[Closed] Big variance in diesel economy.

Has the warmer weather reduced the amount of clothing worn by younger females, thus cathcing your eye and menaing you are easing off with the acceleration to get a better look?
Or are you wearing less clothes thus reducing the weight inside the car? or on a similar note, did you get divorced/lose a child/lose a leg?

Vectra - not very accurate
CMax - more accurate

simple reaaly

Mine varies wildly with different fuel. Never use the supermarket stuff but BP ESSO and Shell all give different averages over a similar weeks driving.

For the record, winter diesel has additives in it to prevent waxing in cold weather, but this reduces the performance of the fuel. So you always get less in winter.

But also, Ford are really bad at reporting MPG if you are going by the computer figures.

530d I drove a few years ago did similar, felt the warmer weather had the aircon chomping a bit more.

Averages are worked out by me with a calculator, not relying on the trip computer because a) The Ford one is not very accurate and b) The Vectra didn’t have one.

I guess that the additives have a greater influence on the newer ford engine than the agricultural and old Vauxhall one.

My Diesel mileage appears to vary with ambient air temperature.

I keep an accurate record of fuel purchased, mileage covered and date.

I see an increase of approx' 10% as the Summer months arrive.

When I plotted Miles per Gallon Vs Date I got a Sine wave form peaking in the Summer months and declining in the Winter months.

I would suggest that's more likely to be down to fuel rather than ambient temperature, in a diesel. Colder air actually helps the intercooler work better, and colder air in the charge helps combustion. If the air's warmer and less dense, the ECU will just boost more to get the same mass of air in. That's why it's got a Mass Air Flow meter and not a Volume Air Flow.

With petrol though, warmer (or less dense) air improves economy because can put less petrol in and then need to throttle less, which is more efficient. You lose max power though. Same as at altitude.

However, warmer air is less dense so air resistance is less, for any vehicle.

Aircon might have something to do with it though too - modern aircon compressors are more efficient than older ones, so you would be losing less fuel to aircon.

I keep an accurate record of fuel purchased, mileage covered and date.

I see an increase of approx' 10% as the Summer months arrive.

When I plotted Miles per Gallon Vs Date...

don't take this the wrong way, but have you considered getting yourself a hobby? something like riding bikes maybe

air temperature means it is more efficient in summer than winter

some of you do need to get out more 😉

I watched some programme, I cant remember what it was, but they said you use more fuel in winter as you have heaters/lights on etc.

I think its because it rains more in winter and the rain hitting the car slows you down a bit so you have to push the accelerator more.

Dogging in the winter means you have to leave the engine running whereas in the summer you can throw open the doors and kick back on the bonnet, getting an expensive hobby in the winter! That's obviously your problem too!

Also colder air is more dense, thus to maintain air:fuel ratio you need more fuel...

DrP

Mine varies mostly depending on my right foot. (my old Focus wasn't so much like that, it wasn't economic but it was the same all the time)

Also recently I've improved the fuel economy by getting it towed places.

This thread a while back, general conclusion that brands/types of fuel can make a difference;
http://singletrackmag.com/forum/topic/shell-v-power-or-bp-ultimate-diesel-fuel-does-it-make-any-difference
Certainly makes a difference in my van, accepting driving style also makes a big difference

Also colder air is more dense, thus to maintain air:fuel ratio you need more fuel...

Not in a diesel..

Also colder air is more dense, thus to maintain air:fuel ratio you need more fuel...
Not in a diesel..

Edit:
Depending on the journeys the OP does I'd guess another significant factor is that many diesel engines take an age to warm up in winter and are less fuel efficient until they do, so if you mostly do shortish journeys, your fuel consumption in winter will be higher.

Heating, lights, rear and front screen elements all use fuel and are more or less constantly on throughout the winter months. IIRC Ford use a fancy alternator that draws no load from the engine unless the accessories are on or the battery needs topping up, this will be in use more throughout the colder months.

Nor in a petrol engine, for any given required output it'll require a more or less fixed amount of fuel so

But, if there's less air you'll want less fuel, surely? With normal driving it'll be as lean as is practical, so less air means less fuel and hence less power. So you'd be using slightly more accelerator pedal, and the wider throttle body would result in less pumping losses - no?

rear and front screen elements all use fuel and are more or less constantly on throughout the winter months

Really? Mine's only on for a few mins at the start of a journey.

Heating doesn't use fuel though, it's all waste heat anyway.