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Reported economy in the toilet after air filter, oil and filter, and winter tyre change

There's definitely scope for a difference of a few percent changing from old to new tyres and correcting pressures. If the old tyres had lost, say, 5mm of tread depth, that's about 1.5% of the radius, so if my earlier argument is correct then your mpg would appear to drop by that much with the new tyres. Plus, actual tyre dimensions for a given size vary among tyre manufacturers. Then there's the question of the effective circumference when the pressure is changed - it's a complex relationship, but the principle of the wheel speed-based indirect tyre pressure monitoring systems is that the number of revolutions for a given distance varies with tyre pressure. So maybe there's scope for a few more percent change, if, say, the old tyres were a bit under-inflated and the new ones are correctly inflated.

But these effects aren't going to add up to 25-30%. The best way to find out if the actual fuel consumption has changed or whether it's just a miscalculation by the car's computer is, as colinallcars suggests, to do a brim-to-brim check.
 
Okay, maybe a different way to ask the question could be this:

After changing old winter tyres to new winter tyres, an oil change and an air filter change, what could cause the reported economy on the dash to drop by 25%?

So even if that's assuming it's gone from pie in dee sky figures to correct now 'correct' figures, what would cause that shift?
Were the filters unwrapped before fitting? Forgive the flippancy but....
Something has changed in ECU mpg calcs, and it surprised me.
.... better IMO to trust the 'gauge' - at least initially than risk overlooking a problem that is being reported no matter how indirectly.
Given the work that has been done I'd be looking for a binding brake (possibly a caliper took a knock during the wheel re-fitting that has affected it) and/or in the engine bay for something that is accidentally disconnected or not re-connected after being disturbed to enable the work done.
Maybe it is the ECU misreporting but I'd suspect a binding brake (easily checked) before it.
 
But these effects aren't going to add up to 25-30%. The best way to find out if the actual fuel consumption has changed or whether it's just a miscalculation by the car's computer is, as colinallcars suggests, to do a brim-to-brim check.

Agree that it's not possible for these small effects to add up to 25 -30%. Not tyres, not winter fuel blend and not the increased density of the winter air which BTW has a bigger winter effect than the fuel blend at cruising speeds. I confess I've no idea as to the real cause other than to note these relatively dramatic changes in mpg always seem to happen to diesels rather than petrol which would suggest it's something to do with the diesel engine management system.
 
Well, not a super long run (that is in the week), but a reasonable 53mpg reported this evening over 30 miles, low traffic but mixed road types driving journey, so it looks like the air filter adaptations reset might have done the trick, and the on board computer is back to it's normal (for my car) reporting of the incorrect and too good to be true good economy like it was before the air filter change. At least it's 'back to how it was', even if the number can't be trusted. Journey was about 20mins in 20-40mph suburban, 15mins duel carriageway, 15mins country lanes with central white lines.

It will be interesting to see on a long run what the delta actually is going to be between the 40ish mpg I'll be getting in reality from a brim to brim calc, vs the on board readout from a long run.
 

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So, I've just done 286 miles over the past two days, as follows:

Computer Trip 1 - 137 miles 54.3mpg (massive motorway traffic jams due to burning truck on M1 & Christmas M25)
Computer Trip 2 - 20 miles 22 mpg (grinding urban traffic)
Computer Trip 3 - 129 miles 67.3mpg (only a handful of traffic-at-standstill incidents on the motorway on the way back)

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This is a combined reported usage (if you crunch the computer numbers) of 5.35 Gal and works out at 53.4mpgs over 286 miles.
Brim to brim was 25.83L or 5.68 Gal, which equates to 50.3mpgs over 286 miles.
So only a delta of 3mpgs on this trip, brim to brim vs computer.
I'm considering reporting a pump error to the fuel retailer, as this is no where near the 36.2mpg I should be getting for an E350 CDI Blue Efficiency that's shown in Post #46
 
I know mine is the lower power version of yours but I cant imagine it getting anywhere close to that. It very odd that your car does so far over even Mercs usually exaggerated figures. Only my ALFA ever got close to the claimed figures and I can drive very efficiently when I want too....but life's to short to drive like that too much!
Many on here could not get that out of a small engine derv C Class.......but lucky you.
 
I know mine is the lower power version of yours but I cant imagine it getting anywhere close to that. It very odd that your car does so far over even Mercs usually exaggerated figures. Only my ALFA ever got close to the claimed figures and I can drive very efficiently when I want too....but life's to short to drive like that too much!
Many on here could not get that out of a small engine derv C Class.......but lucky you.

You might if you reset the trip once the engine temps reach 90. Just saying. 🤔🤔. Best I saw was 55 mpg over 160 miles sticking to 65 mph. Personally I don't care about economic driving , what I get per £50 I get , if it needs more I put more in. 👍 Or buy a VW golf . I just wish I had to take long journeys to appreciate the vehicles I've owned, unfortunately I don't . Work is only 6 miles away .
 
LOL....What's the point of that?...warming up is part of the overall MPG.......proves nothing about the economy of the car if you just choose the best bits....i could get 99mpg going down hill!!!!
 
LOL....What's the point of that?...warming up is part of the overall MPG.......proves nothing about the economy of the car if you just choose the best bits....i could get 99mpg going down hill!!!!

Some do. That's my point ☝️.
 
Some do. That's my point ☝️.
I assume your assuming I've reset the trip computer once warm? Why would I do that? And wouldn't that impact the brim to brim figure, as I would have used more fuel that was not accounted for by the 'fake' mileage after resets?

And this thread was to try and understand why my reported economy was in the toilet. Which I think we have broadly concluded is due to the impact the air filter change made vs. not doing the adaptations reset.
 
I assume your assuming I've reset the trip computer once warm? Why would I do that? And wouldn't that impact the brim to brim figure, as I would have used more fuel that was not accounted for by the 'fake' mileage after resets?

And this thread was to try and understand why my reported economy was in the toilet. Which I think we have broadly concluded is due to the impact the air filter change made vs. not doing the adaptations reset.

It never crossed my mind. 🙄
 
Trip 2 - are you sure that wasn’t in your V8? 🤣
 
Trip 2 - are you sure that wasn’t in your V8? 🤣
Yes, it felt like V8 economy! Shocking mpg figures. It was actually 4 shorter journeys in the late afternoon/evening through grinding Christmas traffic, but they were all within close enough time that the computer just counted them all as the same trip.
 
I track all my fuel consumption with 'Road Trip' on my phone.

I previously had a 2013 BlueTec E350CDI Cabriolet with the 265bhp engine and 7G gearbox..

I drove it 55,254 miles in 'ECO' mode over the course of 6 years and averaged 40.65 MPG. Mostly on a 122 mile round trip commute, with occasional other trips out.

I only ever saw the onboard computer show 50+MPG in the middle of summer if I had averaged 50+mph when driving home from work.

I now have the later 2018 E350d with 9G gearbox. The car has 4Matic, so I wouldn't expect to see the economy that I could previously achieve. I drive this one in 'ECO' mode as well, although I have the air suspension on Comfort, so it is not dropped down unless at speeds of over 70MPH. The gearbox disconnects drive if I am coasting, (unless it decides I am too close to the car in front). I've done 26,634 miles over 2 years and am averaging 40.42 MPG.

I don't like the idea of the engine / turbo sooting up, so I will regularly throw away economy by blasting up a hill on my return commute to get the RPM up and exercise the turbo a bit. After all, what is the point of a car with 620NM of torque if you never use it?
 
Yes, it felt like V8 economy! Shocking mpg figures. It was actually 4 shorter journeys in the late afternoon/evening through grinding Christmas traffic, but they were all within close enough time that the computer just counted them all as the same trip.

Stop start and idle take a hammering on the over all mpg figures. As does the cold start miles. 👍
I track all my fuel consumption with 'Road Trip' on my phone.

I previously had a 2013 BlueTec E350CDI Cabriolet with the 265bhp engine and 7G gearbox..

I drove it 55,254 miles in 'ECO' mode over the course of 6 years and averaged 40.65 MPG. Mostly on a 122 mile round trip commute, with occasional other trips out.

I only ever saw the onboard computer show 50+MPG in the middle of summer if I had averaged 50+mph when driving home from work.

I now have the later 2018 E350d with 9G gearbox. The car has 4Matic, so I wouldn't expect to see the economy that I could previously achieve. I drive this one in 'ECO' mode as well, although I have the air suspension on Comfort, so it is not dropped down unless at speeds of over 70MPH. The gearbox disconnects drive if I am coasting, (unless it decides I am too close to the car in front). I've done 26,634 miles over 2 years and am averaging 40.42 MPG.

I don't like the idea of the engine / turbo sooting up, so I will regularly throw away economy by blasting up a hill on my return commute to get the RPM up and exercise the turbo a bit. After all, what is the point of a car with 620NM of torque if you never use it?


You are a star Sir . 👍👍
 
Highest I saw today was 40.3 mpg . 65 to 75 mph , sometimes using cruise control . 4 matic plus all season tyres. I'll be changing to summers and see what difference it makes if any around march.

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That nice high average speed will help....thing mine is around 26mph in the few months I've owned her!!!
 

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