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Fuel Consumption / Economy

nitrosubaru

nitrosubaru

Member
Joined
Jul 28, 2005
Messages
36
Location
Newcastle, Australia
I recently bought a 5yr old Subaru Liberty Wagon, 2.5L auto.
I have had it for 3 and a bit weeks now and I am a bit disappointed about the fuel economy...now i dont drive like a rev head...so thats not the problem...

i rang the previous owner and he said he couldnt remember what economy he got...

on a 50/50 highway city tank i get about 450km from 45L which is average...
but i just finished my first tank of city driving and im only up to 280km from about 35L...now that is bad...

I checked the tire pressures...im running 205/45R16 @ 40PSI...so thats not a problem (they are the recommended size for the car according to subaru)

now, the previous owner told me he was running premium 98RON and i only put 90RON in it....and its only just finishing its 3rd tank of fuel which is about 1200kms....could this be the problem? will the ECU retune itself to 90ROM fuel and fix the economy or will i need to run 98RON and see how it goes?
 
For a 2.5L engine, 12.5L/100km doesn't seem all that bad for city driving and 10L/100km for combined cycle is good!

I have no idea about how the petrol would affect it but I'm guessing it does play a small part. Oh, and there's nothing as low as 90RON. Normal ULP is 91. Why don't you try a tank of 95RON, see if that makes a difference?

EDIT: Actually, I've been thinking. It's a 5 year old model. You only say it's got the 2.5L not a whizz bang Subaru turbo charge witchery engine. Why would that engine have been run on 98RON? I can't imagine it'd improve power at all. An engine like that would be designed for 91RON...
 
98 will not hurt it, but will hardly improve anything. Do you know what compression ratio engine has? Because if it's really high (not likely though) and you use low octane fuel, ECU will adjust igintion timing backwards to prevent early detonation, and this may couse high fuel consumption.

Are you sure you put 90RON? We didn't have 90RON here for a while, it's 95 or 98 now. Maybe you mean (R+M)/2 (or PON whatever you wanna call it) which americans use. RON 90 seems a bit low.
 
You don't buy a Suby for the gas mileage (cause it's not amzing as you've found out), you buy them for the AWD. ;)
 
its got the standard 2.5L non-turbo motor
and yeah, i meant 91RON
according to the book it should bet 8.8L/100kms for combined cycle...7.7 on the freeway and 10.1 in the city

the compression ratio is 10.1:1
 
10.1:1 is fairly high, so it might be the case when ECU adjust itself to lower octane to prevent knocking. Which means it's less efficient and less powerfull.

Try 95 at least and see what happens
 
Only use the octane specified in the owner's manual.
Higher octane fuel will only prevent knock and detonation.
It does not magically give you better mileage or more power automatically.

I'd check simple things like:
spark plugs, spark plug wires, distributor cap/rotor, timing, fuel filer, air filter

If those don't help, it might be a dirty connection on one of the many sensors on your EFI or ignition system.
 
Some say it does give you better mileage, some say it doesn't. There might be the slightest difference because 98 RON has more octane, and is a denser fuel than a lower octane variety. Therefore, theoretically, if you fill up the same amount, you are getting more fuel. The reality is that it is fairly insignificant.
 
higher octane gas than the manufacturer recommends, in general, will *NOT* improve fuel consumption or performance in an unmodified engine. All octane references is the gasolines ability to prevent "knocking" or "pinging" (detonation, and worse, pre-ignition)

So X car with a compression ratio of XX:Y produces a cylinder temp of Z and therefore the manufacturer recommends an octane rating of W, because lower than that will increase the chances of harm because the lower octane gas will combust at a lower temperature/pressure/mixture.

Using a higher octane fuel than required is simply a waste of money.


Now, the BRAND of gasoline you buy is very instrumental in engine performance. At least in the US, the way it works is that our pipelines will run different grades of gasoline at a time (say, 10,000,000 gallons of 89 octane in the morning, then a run of 87 in the afternoon. So all gas companies will get fuel from the same pipeline, so they will get the same gas. But the difference comes in the proprietary chemical mix that the drivers will add to the tanker truck AFTER filling. These additives are what differentiate gas from different companies, and there can be tremendous differences between them.
 
In the US, there are only two types of gasoline that is actually produced--lower octane (87) and higher octane (91). The two is mixed at the pump to produce the mid-grade (89).
 
Z Draci said:
In the US, there are only two types of gasoline that is actually produced--lower octane (87) and higher octane (91). The two is mixed at the pump to produce the mid-grade (89).
Click to expand...

Sunoco also produces a 94 octane. and you can find a few stations that carry 100+ octane "racing" fuel...
 
I actually own the model year after yours nitrosubaru (from new) and that is about right. The fuel consumption although should be a little better around town. I used 95 RON for the first 3 years of ownership and the fuel economy is only marginally better. In the last year I've used 91 RON and fuel economy around town is about 10.2L /100kms. while it was 10.8L/100kms on 95 RON... so don't expect a huge difference. Now that is driving the car pretty hard... usually changing gears around 4000 to 4500 on the tacho.

With that said said the Liberty 2.5 seem to be a little more responsive when I put the foot down with 95 RON than with 91 RON. Like Raven18940 said... I bought the car for the all round package! It's ultra-reliable (even after 2 accidents) relatively cheap to service (well the 100K service is around 800 bucks so look out for that one... usually Subaru owners change over before that service!) and for it's handling (absolutely amazing (even in the wet) for this money).
 
A car will very rarely get close to the quoted fuel consumption figures.

Those figures are derived from a "rolling road" test, where a car is put through a regime of high speed cruising, and acceleration runs whilst sitting on a car "tread-mill".

Because the car body isn't producing any aerodynamic drag (amongst other things), the fuel consumption will always be lower than the real world figures.

The real world consumption you've mentioned isn't great, but the car is a little heavy, is endowed with a large-ish engine, and has an automatic gearbox. Having AWD also costs you fuel.
 
Wow, that's some low octane fuel you've got in the states. I'm not sure if we use a different system over here in Finland, but they only sell 95 and 98 unleaded here.
 
Maxi said:
Wow, that's some low octane fuel you've got in the states. I'm not sure if we use a different system over here in Finland, but they only sell 95 and 98 unleaded here.
Click to expand...
Octane is calculated differently in the US vs. Europe.
93 Octane in the US = 98 Octane in Europe.
 
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