Sudden drop in MPG?

J

junglist

Member
I seem to be experiencing a sudden drop in MPG in my 2003 3.0i. I didn't buy the car for economy but have always been impressed on how good it is for a 10+ year old 3 litre engine. Sticking to an indicated 70, it'll hover around 39-40mpg on the trip computer, slightly higher if the roof is up.

Last weekend I was coming home on the motorway and I thought I'd have a look at the figures out of interest. I had the cruise set to 70mph and the mpg was sitting at around 31mpg. 10mpg down on what it used to be. This was with the roof up.

I did a 30 mile stretch on the M1 this morning and loaded up the computer, again sticking to 70mph and I didn't get more than 32mpg. Even on the 50mph average speed camera sections it was struggling to get over 36mpg. No passengers, boot was empty and there was less than half a tank of fuel in.

I know the OBC is slightly optimistic so it probably works out I'm not even getting 30mpg at 70mph which seems very low.

Car seems to be running fine, tyre pressures are spot on. Running on Shell V-Power as always. The only thing I've done recently is install an Eibach Pro Kit and add 20mm rear spacers but I wouldn't have thought they'd have such a big impact on fuel consumption, if at all.

Any ideas at what I should be looking at?
 
No idea on the drop but I've got a 2004 3.0 and only get the same MPG as your getting now and always have.!!
 
I guess a drop in height could affect the surface of the tyre on the road due to the increased camber caused when lowering, so its entirely possible this could be your answer.

Also worth checking tyre pressures.
 
Try resetting the indicator and then go for a run and check it again because I'm pretty sure it will record your average mileage overall so if you have been stop starting in town for a bit this will bring your average mpg down
 
simonlpearce said:
I guess a drop in height could affect the surface of the tyre on the road due to the increased camber caused when lowering, so its entirely possible this could be your answer.
Click to expand...

Not enough to cause a 20% decrease though surely?

I leave the first MPG readout as a long term average and have been using the second one to calculate these motorway stints. I've even been resetting it as I've got upto speed so there's not even the initial acceleration from 0-70 to bring the reading down.

As stated, tyre pressures are OK. 32F / 36R on non-RFT Goodyear Eagle F1s.
 
I've never seen more than 32 from my si, but generally it stays around 27. But then I hardly ever do motorways, and almost never drive boring roads!

A million things could be affecting it; have you been sat in traffic lots this tank, have you had it idling for long periods? Ambient temperature will also have an effect to some extent....
 
I average 20 in the M but when I took it to France (drive from Sheffield to Dover then driving around Calais) I averaged 32!
 
Sim_Simma said:
I average 20 in the M but when I took it to France (drive from Sheffield to Dover then driving around Calais) I averaged 32!
Click to expand...
..but ya cant trust them foreign miles, cans ya, theys much smaller than proper english miles... evr'one know that... :wink:
 
The car has been taken up to 70, where I have then reset the computer and then done a 20-30 mile stretch at a continuous 70.

No idling, no town driving, nothing extreme in terms of temperatures.

Appreciate everyone saying they average similar but I'm on about the mpg during these particular motorway journeys. I think my long term average is around 34mpg but on a motorway at 70 as previously stated I was getting 40mpg, quite often more.

Reading through other forums people have suggested checking air filter / spark plugs but these were done a few months back.
 
Have you pulled the codes to see if anything has been logged?

Is the engine getting up to temperature as a stuck thermostat can reduce mpg

A faulty O2 sensor can lead to the mixture being too rich, as can a faulty MAF
 
One other thing occurs, you say you've changed to Eibachs and added spacers, have you checked the wheel arch liners for any evidence of rubbing?
 
Is this the M54 engine? If so check intake boots for splits too. Will need to remove them from MAF back to throttle body to properly check.
 
My vote goes to upset aerodynamics.
Try taking the wheel spacers off and see how that goes.
 
Dirty air filter causes reduced performance(bhp) but has little effect on fuel efficiency as the ECU compensates for the lack of airflow by putting less fuel in.

Have you checked for things like excessive brake drag, loose/missing underside parts, wheel alignment?
 
stlymch said:
Dirty air filter causes reduced performance(bhp) but has little effect on fuel efficiency as the ECU compensates for the lack of airflow by putting less fuel in.

Have you checked for things like excessive brake drag, loose/missing underside parts, wheel alignment?
Click to expand...

Fair enough :thumbsup:
 
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