Car MPG

[ScienceBoy]

Real unusual one here, hopefully someone here can shine a light on what is going on.

Car MPG has steadily declined over time, various annual services have restored to some degree, it which is expected.

Last time nothing unusual was found, MPG still low, I guessed thats just the way with an older car.

Last week I had some battery issues when I didn't use the car for a week, recommended to replace it as it had some corrosion. It's the original battery which was over 9 years old.

New battery and MPG jumps up from high 30s-low 40s to over 50! A huge change, back to where it was when I got the car about 5 years ago (was already 4 years old).

Anyone know why this happend, quite curious as dealer garage and others have not recommended a battery change all this time. I wouldn't say I have spent a lot on the car since I got it either, less than £1000 in anything outside MOT costs.

 

[sev112]

Is it because you had eco stop start which wasn’t kicking in because the batter was too low. You get a new battery and hence eco kicks in and you stop start works again?

 

[Slab]

I may be well off here but while not directly, ultimately isn't it petrol that charges the battery? Faulty battery so it takes 'more' to charge it

Just like MPG goes down when using aircon/lights in a car, these are electrical items but ultimately petrol powered

 

[ScienceBoy]

Is it because you had eco stop start which wasn’t kicking in because the batter was too low. You get a new battery and hence eco kicks in and you stop start works again?

I can’t disprove that but stop start was working when I noticed the low MPG. It’s possible it had stopped but it wouldn’t have been for more than a few months, I am sure I would have noticed it not working!

I would stay it’s a possible contributing factor but I have a feeling something else is involved too.
Its all fixed at least, I’m just curious and also happy I’m going further with my tank of gas!

 

[Jimaroid]

How are you measuring MPG? I'm assuming it's through the car trip computer or what have you?

Swapping battery may have reset the ECU and if its MPG is a stored average maybe it's just gone back to a factory default. You may find it slowly crawls down to high 30's again in time?

I can't imagine it could cost 10 MPG to charge a faulty battery, that's a lot of energy disappearing.

 

[ScienceBoy]

How are you measuring MPG? I'm assuming it's through the car trip computer or what have you?

Swapping battery may have reset the ECU and if its MPG is a stored average maybe it's just gone back to a factory default. You may find it slowly crawls down to high 30's again in time?

I can't imagine it could cost 10 MPG to charge a faulty battery, that's a lot of energy disappearing.

I reset it occasionally using the trip button and it seems to reset after filling up to a full tank sometimes, never sure why. I guess it could have cleared something deep in the memory that the normal trip clear doesn't.

I think your onto something there!

 

[Smiffy]

Routine servicing would not have contributed to better mpg.

 

[ScienceBoy]

Routine servicing would not have contributed to better mpg.

Correct.

It was probably only once or twice when they changed a few parts, it may have been all in my head but it felt like they went up.

Unless anyone knows how or why the battery could have affected it I would say those who guessed it was just the trip getting some kind of hard reset?

 

[Wolf]

Can't see how the battery would improve mpg other that a total memory reset of your readouts due to disconnecting the old one and the time frame to add the new one. The car would run if battery anyway, once started the alternator would be doing all the work to provide electrical power to the car, the battery in effect is just there to help start the car.

 

[ScienceBoy]

Can't see how the battery would improve mpg other that a total memory reset of your readouts due to disconnecting the old one and the time frame to add the new one. The car would run if battery anyway, once started the alternator would be doing all the work to provide electrical power to the car, the battery in effect is just there to help start the car.

Thanks all! Looks like we got to the bottom of it! Team effort!