After reading a couple of threads on testing "actual" mpg I decided to try the brim to brim method as the car's computer cannot be relied on.
On my first try I was pleasantly surprised to find that I was getting 43 mpg and not the 39 mpg that the computer said.
On the second go it only worked out to 40 mpg .

Conclusion;
The brim to brim method is no more reliable than the computer as you cannot tell by looking at the fuel level in the pipe if it is exactly the same as when you filled it the first time.

But there will always be anomalies. No two tankfulls of use will be identical, and weather will play a part. I'd say you were getting just over 40mpg, which sounds reasonable. It would be interesting to do the same test in mid summer.

I let the needle drop to the red mark then add 40.5 litres of diesel and 5 litres of veg oil (45.5 in total (10 gallons)).
Around town, I travel between 430 and 450 miles depending on how I drive. If I'm feeling naughty and some chav wants a race then I can drop this figure to around 400 miles - even one decent burn off can suck the juice!

Although the tank is listed as a 60litre capacity, this is not actual fuel capacity, it is simply the volume of the tank - you always have to have a head of air above the fuel. I've never tested the theory but the most I've squeezed in is about 50 litres all told.
Alternatively, if you don't like the idea of dropping the fuel to the red mark, simply record mileage and amount of fuel added. Over time you will get a good average consumption.
You have to remember, though, that every tankful will differ - you may get an extra 5 mpg using Shell supremo diesel or lose 5mpg by using Tesco value diesel.
You can lose 1mpg for every 1 psi down in a tyre.
You can also lose mpg by leaving a roof box - or even a roof rack/rails on.
Any unnecessary weight in the boot also increases consumption.

Take it as an average & reckon anything over 40 is good.

sludgeguts said:

You have to remember, though, that every tankful will differ - you may get an extra 5 mpg using Shell supremo diesel or lose 5mpg by using Tesco value diesel.

Click to expand...

I think the first tank full was Shell (but not their posh stuff) and the second time it was Sainsburys. No idea where Sainsburys buy their fuel.

I have heard people say they get an exta xxmpg when they use [insert name of expensive brand] fuel - but how much of this is the placebo effect? You're using expensive fuel so you try to get more out of it?
As I regularly drive to Hemel, I tend to fill up there as it is always cheaper than here - around 5p/litre. I also find big brands for the same price as supermarket - so I use the big brands just because they promise more (supposedly use more additives?). Otherwise, I just shove in whatever is cheapest.
i also like the big brand (additives) stuff to help as I always chuck in a gallon of veg oil with 9 of diesel.

I can get 90-100 miles on £10 of diesel if i drive steady and nurse it along.
not bad 4 a 1.9jtd,but when you put the boot in....well it comes down quite a bit ;o)

Is anyone on this thread a member here ?

Might find it usefull. I know a few FF members are already on there.

this may sound like a VERY stupid question but what is the whole brim to brim test

FractalPanda said:

this may sound like a VERY stupid question but what is the whole brim to brim test

Click to expand...

It's the only way to manually check your MPG, by filling the car to the brim, driving, and then working out how much fuel is needed to refill the car to the brim and miles covered to calculate the MPG.