UKC

Geeky question. If anyone else keeps track of their car's exact fuel consumption, can you let me know whether a +/- 3mpg variation is about right (allowing for all usual obvious parameters like long runs vs a scrappy January full of short journeys)
I seem to vary between 33mpg and 39mpg, at first I thought something was up because it was just decreasing, but then a few long runs put it up again so I guess I can set a median of 36mpg.
(Y reg Vectra 1.8 LS, btw, and I got 46mpg out of it on a long run with the back box snapped off once!)

In reply to Blue Straggler:

I drop 4 or 5 mpg if I use tesco diesel instead of shell.

For normal driving my mpg is +/- 1 over a tank (550 miles).

In reply to Philip:

Thanks. My fuel is nearly always from Sainsburys so that's not a factor - not is heavy loading. It is probably to do with whether a full tank saw mostly short journeys or longer runs. I did notice a decrease in economy when I fitted new front tyres and I think that's because I did run the old ones pretty thin (still legal!) and had three months of "drive as if you have no brakes" which is conservative driving, and the new tyres improved the handling so much that I started "playing" a bit more, hooning it around like a brat...

In reply to Blue Straggler:

Mine (57 Focus 1.4) varies dramatically between wet and dry, and between summer and winter - if only I could always drive on warm dry sunny summer days, I'd be sticking petrol into it considerably less frequently than I am at the moment! And before anyone asks, I don't use the aircon, I try to avoid the rear screen heater wherever humanly possible, I don't drive with the windows open, or any other of the well-known fuel sappers. Ho hum.

In reply to Blue Straggler:

Length of journeys makes a massive difference over a tank full.

This is mainly because cars are less efficient with a cold engine, so lots of short journeys mean a greater proportion of the running is on a cold engine.

Euro4 compliant engines are especially bad as they run richer when cold in order to warm up the cat quicker.

diesels are worse than petrol as they take longer to warm up.

for my commute (10miles on dual carriageway) i tend to get around 40mpg, if I do a longer run to the peaks etc I get around 50

In reply to John Ww:

Windows open and air-con operating, at full pelt down the motorway, increases consumption by not much more than 4%. The AA did a test more than a decade ago. Where air-con costs fuel is in the weight of the unit.
I also doubt that the rear screen heater uses petrol. Or am I missing something, have I been misinformed?
I don't have aircon and rarely drive with windows open btw.

In reply to Blue Straggler:
Don't know about petrol, but my diesel Fiesta and Clio both did measurably better on supermarket fuel compared with Shell. Cold weather degrades consumption considerably (upto 10% worse at this time of year compared with summer). I do the same 80 mile round trip daily on a quiet A3 and the figure is stable tank to tank to within +/- 1mpg for a given time of year. 02 Clio was around 67mpg in summer & 62 in winter,57 Fiesta is about 61mpg in summer and 54 at moment.

In reply to balmybaldwin:
Yep, but in a hand-wavy way (and I know I didn't mention this!) I'd sort of factored that in and felt I was still seeing a decrease (I have a spreadsheet and I make a comment next to each calculation e.g. "mostly short hops, just one long motorway run to see client"). Maybe the winter months mean the engine is physically cold for longer so working harder? Or I need to be more fastidious with the calculations. Or just return to "having a life"

In reply to Blue Straggler:

Unless my knowledge of basic physics is even more sketchy than I thought, all energy has to come from somewhere; even turning your lights on will increase your fuel consumption, even if only marginally.

In reply to John Ww:

Depends how well the alternator is set up I suppose, but overall unless you were VERY exact about your measurements, the increase in consumption from using electrical gubbins would be buried in the noise in your data, I think...but yes, there is a use of fuel, albeit rather indirectly

In reply to Blue Straggler: tyre pressure? This has a big effect on my consumption, even just a small drop in pressure in one or two tyres.

Wind can kill efficiency too - driving down to Oxfordshire from the Highlands after Christmas, I was against the wind the whole way, and used about 15% more fuel than normal. Going home again was noticeably more efficient.

In reply to Blue Straggler: One of the biggest factors is the calorific value of the fuel you use. None of us will ever know how consistent that value is and the exact storage parameters that inluence the calorific value at the time the fuel is delivered to your vehicle tank. Distillation values and storage temperatures will have an inluence as will volume and density of product. Probably a little too scientific but don't assume it is your driving style or vehicle dynamics, it could well be the characteristics of the fuel and the engine at the precise moment combustion occurs.

In reply to balmybaldwin:


You have that the wrong way round, Diesels are more efficient for short journeys, as fuel metering isnt reliant on engine temperature like petrol engines.

In reply to balmybaldwin:
>


My dad always told me that diesel engines were more or less operating efficiently from startup (i.e. even when running cold) whereas a petrol engine needed to warm up to reach maximum efficiency.
But, being a diesel fitter, he was probably a bit biased.

In reply to Blue Straggler:

Log it all. Any change for which there is no known reason may be a sign of trouble.

In reply to Blue Straggler:

But how do you measure lights, heated rear window, fan etc. they all have an effect and none of it is free energy, the greater the load the more the engine has to produce, the more fuel you'll burn.

In reply to smuffy: I often wonder how much temperature variation of density affects the total calories stored in the tank when full - if its hot in summer a full tank will contain less chemical energy than a full cold tank in winter. So I presumably in summer I should get lower mpg but still the same miles per calorie.

I guess this only affects the fuel once its in your tank as when you buy the fuel it is stored underground and is always cold?

In reply to krikoman:

My car has a Driver Information System which tells you exactly how much more fuel is being used with each bit of kit you have on. It even tells you off for revving your engine when stationary or accelerating with the window open!!

AC and rear de-mister are the worst culprits...

In reply to Blue Straggler:

In my 2.0 tdi Octavia, on an 80 mile round-trip, I get 50mpg if I drive "with purpose". If I dawdle, then it's more like 58-60mpg.

The drive is mostly decent A roads and dual carriageway.

In normal traffic, taking the dawdle approach only adds a couple of minutes to the journey, so I tend to dawdle most of the time.

As another poster mentioned - if I'm driving into a strong headwind, consumption increases quite a bit (10-15%).

In reply to Blue Straggler:

What about the weight of the car? If you only ever fill to half a tank rather than fill up to a full tank every time I would expect that over a period of time your fuel consumption always filling right up would be greater than topping up to a half tank?

Also the number of occupants, amount of luggage etc must have an effect?

I am always suprised at the number of people who leave a luggage/bike rack on all the time, even when they are not using it. The drag must have a significant effect on mpg?

Other factors include, as mentioned a/c or windows open. Also style of driving eg too much heavy breaking or acceleration.

It's interesting to watch the prats on the A20 duel carriage way leading into London (which has a 50 mph speed limit) braking for every speed camera then accelerating away immediately afterwards, only to repeat the process at the next one, and so on - there are lots of cameras! Keeping to a steady 50 mph must be more economical and by the time you get to the Blackwall tunnel you find that you are only about a dozen cars behind.

My car is a convertible and fuel consumption is worse with the hood down against up.

In reply to Philip:

I always go for Shell, if possible, then BP - supermarket fuel never gives me as good mpg. In fact, last time I had a tank of Sainsbury's fuel, it was utterly gutless and took 3 tanks of Shell and a dose of injector cleaner to get it back to normal!

In reply to OP:
Yes, +/- 3 mpg is about par for mine - Focus 1.8 tddi.

Mine seems very sensitive to tyre pressures, so I do check them regularly.

In reply to Jim Fraser:

Definitely - recent dramatic drop in mpg was a sign of seized brake caliper

Interestingly, it coincided with having new tyres fitted, which masked the cause at first, as I wasn't getting any other usual symptoms, like pulling to one side.

In reply to Blue Straggler:


If your aircon is drawing say 1kW (seems reasonable but the number is a guess) then that's a pretty big increase in power requirement given a modern car cruising at 50 is maybe only using ~10kW. Of course as you say the weight costs you every time you accelerate.


Again, it's going to draw a few percent of the fairly modest cruise power of a small modern car. A few percent here, a few percent there all adds up. Wet roads cause more drag too.

That said, I think you've hit the nail on the head when you've said each tank see's a different set of journey types and that makes a big difference. One of my cars simply will not go over 225mi to a tank no matter how I drive it whether around town or long motorway jaunts. A year or two back I got caught in crawling constant speed traffic through calming on the A1M and both ways over the A66, it did 320mi on that tank before I filled it up. +-3% seems pretty consistent to be honest.

jk

In reply to LastBoyScout:

Fuel comes from the nearest distillery, it may be owned by Shell, BP, Esso etc so it all depends on your location. With regards to supermarket fuel, I am not sure where they are supplied from.
There is a BS for petrol and diesel so all fuel should be fairly consistant.

In reply to jkarran:


An RX8 by chance?

In reply to Blue Straggler: <checks Excel spreadsheet>

The lifetime average MPG on my car is 37.5. I've had a fill-to-fill mpgs as low as 30.6 and as high as 47.2 (the latter was on a single 350 mile journey on a mixture of A roads and motorways). I suspect that fill inaccuracy is also a factor in some of the variation - some pumps cut out a lot more easily than others.

So I'd say that your 3mpg variation either way is entirely normal.

In reply to balmybaldwin:

Similar, Z3M.

In reply to LastBoyScout:

And when the temperature changes, so do your tyre pressures. Remember you need to add some air when it gets colder.

In reply to John Ww:

There was a top gear challenge, in which JC (clarkson) drove from London to Scotland (Edinburgh) and back on a single tank of diesel in a big Audi A6. He eaked out the miles by turning all electrics he didn't need off, along with the more standard hypermilage driving style. Sadly there was no control car so we don't know what the effect of the elecrical gizmos would have been, but I am sure it helped.

In reply to Jim Fraser:

Yep!

In reply to kipper12:

That's Top Gear for you

Yes, the electrics will increase consumption, I always accepted that. I just didn't think they'd contribute SIGNIFICANTLY to the VARIATION that I am asking about.

Was chatting to a colleague today about this, and he pointed out something that I am now surprised that nobody else has mentioned on the thread already - driving in the wet. You are using your fuel to move quite a lot of water through your tyre treads.

In reply to Blue Straggler:
I get far more variation than that just depending on if I'm listening to a noisy CD or radio 4; loud fast music makes me drive like a fool so I stick to radio 4 or 2 that makes me drive quietly and sensibly.

Everyone goes on about turning stuff off, not using AC etc. but just changing your driving style is going to be a lot more beneficial.
My focus mk1 burns about 10% more fuel per mile at 85 than it does at ~70.

Lets all listen to radio 4.

In reply to Blue Straggler:

I know that in an older car I had (mazda 121, original style), when sitting with the engine idling if I turned on the rear window heater the engine tone changed, so it was in some way a noticeable burden. How much, I don't know.

If you could get the wattage of the various systems (would be listed somewhere) you could work out total burden of various electrics in power. You could then look at the joule content of the fuel, run that through the efficiency of the engine (20%?) and the alternator (80%/90% ?) and work out how many litres per hour you'd use to keep those systems running. Then with your average journey speed you can calculate an effective mpg impact (faster you drive, quicker the journey, less important the electrics are).

Quick google indicated alternator might be giving somethign like a capacity of 650W, say 500W for ease of calc
http://www.physicsforums.com/showthread.php?t=267622

Say the alternator is 90% efficient, this means need shaft power of 556W into it. Say Engine is at least 20% thermodynamic efficient, then you need at most 2.8kW of fuel calorific energy in.

If you were driving for 1 hour, or 3600 seconds, you would then use about 10MJ. A litre of petrol has about 36MJ (http://en.wikipedia.org/wiki/Energy_density ), so in one hour you'd use maybe 0.3l of petrol.

In UK gallons, that's 0.07 gallons.

In that hour, you might have driven 60 miles, maybe at 30mpg, so you'd have used 2 gallons of petrol.

So you end up that the electrics are maybe 3% of the consumption (assuming you're really using 500W of electrics... is that realistic/high/low?)

Then again, Futurama has a lesson to teach:
http://www.tvfanatic.com/quotes/this-ac-is-incredible-i-better-turn-on-the-...
(can't search youtube for the clip right now...)

In reply to michaelc:

I had to do environmental testing of one of our systems once. For the low temp, we put an air conditioning unit into the test chamber (a metal room, about 3.5m * 2.5m * 2.5m). The heat sink / outlet of the air con was put on a windowsill under an open window. So far, so good.
Part of the equipment I was testing needed a constant flow of cooling oil (under any conditions). The oil pump and heat sink for this were mounted under that same open window. So far, so good.

The oil pump had a flow switch which would trip if it thought it detected an interruption in flow. So far, so good.

However, the switch was dodgy and temperamental - there was a poor connection somewhere. In short, this meant that if it was cold, the contraction of metal somewhere in the switch would break the contact erroneously. For it to work properly, it needed to be warm. So I had to put a fan heater next to the oil pump beneath an open window in winter when the outside ambient temperature was lower than what our aircon unit was achieving inside the test chamber. It was about as ecologically bad as you can get!

But the end use was related to not blowing up nuclear reactors so I think the end justified the means

In reply to johnwright:


My oil industry friends tell me same. The only differences are due to the proprietary additive packs that the various 'branded' suppliers add to the tankers. Supermarket fuels won't generally have these additive packs.

In reply to michaelc:

I was about to propose the same method; look at the rated power of the engine (should be specified in kW somewhere in the hanndbook, or in a form that can be converted to kW). Then look at the power required by the various electricals; one way of getting a first-order approximation would be to look at the fuse for each equipment, then calculate electrical power used by VI. Maybe assume fuse rating is twice the operating current, or just be pessimistic.

In reply to michaelc:

Thanks for the maths! Ok let's go with 3% if using all your electrics.
I am seeing a +/-3mpg variation around a central value of 36mpg.
That is a +/- 8.33% variation actually observed by me since June.
Your calculations show that electrics COULD be contributing +/- 1.5% to this (let's say for arguments' sake that on average I have half the electrics on anyway)

So looked at that way, it is not as insignificant as I thought - even allowing for all the assumptions and rounding up, it's not like it's orders of magnitude out.

I did kind of know this, from my experiences on cheap buses travelling through Indonesia (why use headlights when you can string up fairy lights and run them off a separate knackered car battery in the cab? why use windscreen wipers when the driver or his mate can lean out with a rag every now and then and wipe down the screen?) Fuel saving measures for sure.

In reply to Blue Straggler:


*Very roughly* cruising at 50 requires 10kW to overcome drag. At 70 it's ~20kW.

The big ticket items the user can turn on or off are the heated screens and aircon, items which are unlikely to be used together. The screens + lights might be 500W, I guess aircon is double that, let's say 1kW. Just estimates but in the right ballpark.

Cruising at 50 about 10% of your engine output is going to costly peripherals. Cruising at 70 it's more like 5%.

Of course most folk don't do a huge amount of steady cruising, a large proportion of the fuel in your tank goes into accelerating (ultimately heating the brakes).


As you've picked up on the rolling resistance changes with the weather too since cold tyres run a little flat and clearing water/snow causes a lot of drag.

jk

In reply to Stuart Hurworth:

The "total calories" delivered to your tank will have a seasonal variation, but the far greater factor than temp/density variation will be due to the biofuel content and whether it's a summer/winter grade fuel.

Biofuel: Ethanol in gasoline and FAME in diesel have less calories per g than fossil fuel derived fuels. This is an inherent factor due to the presence of the oxygen atoms in their structure.

Summer/winter grade fuels: Winter grade is made from Oct to Mar and has a lower freeze point (diesel) and higher Reid vapour pressure (gasoline) than summer grade. These factors reduce the calorific value of the fuel in winter.

Also, one thing that has not been mentioned here is that the viscosity of your engine oil in winter during a cold-start will have a dramatic increase in the friction losses in an engine until warmed up fully. This could easily add 10% to an otherwise identical short journey, such as the school run.

To the OP; +/- 3% accuracy in fuel economy is pretty darn consistent. The industry standard NEDC drive cycle used for testing new vehicles fuel economy is only accurate +/- 1%!!