Rain and its effect on MPG

[Diagro]

hey all

On sunday i drove to Hull and back for work, was just over 300miles round trip on the M1.

On the way there it was nice and sunny, the car managed a nice 57MPG on average.

On the way back it was horrible and raining heavy, and the car did 37MPG....

This is a LARGE difference in average consumption which caused at the pump to show a 44 trip average.
The MPG is back up there today at 47MPG (mixed driving to normal work location)

Has anyone else noticed such a large difference due to the rain?

What on earth causes it? I know that water droplets alter the air density but only by a small amount. No where enough to cause a 20MPG drop of MPG.

 

[Diagro]

Your route was different, your speed was probably different, even if the traffic levels were the same (which they probably weren't). Increased rolling resistance and electrical load will have added to it.

Not really fair to compare the two trips like that.

Same way back. Same Speed (CC at 70Mph) No traffic (6am there, 9pm back)

I dont use heaters, so only lights.

 

[Diagro]

Was there any wind either way?

Was a little extra on the return leg, but im sure it was a south bound wind. (the way i was going)

nothing is wrong with my car it seems, just surprised at the difference

 

[Diagro]

If youre reading it from the computer then dont. We had a load of 1.6 diesels as pool cars in my last job, where mileage was logged against fuel spend. The computers all showed averages in the high 50s, but in reality they were doing mid 40s at best. Very few trip computers are accurate but it seem that the mazda ones are one a completely different planet.

Its normally 2-3Mpg out. I normally work it out at the pump.

Unfortunately i didnt think of doing that till after i got home. next time i will refill when i arrive and again when i return.

Whats annoying is that i was on auto pilot and put BP Ultimate Diesel in the car instead of normal... £1.29 per L! that was an expensive 30L