Rain and its effect on MPG

Diagro · 21 Jun 2016 at 10:40

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.

paradigm · 21 Jun 2016 at 10:43

There is also increased drag on the road surface, typically you'll find that you are braking more (slowing down further) in the wet, which in return means you spend more time re-accelerating.

That said, I don't tend to see as much discrepancy as you do between a run in the dry and a run in the wet, I don't think you can compare a return-leg of a journey to the outbound-leg though, as the roads whilst the same physically, do not necessarily have the same impact in one direction as opposed to the other (one direction may average out as downhill for example).

Skeeter · 21 Jun 2016 at 10:44

Water on the road is going to significantly increase the rolling resistance of the wheels, so the car will be working harder to maintain the same speeds as in the dry.

Then add on the extra electricity requirement for wipers and heaters, plus you probably had some lights on, and it all adds up.

dacads · 21 Jun 2016 at 10:52

Skeeter said:
Water on the road is going to significantly increase the rolling resistance of the wheels, so the car will be working harder to maintain the same speeds as in the dry.

Then add on the extra electricity requirement for wipers and heaters, plus you probably had some lights on, and it all adds up.

Or in this case minuses

Minstadave · 21 Jun 2016 at 10:54

It's usually cooler when it rains too which increases air density.

paradigm · 21 Jun 2016 at 11:03

Minstadave said:
It's usually cooler when it rains too which increases air density.

That's a good thing though, surely?

HangTime · 21 Jun 2016 at 11:21

Not for economy, as there will be more drag in denser air.

Uriel · 21 Jun 2016 at 11:41

Big fat blobs of water falling at you also effectively increase the average density of the medium through which you're moving.

DRZ · 21 Jun 2016 at 11:45

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.

GravyMonster · 21 Jun 2016 at 12:04

I'd guess the cooler air had more of an effect than the rain.

Semple · 21 Jun 2016 at 12:14

That is quite a dip.

My route from Bristol to Manchester i can average about 61/62 MPG in the A3. On the way back on Sunday (in the rain), it had dropped to around 56/57 which i guess i expected a small drop in the wet.

Diagro · 21 Jun 2016 at 12:31

DRZ said:
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.

Uriel · 21 Jun 2016 at 12:45

Was there any wind either way?

Diagro · 21 Jun 2016 at 12:49

Uriel said:
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

Iamzod · 21 Jun 2016 at 12:50

I only see a few mpg difference in my car.

I average in the mid 50's to low 60's most longer journeys ao not sure how you got it down into the 30's. Is it possible you have a caliper starting to play up and sticking intermittently?

I'd probably have to sit in 3rd gear to get such bad economy or be driving with the handbrake partially engaged.

Clarkey · 21 Jun 2016 at 12:51

Never noticed a difference before tbh.

iaind · 21 Jun 2016 at 12:54

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.

Diagro · 21 Jun 2016 at 13:50

iaind said:
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

[TW]Fox · 21 Jun 2016 at 14:19

iaind said:
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.

I always see this sort of thing posted but I have never had a car that's.nore than 1mpg out...

Jono · 21 Jun 2016 at 14:26

It's probably mostly surface water resistance? Given the big shove you get when you hit a decent sized puddle.

Guess it depends how bad it's raining and winding in your general direction.