Fuel consumption puzzle

[QBee]

My daily driver diesel auto has an MPG readout on the dash. I make no apologies for owning a diseasel for the daily drive. It is 90 miles each way , up and down the A1/A14/M11, so it is a large car for safety. I stick the car on cruise control at a steady 70 mph, slowing for the occasional 18 wheeled roadblock in the outside lane. By the way, I have a 5 litre TVR for the B roads and proper road and track driving on my days off.

So, most times I put fuel in the car it is at a BP or Jet station, sometimes Shell. Always standard diesel.
And most journeys the readout for the entire journey is 44.5-45.5 mpg.
Monday evening I put Tescos diesel in the car.

Yesterday I stuck 4 wheels and tyres in the boot, total weight 80kg, to get new tyres fitted. This morning, normal drive to work at 5.30 am, only modest amounts of traffic, I got 40.9 mpg.

So the question (and obviously I will do more tests) - is the 10% worse fuel consumption down to the extra weight (equivalent to a body in the boot), or down to inferior fuel?

[numtumfutunch]

My old E46 330d always got better mpg on the 'premium' stuff versus the generic - although the price differential wiped out any cost saving - so Id say its possibly due to fuel assuming everything else is standard, which it isnt in the case you used.......

[E65Ross]

I'd say if it was more stop start I'd say the extra weight would be the likely cause. But sat at 70mph I'd say it was more likely to be the fuel.

[QBee]

I appreciate I am not comparing like with like.....so will test again on Tesco diseasel without the load in the boot, but your thoughts are appreciated while it is fresh in my mind.

I have noticed that I generally get slightly better fuel economy travelling south (ie down the map) than travelling north (ie up the map) , but have put that down to cooler air in the mornings (southbound) than evenings (northbound) combined with worse traffic in the evenings. Start and finish locations are both at the same height above sea level.

[heners54]

Depends on the car IMO, extra 80kgs in the boot of a 1.6 diesel in something fairly large like a grand picasso for example wouldn't surprise me by lowering the mpg at motorway speeds.

[QBee]

Its an Audi A8 3.0 TDI (2004 vintage, a snip at six grand), so about 1.9 tonnes due to the aluminium body.

[heners54]

I change my vote to filling up at Tesco then
Slightly biased as I normally knock supermarket fuel, but recently my car is running better on Tesco momentum than my local Esso; old rattling pumps and I reckon water in the tanks, so I had poor mpg and slight kangarooing in traffic all last week.

Do tell us how it is after another fill!

[JM]

heners54 said:

I change my vote to filling up at Tesco then
Slightly biased as I normally knock supermarket fuel, but recently my car is running better on Tesco momentum than my local Esso; old rattling pumps and I reckon water in the tanks, so I had poor mpg and slight kangarooing in traffic all last week.

Do tell us how it is after another fill!

Changing from 95ron fuel to 99 should make a difference, or even it it was Esso super, the improvement from 97 to 99 would be beneficial.
Depending on the car it could make a noticeable difference.


To the OP, I used to find a mileage difference between Shell and Morrisons fuel, 40-50 miles more per tank in favour of Shell.

[mygoldfishbowl]

When I drove for a living I found I got more MPG from Esso diesel than I did from any other diesel.

May I ask why you go A1-A414-M11? Are you going north on the M11? Otherwise I can't think why you wouldn't use the M25 at that time of the morning.

Edit. Reading failure A14 not 414.. Makes sense now.

Edited by mygoldfishbowl on Wednesday 23 October 09:00

[QBee]

mygoldfishbowl said:

When I drove for a living I found I got more MPG from Esso diesel than I did from any other diesel.

May I ask why you go A1-A414-M11? Are you going north on the M11? Otherwise I can't think why you wouldn't use the M25 at that time of the morning.

Edit. Reading failure A14 not 414.. Makes sense now.

Edited by mygoldfishbowl on Wednesday 23 October 09:00

Thank God someone else is as dopey as me this morning! It's Newark to Abington (7 miles south of Cambridge)

[Mr Gearchange]

You are doing better than me.
My A8 3.0Tdi (07) has averaged 35mpg over the last 20K with most of that motorway driving. I can see 40mpg if I am really pootling.

But yes - I get better mileage with the premium deisel - and the car runs much more smoothly

[QBee]

heners54 said:

I change my vote to filling up at Tesco then
Slightly biased as I normally knock supermarket fuel, but recently my car is running better on Tesco momentum than my local Esso; old rattling pumps and I reckon water in the tanks, so I had poor mpg and slight kangarooing in traffic all last week.

Do tell us how it is after another fill!

I run my TVR on Tesco Momentum or Shell V Power. Despite being an old Range Rover engine, with the state of tune (over 300 bhp) it runs noticeably smoother on the 99 RON fuel.

The Audi however is a diesel, but I will try a tank (=80kg, by the way) of superdiseasel to see what difference if any it makes. I generally run the car with the tank about 1/3rd full, partly cash shortage, partly as I am aware that a full tank is 80kg. However, I have to say that i have not previously noticed reduced MPG with the tank full, so maybe my 80kg of wheels and tyres in the boot are not the cause of the 5mpg drop this morning?

[QBee]

Mr Gearchange said:

You are doing better than me.
My A8 3.0Tdi (07) has averaged 35mpg over the last 20K with most of that motorway driving. I can see 40mpg if I am really pootling.

But yes - I get better mileage with the premium deisel - and the car runs much more smoothly

My Audi, in mileage terms, is mostly used up and down the A1, so 75%-85% of my mileage is motorway cruising. I do use the cruise control, and stick to 70-72 mph, not 80-85 like many drivers, as that results in far less speed variation, which is the MPG killer on motorways. Most of my journey is on 2 lane dual cabbageway, so the 18 wheeled overtakers have a significant effect on average speed for the faster driver.

The long term average (readout 2 on the dash) is just under 39mpg, which includes our shopping trips (6 miles return) and other family stuff. I think IIRC that the 07 Audi has more power than the 04 (231bhp). The 04 Audi is still a lot nicer to drive than my old 1.9 TDI Auto Saab, whose get up and go had got up and gone where nipping out of junctions was concerned.

I will take your advice and seek out super-diseasel for a trial. My son in law now has the Saab, and reports that it runs a lot better on super-diseasel. The Saab does 50mpg with me driving it on the motorway to South Wayels and back on cruise control.

I know I sound boring, but my morning 90 mile journey could be shortened by as much as 5 minutes by standing on the gas pedal, but I used to arrive so much more stressed. Now I head off early and relax - this morning's commute was 92 miles in 89 minutes.

On the race track in my TVR I can get my fuel consumption down to 8 mpg if I really try!