Why does mileage drop when it's raining?

Can you run in the rain as fast as in the dry? For same reason, water increases drag on both tires and the exterior of the car making you and the car work harder.

 

Pumping water out of the tire track, accelerating it to considerable speed as it is expelled, takes a considerable amount of power or energy.

 

You are lifting a considerable quantity of water as spray, lifting all that water takes power and power takes energy. The only source of energy available is gasoline. (Except the Prius PHV)

You are running wiper motors and air plus rain is thinker than just air, but those are minor.

Many claim the Prius engine likes it hot, in general rainy days are cooler, and water is a much better conductor of heat than air is. so it may not make as much power per gallon on rainy days.

 

Think of how it feels when you hit a long puddle--the drag on the car speed. Same thing but to a lesser extent on normal wet pavement.

 

man, the rain really hammered my mpg's today. and the cold. 45 degrees and pouring. that was a fast summer!

 

i'm looking at ev miles with the plug in. i was up to about 16 with the warmer weather, and today i only got 12.

 

Yea! I Just about missed it!

 

but 90 on friday! not digging climate change.

 

Less friction and consequently more wheel slip.

 

Was it windy? Was there lots of water on the road? Lots of accelerating decelerating? how much tread remains on your tyres?

That's between 8 and 10 mpg drop,

 

As an aside, the Energy Savers are great in the rain. There was no hydroplaning or skidding on turns at all.[/quote]


Weather can really make the MPGs unpredictable. We don't have Energy Savers here. We have Energy XM2s, one of their "Green" LRR. That's the tyre I use and I'm very happy with them for the same reasons.

 

Driving in the rain a Prius, any car for that matter, becomes a pump
forceably spraying water out from under the wheels and lifting water
up off the road surface in the spaces in the tire treads before it is flung
off against the wheel well.

There is also the mater of accelerating any water that strikes and adheres
to the car up to the velocity of the vehicle.

Here are some simulator figures showing probale MPG losses resulting
from light medium, and heavy standing water on the roadway.
FWIW, In the latter case , 2,595,546 lbs (311,590 Gallons) of water is being
lifted/sprayed 4.5 feet off the highway per hour! How about Rain

Generally, all but the gentlest rain involves wing gusts. Thesse gusts result
in the airflow over a Prius being delflected from the most aerodynamic
route, moving essentially straight from the front to the back of the car.
Instead, the airflow moves at an angle and tumbles over the side of the car,
wrecking the smooth flow and adding considerable aerodynamic drag.

All this takes energy, which ultimately comes out of the gas tank, which
means reduced MPGs. It's the way things are. Theres' nothing to be done
about it.

 

Most of the time if it is raining the temp. is down 10-15 degrees, this makes the MPG down a fare bit also! So rain & temps. the double wammy!

 

Last week in rain and strong southerly winds (we live North of Melbourne) my 260 km (162 Miles) round trip to Melbourne netted 5.2L/100 Kms (45.2 US mpg). Today with a light breeze and fine sunny weather I netted 4.7 L/100 Kms (50 US mpg). My long term average is 4.8 L/100Kms