Car fuel use, actual vs. stated

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Car fuel use, actual vs. stated

Postby zzsstt » Fri Oct 01, 2010 9:12 am

Car manufacturers quote fuel efficiency of motor vehicles, measured according to ADR 81/02. Having looked up this standard, I find it is a complete joke as it is based on a very short cycle of preset driving on a flat surface. The "extra urban" cycle, for example, is of 400 seconds duration, of which only about 60 seconds is at or above 100kph, and more than half is at or below 70kph. I'm not sure how that is classed as "extra urban" in a country where most out of town driving involves sitting at 100kph or 110kph for hours at a time! In fact even the math of the regulation is incorrect, as they cannot calculate acceleration (for example, 0-15kph in 4 seconds is stated as 1.04m/s/s acceleration, it is in fact 3.75m/s/s!)

However, I am interested to know what experiences people have with actual fuel economy as compared to the manufacturers stated numbers on recent model vehicles.

For example, driven economically in a rural area - mostly 100kph cruising (18km to town, 18 km back, with perhaps 1km of 50kph town driving) - our vehicles return;

Hilux that is stated to use 8.3L combined cycle. Best ever economy is 9L/100km on a 4 hour run. Normal economy is about 10l/100km

Prado that is stated to use 8.5L combined cycle. Best ever economy is 7.8L/100km on 4 hour run (same run as Hilux above). Normal economy is about 8.8L/100km. This vehicle has an econo-meter, and on a calm day/flat road/unladen/no air-con it will return real time values of below 6L/100km at 80kph

Whilst I am actually reasonably pleased with the real world performance of both vehicles, the Hilux especially is way off the supposed "official" economy figure and given the nature of our usage even the Prado falls short.

So, what fuel economy do you achieve compared with the stated ADR 81/02 economy?
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Re: Car fuel use, actual vs. stated

Postby zzsstt » Sun Oct 03, 2010 3:22 pm

OK, so posting before coffee is not a great idea.... 0-15kph in 4 seconds is indeed 1.04m/s/s!
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Re: Car fuel use, actual vs. stated

Postby Gordon-Loomberah » Sun Oct 03, 2010 4:57 pm

I reckon I can accelerate faster on my mountain bike than that... and use no petroleum fuel at all ;)


2005 Peugeot 307 2.0l turbo diesel, claimed 4.7l/100km ... really only achievable by driving at 80-85km/hr with no significant hills. Close to 5.8-6.0l/100km is more realistic for highway driving... unreliability means its in for repair fairly often, and it uses no fuel then! :(
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Re: Car fuel use, actual vs. stated

Postby moemoke » Fri Oct 15, 2010 12:16 pm

We brought a Holden Captiva Diesel last year and have been a bit dissapointed with its fuel consuption.
Around town we usually get 9.5 to 10 lt/100 average per tank, which is what it should.
this figure is using the onboard computer which seems accurate,
I always fill the tank to about the same spot also.
but my mine gripe is with highway driving where sometimes we get around 9 lt/100 on my trip to work on the highway, if I drive very, very gently I can get it into the 8's.

We recently went to Tamworth towing a moke on a trailer and got around high 11's to 12's for most of the trip but coming home we fuelled up at BP Wallan, pressed the computer reset as I was leaving the carpark and we were into the 9's within about 1km, I think we got back to Moe at around high 9's,
Is some BP fuel better than others?
One silly thing our Captiva has is a 'Diesel Particle Filter' and warning light, which requires heat to burn off the nasties, this light comes on quite often and requires us to drive at over 2000rpm for 20 minutes or so which seems just silly, it came on the next time we used the car after coming back from Tamworth so
I think it may come on when I am driving to gently.
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Re: Car fuel use, actual vs. stated

Postby zzsstt » Fri Oct 15, 2010 3:15 pm

There are many possible causes of changeable fuel economy. I always get slightly better economy when driving to Sydney than when driving back. Could be uphill, prevailing winds or perhaps just enthusiasm to get out of the city?

Today I drove to Forbes and back, and got worse economy on the way back, but the journey home was in to a northerly gale!

I also find the air con makes a fair impact on economy.

BP diesel fuel is generally good. The supermarket fuel should be avoided!

As for the particulate filter, remember that modern turbo diesels are designed to make lots of power, and to do that they "want" to spin faster than old style diesel thumpers. They need to be spinning at above 2000 to 2500rpm to make the power, and the power band is quite narrow which is why they also tend to have at least 6 gears. The big problem is that at these high rpm's they become less economical (more fuel burned per distance travelled). So to get the best economy they need to be driven at lower rpm's, and therefore don't generate the heat required to burn the particle filters clean - diesels run at lower temperatures than petrol engines anyway. So there is a trade off, if you use the power you don't get such good economy. If you don't use the power, you get economy but the particulate filter doesn't self clean....

The new generation of super-clean diesels also have urea (AdBlue) injection to further reduce emissions.
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Re: Car fuel use, actual vs. stated

Postby munter » Mon Oct 18, 2010 12:32 pm

OK - I'll bite :)

Our 2005 model Prius has a sticker fuel economy of 4.4L/100km. In practice we seem to get between 4.5 & 4.9 L/100km for the whole of a tank. Mileage is best cruising around town at 60km/hr and probably the worst out on the freeway where it is hard to get much below 4.8/4.9.
Individual trip fuel economy has gone down to about 2.0L/100km (not really a trip, just a predominantly downhill roll between two locations), just as a short vertical climb might see it go up to mid-30s.

My wife tends to use slightly more fuel and I find that this is because she anticipates the surrounding traffic movements to a lesser degree. She's not a bad driver, but she tends to accelerate a little harder than is necessary given the general flow of traffic and perhaps brake later than is ideal to roll through slower spots.
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Re: Car fuel use, actual vs. stated

Postby Smurf1976 » Fri Oct 22, 2010 9:52 pm

10 year old Nissan Pulsar manual.

Firmly stuck at 6.7 litres / 100km no matter how I drive it. A/C on or off, gentle driving versus flooring it at the lights etc makes virtually no difference to actual fuel use. Even with a 400kg load in the back (!) and driving at 100km up quite a few hills made no real difference.

The only thing I've found that does kill economy is poor quality petrol. It went to over 10 litres / 100km once, and came straight back down with the next tank of fuel. Engine performance suffered with that bad fuel too, I had to keep my foot almost all the way to the floor simply to maintain 110 km/h on the highway.

I normally use premium unleaded (RON 95) as recommended in the owners manual. Consumption goes up running regular and, given that it is designed for premium, I'd guess that lower grade fuel wouldn't do the engine too much good in the long term either.
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Re: Car fuel use, actual vs. stated

Postby Smurf1976 » Fri Oct 22, 2010 9:57 pm

10 year old Nissan Pulsar manual.

Firmly stuck at 6.7 litres / 100km no matter how I drive it. A/C on or off, gentle driving versus flooring it at the lights etc makes virtually no difference to actual fuel use. Even with a 400kg load in the back (!) and driving at 100km up quite a few hills made no real difference.

The only thing I've found that does kill economy is poor quality petrol. It went to over 10 litres / 100km once, and came straight back down with the next tank of fuel. Engine performance suffered with that bad fuel too, I had to keep my foot almost all the way to the floor simply to maintain 110 km/h on the highway.

I normally use premium unleaded (RON 95) as recommended in the owners manual. Consumption goes up running regular and, given that it is designed for premium, I'd guess that lower grade fuel wouldn't do the engine too much good in the long term either.
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Re: Car fuel use, actual vs. stated

Postby munter » Mon Oct 25, 2010 8:19 am

Interesting - less than 7 is a pretty good result for the Pulsar. I have found that E10 seems to return poorer economy numbers than regular unleaded and it almost feels like I would get better $/km if I paid the little extra for regular unleaded (while I still can!).

I drove from Canberra to Sydney last night and the Prius returned 4.6L/100km for the trip fully loaded with 3 people and their luggage. Perhaps the downhill slope improves the economy a little?
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Re: Car fuel use, actual vs. stated

Postby zzsstt » Mon Oct 25, 2010 10:11 am

munter wrote:Interesting - less than 7 is a pretty good result for the Pulsar. I have found that E10 seems to return poorer economy numbers than regular unleaded and it almost feels like I would get better $/km if I paid the little extra for regular unleaded (while I still can!).


There's a lot less energy in ethanol than in petrol (21MJ/L vs. 34MJ/L) and even less in methanol (18MJ/L). So replacing 10% of the petrol with alcohol will reduce the overall MJ/L of energy in the fuel. The knock-on affect is that because the energy requirement of the vehicle remains constant, the fuel usage will increase. This can be overcome by using a higher compression ratio in the engine, as alcohol has a higher octane rating and therefore can be used with an increased compression ratio which gives better efficiency, but then the engine will not run "ordinary" octane fuel.

From a CO2 viewpoint, both ethanol and petrol produce much the same amounts of CO2 to generate the same energy, so in a standard engine E10 will generate much the same amount of CO2, and use slightly more fuel. However ethanol is classed as a "renewable" fuel, and if the math is done correctly it can be shown to be beneficial to the environment. If does also burn cleaner, producing less other emissions, although as petrol engines improve that gap has narrowed somewhat.

At present the main issue with vehicles being "E10 compatible" has nothing to do with emssions, it simply relates to the alcohol degrading many of the rubber hoses and seals in engines not designed to use it.
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