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

[ChrisPDuck]

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.

 

[Bellow]

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.

 

[190]

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.