel - .

I don't believe that any kind of diesel could cause that kind of change in Mpg, so it must have been something else, presumably petrol. That error could have been the buyer's or the seller's (I still suspect the price may indicate petrol).

How does a diesel engine run on a petrol/diesel mix - will it, and what might be the mpg ? It won't do the lubricity of the innards much good either ....

Edited by Andrew-T on 09/11/2021 at 09:53

Just refilled the tank with Shell V Power Diesel, and to my amazement, the MPG come back to 46, from the fuel station in Preston to home about 20 miles, but on a warm engine.

The tank before got down to 33.6 mpg on the whole tank before fuel warning light came on.

With this evidence, In my opinion, the fuel was not Shell V Power Diesel, sold me in Whitby Shell Station.

Q1 these are the cars calculation over a sample of at least 180miles.

Q2 Got the receipt that says so. Pump 5 Fuel - Pre ( 1) 31.48l @ £1.559/L

Nice one, it was definitely V-Power diesel on the pump and its markings. It was over 30 years ago when i had my last petrol powered car.

The main points are, the quality of the fuel grades being consistent?

How can we, the consumers, check we are getting what it says on the pump?

And what the recourse if it not up to standard and proving that?

One Idea is to have a coloured Dye for fuel save and supermarket fuels, and not dye for premium fuels? Thoughts?

It was probably driving round Whitby trying to find a parking space that caused the excessive consumption.

Going to have to wait for Shells investigation.

Will not be filling up for next 10 days, but when I do, I will be putting Shell V power Diesel in from the previous fuel station where i was getting the 40 to 44mpg.

Will post the results.

Most forecourts will now be dispensing winter diesel, which is more resistant to waxing. It also gives less energy per litre than summer diesel.

Perhaps you got a tankful of winter blend, or maybe it wasn't premium but ordinary diesel in the tank that your pump dispensed from. That, combined with the colder ambient temps in the past couple of weeks, can absolutely cause MPG to plummet.

All of the turbodiesels I've run (Passat PD130, Mondeo 2.0TDCI, 2xVolvo V60) have all shown similar falls in MPG in everyday driving as soon as the daytime ambient temps fall below around 8 or 9C, simply because the engine takes much longer to warm up properly.

That's curious, as I keep a detailed record of miles and fuel used, and over the last few years I have seen no notable difference in mpg varying according to the time of year - and I use winter tyres in the winter that have a higher rolling resistance. It's a Mazda6 2.2d which has now >160k on the clock, doing ~20k miles each year.

Back in the late 1980s I had a Renault 18 diesel 2.1L, During the winter it got down to -20degrees C. I just managed to start the engine and had to keep my foot fully depressing the throttle, just to keep the engine ticking over. After 20 mins the fuel return from the injectors managed to unwaxed the diesel in the tank.

After that i put a heating element designed to go on the fuel filter.

Look at most car fuel tanks and they are plastic, which help insulate the fuel better, than steel tanks which at the time the Renault had.

There was no mandate by the government then, when to put anti-wax additive in the fuel, now there is.

Waiting to see what Shells investigation comes back with.