I drive my 07 520D Auto 90% on Motorways @ a steady 130kmh.... I bought the car 6 months old with 15,000km and at first my average consumption ranged between 49 - 52 MPG (apologies for mixing MPG & KM!).

Over the winter consumption increases to about 46 - 48 MPG.

The car has now covered 74,000 KM, and had an oil change, fuel, air and microfilter change done about 10,000KM ago as per the iDrive.

Over the last 2 tanks of fuel, on identical routes, the consumption has increased to 37MPG! I have checked the tyre presuures and they are OK. I have also checked the engine temp as per the thread on poor fuel consumption on 525D and this is a consistent 86 degrees (was 86 degress when consumption was OK).

The only recent change I have made is replacing the front tyres with non RFT's a couple of weeks back......... I can't believe that this change could increase my consumption soooo much!

Any ideas on any other components or faults I can check?

Are you going by the on board computer or brim to brim fills? If using the OBC then try checking via brim to brim as this will be more accurate.

i was getting poor gas consumption before my 60,000 mls service i got my spark plugs replaces & cleaned my MAF sensor and got great gas consumption average 19 mpg... this passed friday i replaced all 4 tires to Falken and i inflated my tires two PSI over from the minimum requirement and my average is now 21 mpg i could not believe my eyes

Yep - next check is to measure brim to brim consumption in case the computer has somehow become messed up.

Will also try over inflating the tyres slightly - from using the Search here, consensus seems to be 36 PSI front + 40 PSI rear.

Definitely do brim-to-brim monitoring. I use quite a detailed spreadsheet for this, which also shows me how far off the on board systems can be in displaying the MPG, particularly. I'll be happy to share my spreadsheet - just shout if you'd like a copy.

Do a search on Progman/CIP updates here too. As I recall, some people have had adverse consumption effects as a result of getting certain CIP updates on certain models. Mine has been OK, but you may want to check which car-CIP combinations encountered the adverse effects.

Thanks MacSid..... I'll try a brim to brim first. Don't think my CIP has been upgraded as the consumption only changed in the last couple of thousand K's and the car was with the dealer >10,000K ago.

I understand that our cars come with low rolling resistance tires. Perhaps the tires are to blame? In any case you should indeed do brim to brim measurements.

when is the last time you changed your fuel filter? lower fuel pressure decreases the mileage.

Fuel filter was changed at last service..........

Have noticed also that inside of tailpipe is far more back than normal.... once I've confirmed the poor MPG through brim to brim calculations, I'll first check out the MAF and then DPF.....