What a difference a day makes.

[JockoT]

Five days a week I drive my wife to her work then return home to enjoy my retirement(!). The entire trip is just over 7 miles in length, through urban streets, and commencing at 06:20 (so traffic is never an issue). I have a ScanGauge E fitted to my 2006 Honda Jazz. What I have noticed is that on a rainy day, the average mpg displayed is 10%, sometimes as much as 15% poorer, than on a dry day. I know it is not a very scientific result, as there are so many other variables, but it gives an idea just how detrimental rain can be to good FE.

[peteo48]

For a long time I naively thought rain would mean less rolling resistance but it causes the opposite as it has to be pushed out of the way. EV drivers will tell you that rain kills range as well.

[Draigflag]

Not just hydrodrag from rain, but slight aerodrag from wiper use (at higher speeds) too, and the fact that you may be using more electrical energy for fans, lights and ac to demist windows will all affect fuel economy too.

[JockoT]

The wipers themselves will use a bit of energy, but as I never get above 30 mph on my daily runs they won't cause much drag. At this time of the year and that time of day the lights are on rain or not. Fan always on "1" and no a/c to worry about, but I appreciate that all these things could influence some people's figures.

[Draigflag]

On the plus side, look what a difference the clocks jumping forward an hour makes, warmer weather and lighter in the mornings and evenings, things can only get better! My last tank was almost 5 mpg more than my previous

[JockoT]

It is light in the evenings, but the mornings are still pretty dark. One of the joys of living in Scotland. At least it is daylight before 9 am now! I always see a big improvement in mpg in summer as against winter. I have calculated my mpg for almost 40 years now, and the difference is stark. Mind you, there is just as much chance as rain in the summer as there is in winter, here in Scotland. Wales must be pretty similar in that respect?

[Draigflag]

Yea we get our fair share of Atlantic rain blowing in from the west, ut then we also get the warmer weather too, some of the hottest places in Europe were in Wales last year, and the waters bring in the highest concentration of dolphins in Northern Europe which is great for tourism too. The only thing I hate about the summer is the lighter mornings, having house parties etc. I remember last July I had this girl at mine, and we didn't go to bed until 4.30 am, really hard to concentrate on anything when the sun's already up if you get me. But then I had another girl in December, went to bed at 7 am (got up at 8) and the fact that it was still dark made a big difference

[JockoT]

This past week it has been light in the mornings, and I am amazed what a difference it has made to fuel economy. Nothing to do with driving without headlights, I wouldn't have thought, but probably more to do with being able to anticipate other traffic better.

[Draigflag]

It's also warmed up alot the past week or so, so your engine will be less likely to go into a cold/fast idle and warm up quicker too.

[JockoT]

Originally Posted by Draigflag

It's also warmed up alot the past week or so, so your engine will be less likely to go into a cold/fast idle and warm up quicker too.

Not here it hasn't. We still had frosts last week and this week the wind is still off the east, straight in from the North Sea, and it is Baltic here.
The Jazz is unusual in that it doesn't have a temperature gauge, just a blue light (when the engine is cold) and a red light (to show it is overheating). The blue light still goes out within 10 feet of the same spot, every morning without fail. If it doesn't that is when I realise I have the cabin temperature knob switched to hot (I usually have it set to cold, but if I do turn it to hot I do so only after the blue light goes out).