My MPG has gone down??

tasticz

As title said my MPG has come down from 35.5 to 29mpg and it is probably still dropping!

2 Weeks ago my fuel tank nearly went empty and the indicator light came on. As soon as it happened I filled it to full tank and from that day it has been going down and now its at 29.4mpg

What could be the reason? How can i get it up to what it was before, My driving pattern hasn't really changed.

Thanks

edit: forgot to mention i got this car 6 months ago, and it has been 35mpg steady until 2 weeks ago

vikingaero

Colder temperatures = more use of lights, heater, rear window demister and the car stays on the cold cycle for longer.

Also check your tyre pressures. When the temp drops so does the tyre pressures and they may be below the recommended inflation level.

Raksha

Where are you getting this figure from? Computer display or calculations fuel/miles

tasticz

Raksha wrote: »

Where are you getting this figure from? Computer display or calculations fuel/miles

hi its from my car computer on the speedometer

Ultrasonic

tasticz wrote: »

2 Weeks ago my fuel tank nearly went empty and the indicator light came on.

You don't think that's a problem do you? It certainly shouldn't be. Some people do that pretty much every tank.

As above the deteriorating weather will be causing mpg to drop, although to be honest I wouldn't expect the drop to be as big as you've seen. Possibly your last lot of fuel has a little lower energy density as well.

There might be something else going on but I suspect it is unlikely.

reddwarf2002

I wouldn't expect the weather (and even fuel change in winter) to have such a large impact. In any case the temperature hasn't dropped that much either; it has been a rather warm October to be honest.

Sometimes it's difficult to diagnose such faults if no code is recorded in the ECU especially if the car is not exhibiting any symptoms of poor running. Having said that, if your brake pads were sticking then your MPG would be affected so it's worth checking whether your wheels spin freely.

Ultrasonic

reddwarf2002 wrote: »

...and even fuel change in winter...

Just to be clear, I wasn't referring to a change in fuel for winter above, just regular inter-batch variability. It's too early for winter diesel yet, I've no idea if or when petrol might change though.

daveyjp

Its perfectly normal for mpg to reduce once the air temp starts dropping, especially on short runs.

Ultrasonic

daveyjp wrote: »

Its perfectly normal for mpg to reduce once the air temp starts dropping, especially on short runs.

A 17% drop already is pretty extreme though to be fair.

Although a little more detail of the drop in mpg would be informative. If it was a single trip at an indicated 29.5 mpg that is less surprising than if the tank average had dropped from 35 to 29 mpg.

(I averaged 56.0 mpg for my 9 mile each way commute last week. The best trip was 58.0 mpg but the worst was just 50.5 mpg. I never see figures that low in the summer.)

tasticz

My journey is 14mile to and 15mile back, I drive a petrol. Does this count as short runs?

I do get stuck in traffic but its always been like this car moves at 15-20mph at 30mph roads and i usually select high gear as possible to keep revs down.

I will check the wheels tomorrow, I will have to lift the 4 wheels 1 by 1 and check all? or just the front 2 as my car is FWD?

Ultrasonic

tasticz wrote: »

I will check the wheels tomorrow, I will have to lift the 4 wheels 1 by 1 and check all? or just the front 2 as my car is FWD?

If you want to check them it should be all wheels. If any of the brakes are sticking on it would reduce mpg, whether on a driven wheel or not.