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Re: Car fuel use, actual vs. stated

PostPosted: Mon Oct 25, 2010 10:49 am
by greg c
I think the tests are biased toward larger cars, but if you know that at least it is a standard all cars are measured against

Citroen C5 HDi sticker says 7.1. I can get 7.6 in the city and 6.5 easily in the country, even down to 5.5 if you really try
Citroen C4 HDi sticker says 4.7. There is no way you can get under that in normal driving. I know a guy in Albury who keeps meticulous records and he can't beat 4.7 either.


Re: Car fuel use, actual vs. stated

PostPosted: Mon Oct 25, 2010 12:34 pm
by zzsstt
I suppose the standard is at least a common point from which to judge vehicles, but the nature of the standard is such that it is not very realistic. The rates of acceleration are very low, 0 to 15kph in 4 seconds may give good fuel economy but if you drove that slowly at any set of traffic lights you would likely be rammed! For extra-urban driving, the reality is that you sit at 100kph (or 110kph as appropriate), yet the vast majority of the "standard" is at or below 70kph.

The result is that the tests are biased towards good economy at low speeds. A vehicle with gearing such that it can be driven at low rpm at 50 to 80kph can give excellent test results, but when used on the roads that economy cannot be replicated because at 100kph the wind resistance has increased massively and so have the engine rpm's. This is not overly detrimental to the test result, because so little time is spent at such speeds, but in everyday use where such speeds are the norm it has a serious impact.

Our Prado can, at 70-80kph, return a fabulous "spot" value from it's econometer, but that drops away markedly when the speed is increased to 100kph.

It is, however, interesting that the Prius returns such good numbers in what I assume is highway driving. I have always figured that in rural/highway driving the Prius would have very minimal electrical generation and so the hybrid concept would become much less beneficial than it would be in stop/start city driving. For it to return a (presumably mostly petrol driven) highway journey economy the same as its in-town result is very impressive. I do not know the Prius, but it would be interesting to know if the battery capacity is available to the driver, to see whether it had also depleted it's battery during the journey?

Re: Car fuel use, actual vs. stated

PostPosted: Mon Oct 25, 2010 1:11 pm
by munter
Re the Prius - the touch screen does show some information about the battery capacity and the power flows inside the powertrain. On the freeway, whether the battery is being charged, discharged or just sitting is determined by an algorithm that I haven't worked out yet. The direction of power flow into/out of the battery changes every second or two depending on the circumstances. On the freeway I basically ignore the power flows as I can't understand what is happening! The car never lets the battery fully deplete and my observation is that the algorithm manages the charge level so that even after extensive highway driving, the pack is within the acceptable range. We pulled up into our driveway after the three hour drive with the battery one bar away from a full 7 (or is it 8) state of charge though that has more to do with our house being half way down a hill rather than the freeway driving that preceeded it.

I think the good freeway (speed limit +- 10) economy comes down to good aerodynamics and the ability to run the petrol motor at an efficient speed through the use of the CVT unlike around town where the regenerative capacity and electric motor boosting is a significant help. The engineering that has been put into the design of the car is quite impressive (though I am obviously biased).

Re: Car fuel use, actual vs. stated

PostPosted: Wed Oct 27, 2010 8:05 am
by PeterC
Our old (1988) Honda Civic 4WD wagon used to be almost 10L/100km around town (Canberra) and just over 8 on highway trip (though usually heavily laden). I think it was supposed to be 7.6. Our former 1L/3cyclinder mid-80s Daihatsu charade would routinely do about 5 on highway trips-barely different from the Prius.
We now have a 1998 Subaru Forester which uses about the same as the Honda Civic it replaced even though it is a little bigger and 300Kg heavier. For a while we had a 1.4L Hyundai Getz rated at 6.1; the best highway we got was 6.7, about 7 was normal for highway and a bit over 8 in town.
To answer some comments about the Prius.... Its standard battery is really only good for very few KM of EV only driving. It is there to moderate demand and loads on the petrol engine, not to store lots of energy. As electric car batteries go it is very small. The prius is a very 'mild' hybrid. IE it is mostly a petrol car with an electric motor and battery used to assist in a secondary way. The good highway performance of the Prius is mainly down to low rolling resistance tyres and good aerodynamics, not being a hybrid. You can get an add on battery for the Prius which has much greater capacity. Plug-in conversion kits (eg. let you mostly use power from the wall for some 10s of km. On shorter urban trips people report getting about 2L/100km of petrol use. When you have run the extra battery down the car seamlessly reverts to its normal mode and the economy is no worse so you have the range for longer trips. When you get home you plug it in so that your next trip is at least partly on Greenpower rather than fossil fuel (or at least it should be!).

Re: Car fuel use, actual vs. stated

PostPosted: Thu Oct 28, 2010 12:37 pm
by TerryB
I have a 2006 Prius (sticker consumption is 4.4) and have kept consumption records since I purchased it using an iphone app.

My average for past 18 months (around 30,000km) has been 4.15

Worst tank = 4.38 (e10 fuel)
Best tank = 3.72

A few things I have noted:

* e10 fuel is definitely less efficient that regular unleaded in the Prius and the saving of a few cents per litre at the pump is nowhere near enough to offset the lower economy.

* Premium (95) unleaded gives better economy than regular unleaded by around 5% on my car (I generally use this now - it costs me about 7% more than regular unleaded so even with the improved economy it still costs a little more than regular but several mechanics have told me that the premium is better for the fuel system (cleaner) so is probably worth paying a few percent more for in my case).

* Tyres make a fairly noticeable difference to economy. The Prius comes with low rolling resistance (LRR) tyres as standard but when they wore out I replaced them with a better LRR tyre (Bridgestone Ecopia) and have noticed a further improvement. I would guess that replacing standard tyres with Ecopia's would save up to 10% fuel on many cars.

* Keeping tyres inflated properly is essential - I inflate mine to the higher pressures indicated in the handbook for "high speed driving" which gives better economy than the standard inflation pressures (perfectly safe but will give a slightly firmer ride).

* On a Prius specific item you can improve efficiency by I would guess around 10% by adopting the "Prius" style of driving which gets the car to coast with engine off more often - it isn't any slower than driving normally but does definitely improve economy noticeably. Obviously you cannot do this with a standard car but by anticipating traffic flow, lights, etc, you can reduce fuel consumption.

With regards to the testing I believe the standard test is actually done on a "rolling road" simulator rather than on real roads (or at least it was a few years ago) which means that wind resistance is not even factored into the test results - so a car like a Prado probably gets better results than it would in a real world environment whilst the benefit for a slippery car like the Prius is not as great. Of course no test is perfect but overall I am sure that they are fairly indicative of what you can expect so if you buy a car with better economy figures on the test you will use less fuel than one with worse figures but you just might not quite get the sticker figures.

Re: Car fuel use, actual vs. stated

PostPosted: Fri Oct 29, 2010 9:35 am
by munter
Very impressive figures Terry. I thought I was driving our car pretty well but your results show there is still plenty of room for improvement. I run our tyres at 40 or 41 psi - what pressure do you use? I am still using the Michelin XM1's but will have to look at other LRR options.

I'm guessing that my poorer economy is perhaps a combination of:
Poorer driving technique
Living in a hilly area where short bursts of high power and the use of friction braking is perhaps more common?
Not as optimal car configuration (tyre brand/pressure/something else?)


Re: Car fuel use, actual vs. stated

PostPosted: Sun Oct 31, 2010 5:21 pm
by karlajensen
I cant be happier with my LPG Volvos
I have a V70T5 with BRC vapor inject -it matches performance with PULP and makes more torque than pulp, down a KW or two here and there (tuning perhaps)
but nothing noticeable.
Drove it to Darwin and Matched the numbers on the speedo
70KM/hr =7l/100, 80=8L/100 90=9l/100 linear to 150KM/hr then
160=17 170=19 180=21 190=2105 200 =22 Didnt go for the 155mph 255kmph speed limiter as I would have been walking home if a Cop pulled me over at that :oops:
Petrol consumption is 15-20% less again.

Round town kids school run it gets 13-14l/100

Then there's the JTG fueled car up 22KW at the wheels over PULP and that drops a full second off the 0-100KPH time down to 6.1 seconds -not bad for a volvo station wagon.

It gets the same figures as the other car even though I probably drive it harder because it goes harder.
very handy to be almost able to keep up with a new SS commodore (5.5seconds 0-100 for one of those)

Most of the time i drive like miss daisy but when you need it, its nice to have 200KW on tap.
My driving is longer than the school run so i usually get under 12l/100 combined

Re: Car fuel use, actual vs. stated

PostPosted: Mon Nov 01, 2010 10:58 am
by munter
Seems like reasonable economy given the amount of power on tap! Driving the Prius is a pretty leisurely affair. Comparing different fuel sources can be difficult. The L/100km measure doesn't really accomodate the different properties of the fuels. An Australian academic wrote up an interesting paper comparing the well-to-wheel fuel economy of various transport fuels which I found quite interesting. Here's a link to a copy of his paper:

Re: Car fuel use, actual vs. stated

PostPosted: Mon Nov 01, 2010 4:26 pm
by karlajensen
Very close with Liquid LPG injection to being the same as petrol.

when driven with the right amount of pedal!!!

Re: Car fuel use, actual vs. stated

PostPosted: Tue Nov 02, 2010 11:04 am
by TerryB
munter wrote:Very impressive figures Terry. I thought I was driving our car pretty well but your results show there is still plenty of room for improvement. I run our tyres at 40 or 41 psi - what pressure do you use? I am still using the Michelin XM1's but will have to look at other LRR options.

I generally set them to around 42 front and 40 rear. I have found that the new Bridgestone Ecopia's have taken about 5% off the fuel consumption (so around 0.2l/100km saving) compared to the standard Michelin's so I would definitely recommend you look at those when you need to replace the tires.

munter wrote:I'm guessing that my poorer economy is perhaps a combination of:
Poorer driving technique
Living in a hilly area where short bursts of high power and the use of friction braking is perhaps more common?

Where you live makes a noticeable difference as well - we were living in a suburb with 40km/h zones and then late last year moved to an acreage property where we have to come out of the driveway and get straight up to 80km/h - that hurt my average consumption by about 5% (so have just made that up with the tires). Small hills can definitely help the consumption figures (as you can save more on the coasting down than the extra you use on the drive up) but if they are too big then yes that hurts consumption as well (if the battery is full before you get to the bottom of the hill then you are going to start wasting energy).

We are lucky enough to live on the Sunshine Coast in Queensland so I very rarely have to use the heater or air conditioner (a lot of my trips aren't long enough for the A/C to really cool the car down so I normally just drive with the window down in summer - prefer the fresh air anyway). Both heater and A/C can have a fairly negative impact on fuel economy so people who use those a lot tend to get worse figures in those seasons.

If you are interested in getting the maximum economy from the Prius there is a particular driving technique that I estimate can save a lot (depending of course on how you drive at the moment). If you are driving normally (ie. constant pressure on accelerator) and come to slight downhill gradient you will often find that the car drops down to around 1.5 - 2L/100km on the usage display but will not actually switch the engine off however if you take your foot right off the accelerator for a fraction of a second and then put it back on gently the computer will turn the engine off and switch to coasting mode (with obviously 0 fuel usage) but provided the gradient is OK you will not lose any speed (and with gentle pressure on the accelerator you can get it to use the battery power to keep the speed constant if necessary). The faster you are going the steeper the hill needs to be but in a 40 to 60km/h zone you only need a very slight hill (or even flat at 40km/h) - at 80 it needs to be a bit more noticeable and at 100km/h it needs to be a significant hill. Once you learn how to do that it becomes second nature and I find I do it now whenever there is a slight hill without being conscious of it. The same technique is useful when you need to slow down gradually (eg. coming up to a slower speed zone or a red light) - so lift off completely then just slight pressure on the accelerator to put it in coast mode. If you put the display onto the energy use you can see from the arrows what is happening so find a nice quiet bit of road with no traffic and give it a go until you get the hang of the pressure required to have the car just coasting (so engine off, no arrows indicating battery use/charging) and then play with the pressure to get slight battery assist or slight battery charge to maintain the speed. (Note that it wont do it if the engine has just been started or if battery is very low).

Apart from that standard defensive driving techniques obviously help save fuel over aggressive techniques as with any car (so driving to speed limit, backing off early if a red light is coming up rather than racing up to it and slamming on the brakes at the last minute, etc).

Good luck!