HELP - My car using more fuel?

tomthered · Post by **tomthered** » 30 Jul 2006, 09:31

just got back off hols and just finished 250 ml journey. Car is a 1996 Xantia 1.9TD with AC. Had a roof rack on with luggage on way down to Isle of Wight and mainly did speeds of about 70-80 mph as an average for msot of way.

When I got there the place is not so big so speeds were about 30 mph average; I left the roof rack on and still used the AC.

Filled up for way back and put 40 litres in, and only did just over 300 miles and petrol light came on just as I got home. I noticed whilst on way there that when I filled up the fuel gauge went right above 65 litre (full tank) mark and stayed there until tank was about a quarter full then went back to registering correct amount.

I am concerned that I may be using too much fuel as I normally get about 40 - 45 to the gallon or 9 - 9.5 to the litre. There appears to be a 15% drop in the above, would this be right for a roof rack with 4 pieces of luggage on and AC on most of time?

When I got back I stuck 5 litres in as garage was a bit pricey!! the fuel light went off but after 2 miles it came on again!! and I should have got about 45 miles out of it before that happened.

My mate reckons it may just need a good service with air cleaner etc changed, does anyone know if this would indeed solve it?

Engine runs sweet and no noise at all. Starts first time.

Whilst there I did drive onto a kerb to park up and scraped the underneath about middle of car. Are there any fuel pipes it could have damaged?, cannot see any leaks and there are no diesel stains when it was left.

JohnD · Post by **JohnD** » 30 Jul 2006, 09:45

Using the aircon will use more fuel - between 5-10% more. Having a roof rack on will also cause drag and increase consumption. The high speed would also add to the heavy consumption. The only way to judge fuel consumption is to brim the tank, reset the trip mileometer and refill to full in 200 miles or so. The fuel pipes shouldn't have been damaged. They are covered by conduit.

tomthered · Post by **tomthered** » 30 Jul 2006, 09:52

thanks for quick reply John. Any ideas why the gauge would show orange empty light, I put 5 litres in then drive 2 miles and it would come on again? Is there a way of checking how much fuel is actually in the tank by a dip stick or some other method?

I feel reassured that I would not have damaged the fuell pipes!

deian · Post by **deian** » 30 Jul 2006, 10:15

hi,

...also while you were there driving doing town driving you will use a lot more fuel than you would doing a constant 70mph, because around town you use gears 1, 2 and 3 a lot, and they tend to use a lot of fuel. I doubt your fuel lines are broken, they are well hidden away so as not to get damaged so easy.

when the red light comes on you still have quite a bit of diesel left in the tank (8 litres in fact, which is 1.76 gallons, and say your car does at least 35 mpg you can get 60miles out of a red light before u totally run out), mine tends to come on quite early, but bare in mind that the fuel level sender in the tank takes an average yea, when you go up hill the fuel goes to the back of the tank, and i've also noticed when you drive to a right corner or park your car on a slant (left side pointing lower) this also gives a false reading too. I guess when the light comes on it stays on and doesn't go off like other cars i've seen. Of course the wise thing to do would be to carry some diesel in the legal black container in the boot for peace of mind.

but yes AC and roof rack can kill fuel consumption in any situation

tomthered · Post by **tomthered** » 30 Jul 2006, 10:26

cheers for that info. I thought you got less consumption at 70mph than at 30mph? Whilst there I did not do many miles, about 70 at most so bulk of mileage was done on motorway. anyone know what it would do at that?.

On my daily drive to work, mostly via motorway at 70mph speed I get 40 to gallon, possibly more. I suppose 15% off that would give the consumptioon I got.

Stewart(oily) · Post by **Stewart(oily)** » 30 Jul 2006, 10:55

My roofbox knocks fuel consumption about too, I have a boost gauge fitted and unloaded the car seems to require about 7 or 8 psi to maintain motorway cruising speed on the flat with no headwind, I put the roofbox on and it needs 10 or 11 psi to do the same thing, it must be adding fuel to this air so I just grin and bear it or squeeze all of the gear inside the car.
Stewart

JohnD · Post by **JohnD** » 30 Jul 2006, 12:16

When I had my 2.1 Xantia it averaged 42mpg over a month of longish runs. Put the roof box on and the caravan on the back and all it could manage was 28mpg. More drag = less mpg.

tomthered · Post by **tomthered** » 30 Jul 2006, 12:44

cheers guys, just hope there is no major problems. gonna change air filter anyways and will prob get fuel filter and whatever else is needed.

deian · Post by **deian** » 30 Jul 2006, 16:33

you'd get better mileage on the motorway, maybe close to 45mpg compared to maybe 35mpg around town

citroenbx · Post by **citroenbx** » 30 Jul 2006, 18:23

30mpg is good at that speed and with the drag factors....Keep your speed between 55 - 60mph remove as much drag as you can try not to use the AC and you should be back to 40-50 mpg.

Sounds like sucking eggs and it is a bit slow but if youv'e got time....

Oh by the way if you keep a safe but close distance behind a coach or lorry you'll save a noticable amount of fuel (they do all the wind breaking for you )
theres a fine art to this learnt it when i was a courier.

Hope this helps.

fastandfurryous · Post by **fastandfurryous** » 30 Jul 2006, 20:42

I have to concur with the above comments. The fuel consumption from a 1.9TD fitted in just about any meduim-large car will be about 40mpg at 70-80 mph, and roof boxes, for all their "streamlining" are not very aerodynamic. You can bet on using at least 20% more fuel with one up there.

Now, if they were aerofoil shaped (fat at the front, with a taper to the rear) then they'd only add a few % on top. But they're not.

The AC on won't use that much. It looses you a couple of MPG, but it uses less fuel than having the car windows open at high speed. The cut off speed for most cars is 40-50mph. Below that, Windows open is better. Above that, AC is better.

jeremy · Post by **jeremy** » 31 Jul 2006, 00:00

The only reliable way to check the fuel consumption is to fill the tank, record the mileage or set the trip indicator, and then measure the fuel added to fill the tank and then work out what has been used and hence the MPG. You have to be honest when filling the tank (its easy to be 1/2 gallon out and on 10 gallons thats 5%) and obviously the longer the test - the smaller the errors.

Aerodynamics will make an enormous difference - with the roofbox at 70 I bet the car felt very sluggish and heavy. - Stick your arm out of the window and feel the drag on it at 70 MPH for a simple illustration!

The mention of getting a 'tow' from goods vehicles reminds me of an 1108 cc Renault 4 that we had many years ago. This thing had very tall gearing which meant that it was reasonably refined at 60 - 70 MPH but not much more speed. What would happen is that you'd come up behind a goods vehicle on a motorway and you seemed to have enough power to get past - only to pull out and find that you were actually falling behind when it hit the still air in front of it!

Post by **Peter.N.** » 02 Aug 2006, 12:41

As JohnD says, aircon will allways use a fair ammount of extra fuel and its much more niticable if you are driving economicaly. Anything that increases drag wil also affect the MPG, but the most signifcant factors are weight of right foot and traffic conditions. I recently averaged 50 mpg in my XM on a trip to the north of Scotland with four adults, two dogs and all our luggage, but I drive neither fast or aggressively .

oscarloco · Post by **oscarloco** » 02 Aug 2006, 16:12

It's been said that "platooning", that is taking some cars and forming a convoy, can save up to 25% fuel if they go almost bumper to bumper.

In fact, there is some research being done on this subject with computer cotrolled cars that comunicate with each other, so they don't crash.

Of course, in real world, you can't go too close