I check my MPG every tank fill and usually have the MFA set on average MPG. Suddenly, over the last 2 weeks, the MPG has dropped from a computer indicated 56-57 mpg on my journey to work to around 49-50.

The MFA is totally inaccurate always reading 12% over what it should but is is contant. My last tank full showed 42 mpg instead of usually 46-47.

The ERG looks like it could do with a clean but this shouldn't suddenly affect fuel consuption.

The only thing I've done is change the front 2 tyres from Rainsport 2s to Rainsport 3s and fitted LED sidelight bulbs.

The fitting of new tyres shouldn't have such an effect and the bulbs shouldn't have ANY affect.

Tyres are correct pressure but it'd need quite a deviation to have a 10% reduction.

 

Is there anything else I can check before hooking it up to see if any faults show.

All I'm thinking is common things like boost leak or dirty fuel filter for example. How about an ECU reset?

All I'm thinking is common things like boost leak or dirty fuel filter for example. How about an ECU reset?

 

Sounds daunting.

Take the battery off. It resets the ECU. Personally I leave mine off overnight to make sure any voltage trickles out, but some people say leave them for 30 mins.

The usual suspects are...

Boost leak from one of the hoses (noticed any more smoke under hard acceleration recently?)
Blocked/dirty injectors (run some FORTE diesel treatment through should help keep them clean)
Wheel alignment (this should have been checked when they fit the new tyres)
Sticking/seized calliper
Cold weather (not much you can do about this)
Dodgy batch of diesel (again not much you can do until you use it and refill)
 

Have you looked at the rolling resistance specs for the rainsport 3 tyres over the rainsport 2s?

 

The tread pattern is fairly different and it may just be the rainsport 3s have a higher rolling resistance, thus decreasing your MPG. Have you tried running them 2-3PSI over what the OEM specs are as this can improve MPG slightly (the theory is it reduces drag and rolling resistance)

Mine is showing about 17mpg around taking it very easy!

Lower rolling resistance according to this  :wonder:

I've not noticed any more smoke. I've added some diesel treatment and I'll see how that goes. Weird though the computer figures are well down???

It'll be another 2 weeks before this tank needs filling for me to check properly. But the tankfull before this was the same, different petrol stations though.

The usual suspects are...

Boost leak from one of the hoses (noticed any more smoke under hard acceleration recently?)
Blocked/dirty injectors (run some FORTE diesel treatment through should help keep them clean)
Wheel alignment (this should have been checked when they fit the new tyres)
Sticking/seized calliper
Cold weather (not much you can do about this)
Dodgy batch of diesel (again not much you can do until you use it and refill)
 

Have you looked at the rolling resistance specs for the rainsport 3 tyres over the rainsport 2s?

 

The tread pattern is fairly different and it may just be the rainsport 3s have a higher rolling resistance, thus decreasing your MPG. Have you tried running them 2-3PSI over what the OEM specs are as this can improve MPG slightly (the theory is it reduces drag and rolling resistance)

Edited 3 March, 201412 yr by vRS G60

My MPG gradually got less and less over time - turned out it was a boost leak that eventually ended up with the pipes separating (with associated 'pop', smoke etc etc).  Cable tied up to get me home then fitted new pipes - end result MPG restored to previous level and a big jump in performance (obviously that was getting worse slowly over time but I hadn't noticed the decline).  

Thanks, I need to check the hoses, not really had time for a good look. Anything specific?

The bottom intercooler one is prone to going. Mine has started seeping oil recently so I need to attempt a DIY bodge when the weather gets better. Best bet is to get it on axel stands, remove the under tray and check everything from the turbo to the intercooler to the EGR.

I had the car checked for fault codes on Saturday and non came up !

After running a dose of injector cleaner through it things are picking up again.

 

Edit, new fuel in yesterday (Shell) and we're back to normal