WTF... is happening to my fuel economy?!

Binding handbrake? Is the engine overfilled with oil? No oil in the engine? Rubbish garage by the sound of it, take it back and get them to repeat the service...

Tyres can, in extreme cases affect fuel economy by 10%, but that's unusual and also not as much as you're seeing. Did you check the pressure when the car was returned?

Would a (decent, something like a K&N) new air filter help?

No, not at all. Air filters are rarely the issue and all this K&N bollocks is just that.

It's hard to imagine what could cause that kind of drop without a warning light on or something.

Could the main dealer have done an ecu update that is affecting your fuel economy?

A new air filter will make a marginal difference, but in steady state driving the ECU will be looking at the MAF sensor and then applying a correction based on the lambda in the exhaust. It won't be looking too hard at the throttle position. That makes a slight difference because in effect a higher flowing filter (new or performance) is (as far as the physics of the engine is concerned) the same as a slightly more open throttle, so they can actually make the ECU think you're taking it fractionally easier than you are.

My guess (and it's a guess because there's not much to go on) is they've unplugged/knocked a sensor off somewhere which means the ECU is having to guestimate the fueling based on it's internal programing rather than having any data or feedback to work from. Most likely the MAF (because it's in the engine bay, near the air filter usually, possibly in the way of the oil filler, and would result in crap MPG if the ECU was forced to run just on throttle position as an input).

Surprisingly unplugging the MAF doesn't always throw up an error! Makes sense on a diesel as it's only function is to provide a limit value so the engine doesn't go beyond a stochiometric mix, but on a petrol I'd have expected it to be more key to the ECU's function.

Had my 71 plate Hyundai Kona hybrid serviced recently and the mpg has dropped a fair bit as well, it was serviced just before 8k and it showed a combined 54.8 mpg, its since done around the 1000 miles since service and I can barely get it above 49mpg , I'm guessing they've done a software update as nothing else has changed ?? but they didn't say and the paperwork doesn't show its had an update ?

what grade oil have they used.

Checked tyre pressure?

Sounds simply like seasonal atmospheric and Winter fuel changes to me.

My hot hatch does 41mpg with Shell VMax Unleaded in mid Summer - I.e. Higher Summer weather temps = better aerodynamics + Summer fuel produces better engine economy results vs Winter fuel.

As a contrast, in mid winter the same car does 35-36mpg and that's probably with more conservative driving style compared to more regular Summer b road hooning here and there.

It's colder than it has been over the past couple of years? Using a lot more power to heat up?

Checked tyre pressure?

The garage our company uses seem to adjust all tyre pressures to 32psi. Mine should be 35psi minimum.

Or any of the other variables mentioned. Including weather.

I'd be amazed if knocking a plug off a sensor wouldn't throw a check engine light.

Suspect the cold weather, new tyres (check the tyre pressure) and more heating on in the car are impacting the mpg. I replaced the 2 Bridgestone all season tyres on the Puggy with some Cross Climate things and the fuel economy improved around 15mpg - which has now settled to about 10mpg improvement overall.

Tyres can have a major impact and at a complete guess, your new tyres have higher rolling resistance and aren't as economic. More heat needing to be used to keep inside the car warm will also be having an impact.

Just the air temps.
I reckon i drop by 6 to 8mpg from summer to winter.
Thicker engine oil on start up, plus 3 x as long to get fully hot.
Ditto gear box oil
Ditto. Wheel bearing grease
Plus more drag on the alternator as you have the hrw on, heated mirrors on, blower on fsa etc.

Check everything already mentioned in the thread... Though I'd be going back to the garage and asking WTF they have done myself too!

TINAS' point about MAF sensor is quite appropriate, as they would almost certainly have disconnected it during the service at some point, and if it hasn't been reconnected properly the engine would definitely be overfuelling...

Surprisingly, most modern cars don't get an air filter replacement these days until 40k miles, sometimes even more! Seems crazy to me when they're generally a £10 part, but there we go... No, a K&N wouldn't improve anything, but personally for a tenner (and about 5 mins under the bonnet yourself) it wouldn't hurt to put a new paper filter in...

But it's surprising just how much more fuel modern cars will use in winter than summer, not just cos of additives in the fuel, but modern engines are much more thermodynamically efficient than old ones, so can take a lot longer to get up to a decent operating temperature, where they are efficient, in winter so can use a lot more fuel than normal getting up to that temp... It isn't just diesels either, modern petrols take longer to warm up too.

Cold weather, new tyres and new, thicker oil until it beds in/breaks down a little will be the reasons. This is especially true if the car is only doing short distances and the oil not getting warm for longer periods.

TINAS’ point about MAF sensor is quite appropriate, as they would almost certainly have disconnected it during the service at some point, and if it hasn’t been reconnected properly the engine would definitely be overfuelling…

Still think the ECU would throw a light at minimum.

I assume 1.0tsi means one litre turbocharged? Cool dense air more petrol?

Can't imagine a small motor like that will be on warm up enrichment for long.

I think it's a combination of little things mentioned above rather than a fault, unless it's a mechanical fault, such as a slightly leaking boost line or binding brake.

Sounds simply like seasonal atmospheric and Winter fuel changes to me.

Same here. My car goes from 50-55mpg to 45-50mpg between summer and winter. The recent warm spell followed by the cold snap last week really showed the difference!

Are you sure that it’s not just something as simple as the fact that it’s been brutally cold for the last fortnight, and now it’s warmed up again there’s a howling southerly gale?

Next time you get out of the car after any significant run, walk around the car and feel for any heat emanating from each wheel, place your hand on the wheel rim, is it unusually hot? Have had this happen a few times now and it was worn brake callipers binding on the disc, not enough to really notice when driving, but enough to generate heat from the friction enough to notice the heat wafting up form the wheel when stationary... it also knocked almost 10mpg off the economy. Worth checking if only to iron it out from the other 20 things it could be...

Why disconnect the maf sensor?
They are a fit and forgot part midway down the inlet tract.

MAF filters can fail or there can be leaks in the intake system after the MAF that totally screw the fuel mix. Something as simple as the air filter case not being put on properly can have some affect too.

I would check the air filter case first asd that is one thing that has recently been fiddled with.

Why disconnect the maf sensor?
They are a fit and forgot part midway down the inlet tract.

Often to get at the fuel and/or oil filters you have to remove the airbox and/or pipe work on these tiny cars.

also... do you have an idea of the ethanol content of the last tank of fuel that gave you those numbers? Given E10 can be 'up to' 10% it's hard to know for sure, but higher ethanol can for sure lower mpg.... which is why mine only drinks the 99Ron VPower good stuff, it sucks on E10.

It's winter, cold weather makes cars more inefficient.

Tyres can make some difference.

Reading between the lines you were averaging 55mpg and now 48. Are the tyres the correct pressure?

An air filter wont make much differnce unless its clogged upp with shyte.. have you opened the filter box and had a look? Classic dealer stealer hack is to say they swapped the air filter out and they just leave the old one in.

I’m flummoxed by why it’s suddenly dropped so sharply!

It's a circa 12% drop in efficiency, done more cold stop start driving, rather than motorway driving recenly? engines will overfuel on short trips, especialy in cold weather.

Has the wife/offspring used the car recenty? 😀 , it could be a cumaltive effect of new tyres (dunno if your new ones are suppsed to be better on economy) but if the pressures are incorrect that will also play a part. And check the air filer is actually new/clean.

It’s winter, cold weather makes cars more inefficient.

Only really on start up, so multiple short, cold trips will easily dent your efficiency. if that's the case then a few hours of motorway driving should bring the average back into line.

I'd be very surprised if they unplugged anything during a service - but it is German and they are pretty illogical at making anything reliable or easy to work on these days.

It'll be little more than oil+filter and some glances at other bits (brakes, tyres, fluids) and then a big bill for a main dealer service at that mileage.

Gearbox oil (If DSG), spark plugs and air filter is 40k on them.
Schedule is here -

https://www.seat.com/datamanual-maintenance/my12_w22/Turismos/en-uk/Maintenance_programm_EN.pdf&ved=2ahUKEwjplPLy9oj8AhVMPcAKHUaeAFQQFnoECAkQAQ&usg=AOvVaw3b0ikrlHPJo508IWYuPT5 Y" title="GOOGLE.COM" rel="nofollow" target="_blank" >

GOOGLE.COM "https://www.google.com/url?sa=t&source=web&rct=j&url= https://www.seat.com/datamanual-maintenance/my12_w22/Turismos/en-uk/Maintenance_programm_EN.pdf&ved=2ahUKEwjplPLy9oj8AhVMPcAKHUaeAFQQFnoECAkQAQ&usg=AOvVaw3b0ikrlHPJo508IWYuPT5 Y"

I'd check for binding brakes and low tyre pressures just to be sure then just carry on as normal using it.

Your current MPG figures seem correct and in line with others;
https://www.fuelly.com/car/seat/arona/2019

The brake caliper sticking would definitely drag causing you to use more petrol but to not notice a wheel was being held back would be very hard to ignore, the car would feel gutless , probably vibrate and definite heat at the wheel
I had to change a front caliper last week £50 Inc brake fluid to change it myself rather than kwikfit price of £300 caliper fit + new discs and pads all round ££££££
My new discs and pads hopefully arriving from Germany soon

Years ago I had a Golf TSI. Mpg dropped by about 10% after swapping all 4 tyres from the standard OEM energy efficient ones
(which were useless in snow) for some Bridgestone All-seasons. And it never snowed again in my ownership of that car (Typical🤣).

It’s not just that they’ve reset the trip computer so you’re now seeing averages over a few miles vs the thousands of miles since it was last reset?

Does a tank of fuel still get you the same distance that you’re used to?

Its the cold snap. My Skoda 1.5Tsi does 40-50mpg normally but during the cold snap last week my mpg dropped to 30-35. As I had just fitted goodyear vector 4 G3 all seasons (excellent tires by the way) I thought the tires were to blame.

Now the weathers calmed down this week I'm getting my usual mpg back now. I'd actually say it is a bit higher by 1-2mpg with the new tires!

I'll bare my arse if the mafs unplugged.

On a 1st service its oil n filter and a visual normally its all paired right down and spread out to fit into lease and service plan costing models

It might have 0.2 of an hour for air filter check but I'd be shocked

As its a petrol fuel filter will be something like 4th year service.

As folk say cold air more dense more fuel also tyre pressures!

Love the fact people automatically assume garage have ballsed up. Admitted they left the cap off and I'd flag that with workshop control.

Admitted they left the cap off and I’d flag that with workshop control.

Lol.. find a new garage

Tyres fuel efficiency varies by about 7.5% from best to worse. So unlikely to be more than a couple of mpg there.

https://www.rac.co.uk/drive/advice/tyres/what-do-your-tyre-labels-mean/

Assuming you are going by the car computer it is just guessing anyway. To get an accurate figure you need to brim the tank. Do a decent amount of miles. Brim the tank again and calculate.

Just the air temps.

That is what I have put it down to. In summer I get over 60mpg (even when using air con) but over last month I have been getting 55mpg.

Unsure where you are in the country but it was minus 10C couple of days here last week - plus it was a very cold week in general. That absolutely eats petrol - especially if you have to sit stationary for 15 minutes whilst the windscreen defrosts.

Also, bear in mind that this year is (i think) the first full year with E10 in the UK.
So you'll be comparing weeks (months) of running with ~10% Ethanol in summer mix, with a few tanks of winter mix, which is closer to ~5% ethanol IIRC.

So, you've lost a couple of points of octane value and a little bit of total energy (Ethanol has higher energy density than petrol).
If you'd driven a flex fuel car on E85, the change is noticeable and significant.
Using E10, it's just a puzzling change in fuel economy.

Last winter many places were still using stocks of neat petrol.

Thanks all! Tyre pressures were checked recently and they were between 32-34 PSI, have topped these up to 35PSI. I had eliminated E!0 as the cause, I've been on E10 for over a year now as it appeared around me in North Wales quite early for some reason and that cost a little to start with, but nothing like as much as this. It's not changed now the weather is warmer either.

The mileage quoted is per journey as well, so not a trip computer issue, the car gives me a claimed MPG figure for that journey and resets it if the car has been left for more than 2 hours, which during the day it is. I brim the car everytime I fill up, used to get 390ish claimed "range" when I did, now it only goes up to 350ish... Something weird and I'll keep an eye on it and check for anything unplugged. For the cost of the filter I'm tempted to do it anyway and if it doesn't improve I'm definitely even more tempted by a remap to try and boost the economy back up a bit!

That absolutely eats petrol – especially if you have to sit stationary for 15 minutes whilst the windscreen defrosts.

Not got an arosa but I do have a 108 with a similarly sized engine.

It's never taken 15 minutes to defrost the windscreen. That little car gets warmer than the gates of hell in minutes.

Mpg for me has gone from 55mpg AVE down to 52ish over the last few tanks now winters hear ..... I reset ever tank and the in dash display shows the trend over each reset. You can see the summer/winter seasons clearly by the dip/peak

It’s not changed now the weather is warmer either.

Sorry, i wasn't clear, you have summer and winter mix of E10. There will be something like a 1-2% change in economy linked to that, completely separate issue from ambient temperatures and humidity.

Also, you've not doing mega miles, so as mentioned above you *might* have missed any software updates to account for E10 until the service you've just had done.

Also, careful with remaps, especially on smaller capacity/turbo/DI engines.
I've dealt with warranty on remaps (we can tell, easily, that it's been done, unless you swap the entire ECU) something like 90% of them switch off half the safeties in the ECU, or block fault codes from being used (but not logged) just to allow their new map to run without turning the dash into a christmas tree.

Seen some right piles of cräp masquerading as remaps.

The biggest thing I think kills my winter mpg is the increased traffic on the roads

Its the cold snap. My Skoda 1.5Tsi does 40-50mpg normally but during the cold snap last week my mpg dropped to 30-35.

Same here on our Leon 1.4tsi.

Cold air, changes in fuel, wet roads etc can all affect engines a lot. I notice my newer engine is much more susceptible to this - perhaps they are now more finely balanced and every last mpg is squeezed from them that any changes leads to a reduced mpg.

Our older, 1.4 16v, Bosch injected, non turbo, non anything, old skool petrol Ibiza with 75 throbbing horses only loses a couple of mpg's - but then it gets 10mpg less in the first place.

Also, careful with remaps, especially on smaller capacity/turbo/DI engines.
I’ve dealt with warranty on remaps (we can tell, easily, that it’s been done, unless you swap the entire ECU) something like 90% of them switch off half the safeties in the ECU, or block fault codes from being used (but not logged) just to allow their new map to run without turning the dash into a christmas tree.

Seen some right piles of cräp masquerading as remaps.

Been looking at TDI Tuning Boxes as they seem decent from what I've read? Also heard some dodgy things about maps so would only go for something reputable!

Having thought about it, you've had new tyres put on. That means the rolling diameter is larger by ~12mm, meaning the roll out is ~40mm greater, that means for the same speed the speedo is now under-reading more, which means the mileage is lower, which means the MPG is lower (the car's still gone over all the same hills, accelerated out of all the same junctions, push all the same air out the way, it just thinks it's traveled a few percent les distance than it did last week).

I had the opposite on my last car, sapping to winter tyres which were marginally smaller made the apparent MPG jump by about 13%, but it was the same if you took the miles from google maps and divided it by the fuel receipts.

Same issue if they were marginally under inflated when you went in and they've pumped the new ones up.

Been looking at TDI Tuning Boxes as they seem decent from what I’ve read? Also heard some dodgy things about maps so would only go for something reputable!

There's not really such a thing.

It's a turbo diesel, any tuning is easy, all you have to do it add more boost and/or fuel. Means it runs hotter, produces more NOx, and more particulates.

Remember 20 years ago when tuning meant tearing the engine to pieces and spending a fortune for make 3 more horsepower. Now it's some spotty Kevin with a laptop quoting some barely believable 50% increase in power, which is easy if all you're doing is increasing the fueling to 150% of the original value. Want more low down torque, easy, just increase the setpoint of the wastegate and more fuel lower down in the rev range. Who cares if higher pressures produce NOx, or the lengthened dwell time of the injectors now means not all the fuel is actually burning.

Been looking at TDI Tuning Boxes as they seem decent from what I’ve read?

Is that one of the ones that frigs with input/output voltages between turbo/fuel pump and ECU, so they can change their operating parameters with no consideration for the rest of the engine, or even the component they are mucking about with?

This would be particularly dodgy but i have seen it happen.

A lot of main dealer services include a ecu flash as standard which often include any standard updates to software. They are supposed to get your explicit permission in circumstances where it includes and emission update and we have to have a copy on file for anything similar on the brand i work for. The customer is within their rights to refuse the update if they wish.

Remember all the vehicles which failed their emission tests due to either dodgy MOT setups or even just dodgy claims. Well the manufacturer got around it by doing recalls with software updates that hugely effected the vehicles fuelling and economy. Those issues havent gone away entirely and it wouldnt surprise me if they were hiding them in ecu flashes at service time......

Car manufacturer: Spends loads of time and money making the engine reliable and efficient
Backstreet remapper: Never mind all that, let me **** about with it, more power!

Spends loads of time and money making the engine reliable and efficientpass the emissions test