Increased fuel consumption?

[DDY]

For a few months my Toyota Aygo has been drinking a bit more fuel than usual.

I think this isn't a statistical blip but I'm not confident enough to rule that out yet.

I reckon I can rule out volume measurement error, climate and fuel quality. I don't use the same pumps at the same fuel stations frequently, I have more than a year of data covering all four seasons in the past, I also fill up my motorcycle (ER6) at the same stations as the Aygo and largely travel the same routes on both car and motorcycle.

I have AC on constantly in the car, it's almost never off, and a tiny bit of heat, mostly to keep the damp out. This hasn't changed since I started recording this data. My driving style or commute hasn't changed either.

I'm thinking worn tyres may be involved - the four tyres have between 2.5-3.5mm of tread, although that doesn't explain the sudden dip... I rotated the tyres last week to give the front a bit more tread and to squeeze a bit more life from them before I have them replaced in a few weeks. While I did that, I checked the free-rotation of all four wheels and I can rule out binding brakes.

What do you reckon I should look at next?



* The cumulative mileage is from when I started recording the data, not the vehicle's absolute mileage.

[kalniel]

Worn tyres would likely have the opposite effect - decreased radius making your odometer count higher miles travelled for the same distance. Rough eyeballing makes me think the miles are increasing more, but that could be a visual illusion as I doubt the effect would be noticeable.

How long since last air-con service/regas? Loss of efficiency will mean the compressor's used a lot more, creating alternator drag. Ditto any number of service items can do it - oil change, spark plugs etc. all have quite big effects on efficiency in my experience.

Otherwise I'd say you don't really have enough data to rule out variance due to factors out of your control - all but the very last reading you've pretty much seen before in winter periods, so maybe a long fill?

[blueball]

I was advised by a tyre centre not to put the best grip on the front. When I quizzed him about this he stated that if you break hard the front will hold but the back end can break away. I'd been swapping best tread to front for years.
EDIT: Front wheel drive car in my case, don't know if rear wheel drive affected in same way.

[kalniel]

I was advised by a tyre centre not to put the best grip on the front. When I quizzed him about this he stated that if you break hard the front will hold but the back end can break away. I'd been swapping best tread to front for years.
EDIT: Front wheel drive car in my case, don't know if rear wheel drive affected in same way.

Yeah you definitely want to lose the front end if you have to lose anything - you'll carry on going straight which is predictable and much more easily fixed by the average driver and car ESP. And given said situation is more likely to occur through aquaplaning it makes sense to keep most tread at the back. But if you want to replace all four tyres at the same time then you might need to rotate to even out the wear rates. Otherwise, always replace in pairs, new on rear, old rears to fronts, assuming you run the same tyres front and rear.

But that's a bit of an aside

[Spreadie]

Silly question but have the tyres lost pressure?

Softer tyre = greater rolling resistance.

[DDY]

Hmm. Maybe not the tyres then.

The AC hasn't been touched since the car rolled out the factory in 2010. I'll put that on the garage to-do list and get it re-gassed soon, maybe when I take the car to do the tyres if the garage does AC too, certainly needs doing because it's nowhere as cold as it could be. For now, I'll run a full tank with the AC off and see what happens.

I don't think the servicing done since the start of data collection is significant in this case considering the timing;

- New auxiliary belt and water pump 2 weeks ago, plus new coolant of course,
- New clutch 9 months ago
- Spark plugs, air filter, oil & oil filter 7 months ago (approx. 3500 miles ago)

I agree the assertion that my car is drinking more is still statistically wobbly, but I've never had five consecutive fill-ups yield less than 43 mpg - I think the sinking feeling of seeing the digits on the fuel pump volume continue beyond what I'm expecting for the last few months may be inducing a bias!

Oh, and I also checked for fuel leaks! Tyre wear patterns are even - balancing and alignment is least not stupidly out. I typically run the car down to near empty and fill up for a full tank for consistency. Tyre pressures are tip-top!

As for where the good tyres should go...

In my mind, under braking, especially heavy braking the best tyres should go at the front? I mean, the front does the majority of the braking so surely the best tyres should go at the front to make the most of the front-rear braking distribution? Not that I'm disagreeing that with worse tyres at the back there is a greater chance for the rear to slide and induce oversteer... But, what if I lose the front under emergency braking on a corner and continue to go straight - off the road and down a cliff? I'd rather lose the rear and attempt to Colin Mcrae it around the corner. I accept that with my skills I'm unlikely to succeed, but spinning sideways, and likely also rolling, down a cliff is more stylish than going down straight forwards

I wanted to keep that 2.5mm tyre legal and >2.0mm for as long as possible, so I put that to the rear. And if things do go wrong, at least I could comfortably say my tyres were legal

Anyway, digressions aside, this is only a temporary measure. I have fresh rubber, literally sat next to me, which I'm going to take to the garage when I get the chance.

[blueball]

Off the top of my head, some things I would check are:

Spark plugs (gap set correctly)
Distributor cap (check points clean)
Air Filter (clean and not damaged)

Probably others to check but these things are really easy and quick to check, costs nothing but time.

[DanceswithUnix]

Watch the engine temperature. Petrol engines like to be the correct temperature, in winter they take a while to get warm so fuel economy suffers but if it takes longer than usual to warm up then that could be what you are seeing.

The basic service items should be fine, modern cars don't have distributers or points to wear and faults are normally pretty obvious if you get something like a fuel metering problem. Check the intake hoses to make sure there aren't any air leaks though.

As for tyre positions, traditionally you want to keep the rears sticky so the back doesn't kick violently around. I thought most modern cars had break force distribution though, in which case each wheel works to the best of it's ability and won't lock up so it becomes pretty academic. If you have a car with basic ABS, it probably still matters.

Edit: Has traffic been worse recently, maybe road works? It might not be you

[matts-uk]

I agree the assertion that my car is drinking more is still statistically wobbly, but I've never had five consecutive fill-ups yield less than 43 mpg - I think the sinking feeling of seeing the digits on the fuel pump volume continue beyond what I'm expecting for the last few months may be inducing a bias!

The quality of any analysis is only as good as the data (RIRO). However looking at the data, the miles travelled gradient has increased in the last few months suggesting a significant change in your 'style' of driving. While all the other factors you have mentioned 'could' affect fuel consumption the style of driving is the most likely to. The first bias you should try to rule out is the psychological one; your own conformational bias. In view of the gradient rise you could be doing a few more, shorter journeys and/or travelling a few miles per hour faster to reduce time spent in the car - Both factors that are generally more significant than all the other factors (you have mentioned) combined.

Oh, and I also checked for fuel leaks! Tyre wear patterns are even - balancing and alignment is least not stupidly out.

Alignment does not have to be too far out before it starts to have an noticeable impact on fuel consumption. If one wheel is presumed correct, the other wheel is being dragged sideways along the road acting as brake.

As for where the good tyres should go...

Best grip should go on the driven wheels. The influence of tread depth is mainly felt in the wet. The tread pattern pumps water from under the tyre increasing the friction co-efficient. Deeper treads allow more water to be 'channelled' away from the contact patch at any particular wheel speed. The most obvious demonstration of the significance of the tyre's friction co-efficient with the road is letting in the clutch. A driven wheel with worn tread in the wet acts similar to slipping the clutch. As the friction co-efficient with the road decreases the wheel rotates faster than the resulting speed over ground. The difference on each wheel revolution may be tiny but it is accrued on each and every wheel revolution, and soon becomes significant. On the other hand, in dry conditions the 'gaps' in the tread pattern reduce the contact patch (below 100%) and so decrease the friction co-efficient.

Wheels without drive are effectively pushed or dragged along the road by the driven wheels. The friction co-efficient stills plays a part but is relatively trivial in comparison to the driven wheels. The motive force for the free-wheeling comes from the driven wheels. All other things being equal, all inefficiencies at the driven wheels are transferred directly to the free-wheel friction calculations by a reduction of the motive force available.

In my mind, under braking, especially heavy braking the best tyres should go at the front?

Tread pattern design is a compromise between efficiency in the dry and efficiency in the wet. Deeper, wider treads are more efficient in the wet and smoother 'slicker' treads are more efficient in the dry. Once the tread goes below the legal limit all bets are off, as the rubber compound protecting the ply cords is generally much harder, with a significantly lower friction co-efficient, than the tread laminate laid on top.

Not that I'm disagreeing that with worse tyres at the back there is a greater chance for the rear to slide and induce oversteer...

"Best treads on the back," is a hangover from when the majority of cars had rear wheel drive. Wheel efficiency under (both) acceleration and braking is somewhat proportional to the rate of tyre wear.

[kalniel]

Best grip should go on the driven wheels.

For speed on a track, maybe, though most cars generate significantly more braking force than accelerative. For safety, no - best grip in a risk of aquaplaning situation (ie tread) should be on the back.

"Best treads on the back," is a hangover from when the majority of cars had rear wheel drive.

Other way around. The advice used to be to rotate tyres for even wear. Now the advice is new tyres on rear.

https://www.theaa.com/driving-advice/safety/car-tyres

[badass]

I think you're worrying about nothing. The reason is probably that cars get less efficient over winter due to both winter driving aids and the fact that engines simply get less efficient (although more powerful) when its cold. Forget A/C in the summer - it's effect whilst measurable in a lab is so insignificant in real world driving as to have no noticeable effect. Less than braking from 70 to 40 MPH once in a 2 hour journey.

The reason the bike hasn't experienced the same effect? Probably because you only ride it on warmer, sunnier days.

However if it continues getting worse I'd be on the lookout for binding brakes. Any funny smells when you get out of the car or stop it at junctions?