Reasons for drop in fuel economy?

[Al115]

I have recently noted a drop in the fuel "economy" (if you can call it that!) of my T5, and I'm wondering why...

Over the last year I have averaged a frankly amazingly consistent 31mpg - for example, four consecutive tanks over the summer gave me 31.4, 31.6, 31.7, and 30.8mpg. For some reason I have now found that I am getting a similarly consistent 26mpg. Yes, I know that's still not bad for a T5 - but can anyone think WHY the change??

I've come up with a few possibilities myself -

change in type of driving (more town / less motorway)
aircon on
inferior fuel quality
under-inflated tyres
wider tyres (<-- I think it might be this one as I have recently changed from 205s to 215s)
partially-seized brakes
faulty air mass meter
some other fault affecting mixture
and of course that long-standing favourite, Heavy Right Foot.

Can anyone with more mechanical knowledge add to these?


Alastair

Alistair,

If you've changed your driving to more town work than you used to do, that alone will explain the drop. Just look at your mpg on the computer as you pull away from lights etc. You'll notice that your actually getting as low as 6mpg for a few seconds on initial acceleration!!!

Whereas,... I drive mine on the Motorway every day, and when crossing the severn bridge, I get about 27mpg climbing one side, which is a very slight incline, but about 38 mpg going down the other side, an equally small incline. These figures are at a pretty constant 70mph.

[kevinbis]

I found the fuel economy drops right off when its due for the service were they change the spark plugs.
The plugs cost a fortune but it makes you feel better when you get instant more MPG.

[Al115]

Thanks guys - both are interesting suggestions.

I think my plugs have been in for about 10k.

Unfortunately I don't have a trip computer so can't see the instantaneous mpg. The only way I can calculate it (and I suspect the only truly accurate way) is to "clock" how many miles I get to each tank, and fill-to-brim each time.

Due for a 6-monthly service in early Oct so we'll see if that helps.

I did try unplugging my ECU last night to "reset" it... but (shamefully) I couldn't remove it from the car! It seemed pretty well wedged in its slot. I suppose I could unplug the battery instead...


Alastair

2 main reasons are the Air Con and town driving. I forget the exact figure but when the Air Con is in use the fuel economy drops by something like 6%.

[sarge]

I've heard this before about air con but I can't say I've ever noticed it having any particular effect. During the winter my car uses more fuel, and I put that down to the colder air temperatures and the car using more fuel on start up and taking longer to warm up. I find that the biggest users of fuel are the power and town driving. I really notice it with the re-map. It's fine if you're just pottering along, but start doing a few manouveres and the fuel economy soon drops.

Sarge
'99 V70 AWD

[Chris_Rogers]

Have a look at the air filter.

This is easy to do and will have a significant effect.

[SIAMBLUE]

>
>I did try unplugging my ECU last night to "reset" it... but
>(shamefully) I couldn't remove it from the car! It seemed
>pretty well wedged in its slot. I suppose I could unplug the
>battery instead...
>
>
>Alastair

Hi Alastair,On your ECU,Did you pull the bar ontop of the ECU over to the otherside?As it slides over so you can pull the ECU out,Reverse the situation when refitting.

[doorman]

Sorry to say this, but if your driving a performance car, what the hell are you doing looking at the mpg your getting. I'm pleased to get 21 fun packed mpg out of my V70 'R' AWD. If I wanted economy, I might consider something boring like a BMW or a Merc
Wait for the flac!!!!!!!!!!!!
Dave