Normally on my daily commute to and from filton to Gloucester I've been getting around 52mpg. I had my tracking sorted out on Saturday however and without really changing my driving style I averaged 58 this morning. However when I drove home (after the rush hour) I averaged 48 on the return journey.

Now I know I didn't get stuck behind anything for 5 minutes doing a more economical speed, but even so, how (unless I've knocked the tracking way out again) can my fuel economy suddenly be so bad?

Is the journey home uphill? Temperature change? Tyre pressures?

They can all play apart.

To be honest, you'd need to monitor your consumption for a bit longer to get a picture of your overall consumption situation.  

 

On my daily commute my consumption record is 78.5 MPG calculated on my PD100......my worst was 49 MPG calculated. All on the exact same route. Mostly my MPG is in the middle of these values. (my tank average for the past few months is 65 mpg......I do drive it fairly carefully). But the point is as danham says, lot of factors can affect the MPG on a daily basis.   

For what it's worth......My daily commute is 46 miles each way. I get around 55 MPG on the trip to work and around 63 MPG on the trip back. It's pretty much all motorway done on cruise control both ways so the speed is fairly constant. I've found the biggest knock to fuel consumption is those really cold Winter mornings when I get high 40's. Diesels really don't like the cold!

Wind affects it badly!

In some cases upto 10mpg

Wind is something I didn't think of. I averaged 52/53 ish on Tuesday and today on the way there. I only got 48 on the way back today, much less Tuesday but I wasn't going gentle. My guess is its a combination of the traffic holding me up at more Eco speeds and the wind usually blowing from the south west

My whole average is 43.4mog but back from Cardiff to cambridge non stop got me 73.4 mpg from my vrs

For what it is worth +1 for wind.

 

If you are almost a cyclist you will appreciate what a difference this makes - same applies to a car.

I live very close to an airfield and work very close to another airfield and found that home is about 38 metres above work, so the journey home does workout to be uphill on average. Might also have something to do with it