MPG in Rain

It takes energy to pump rain through your tire tread and off the road or at least push it aside. That energy comes from gasoline so yes, water on the road will cause you to lose mpg. You also increase rolling resistance. That is one of the many reasons winter mpg tends to be much lower than summer mpg.

FWIW I am observing the same drop right now because of all the rain we had this week. I went from about 49mpg to 45mpg.

 

Not to mention pushing the falling water drops out of the way. (a smaller factor than the water on the road already)

JeffD

 

You should try below zero temps and snow!

 

My first big rain storm....I found out the same. I lost 3mpg over the 30 mile drive. It is quite easy to tell it will happen because you have to apply much more throttle to keep the same speed and the real time MPG meter drops down quite a bit.

Sucks, but not much you can do I suppose.

 

Yep, on the points people already stated.

The OP might be well served by running some gallonage comparisons (http://priuschat.com/forums/gen-iii...mileage-gets-worse-in-winter.html#post1411488).

 

You also displacing the water which is falling down, constantly accelerating it from 0 to 60MPH. IIRC from physics many years ago that alone will account in ~10% loss, give or take.

Per Nokian less then .5mm water on road will double rolling resistance

 

I see this effect on all of my cars during the wet season. We get plenty of wet days in the NW. Try to drive on the crown of the road so the water isn't as deep as the tracks in the road.

 

Plus, during the storm we had here in L.A. area, we had some strong headwinds which would factor in to some mpg loss.

 

In heavy rain and headwinds you can kiss good mpg goodbye. In those times I mpjust crank up the heater and give up trying to get good mpg on that tank. It's sort of nice not having to stress every once in awhile.

 

While a driver could accelerate at a slower rate and go at a slower top speed to mitigate some of the effects of this additional power drain - the negative drop in fuel efficiency is unavoidable (unless one doesn't drive in the rain). Starting with a cold engine, I've seen my 2010 Prius MPG drop as much as 20% on my 16 mile/1 hour regular commute when it rains. When there's ice and snow the drop in fuel efficiency is much worst. In the winter before last, I got stuck in a snow storm and my mpg for my normal 16 mile commute was extended to +2 hour and my fuel efficiency dropped to 22 mpg.

 

I would never notice loss in fuel economy with my other cars if I never own a Prius to this extent. All these little things add up and makes a huge difference in the end. This is also because of the digital read out on the MFD.

 

I think what catches your attention is a potential 5mpg swing from a general 50mpg car (good verses bad conditions). In a 15mpg guzzler...a change of 1.5mpg does not seem like a big deal. But both are 10% on the calculator.

 

Right you are. It's like the moon illusion, when you look to the sky at night and when the full moon is just above the city skyline it looks much bigger than it is when it's all the way above the city skyline...the distant from the moon in relation to the earth has not changed but the perception of size has because we have a point of reference.

My point of reference is that MFD in the Prius and yes driving a 50mpg car and losing 5mpg is the same as driving a 15mpg car and losing 1.5mpg but my perception tells me that I'm losing ahlawt more eventhough percentage wise it is exactly the same.

 

On a percentage basis, rain losses may be worse for a Prius. A Tank Class SUV has drive-train, engine, aerodynamic, and air conditioning losses far greater than a Prius. While the absolute losses due to rain will be equal or larger for an SUV, they may be smaller as a percentage, due to the larger denominator in the equation.

If you have $100 and lose a dollar, you are out 1%. If you have $10 and lose a dollar, you are out 10%. It's a dollar either way, but it seems bigger when you have less.

Tom

 

I recently filled up my tank & was avg. 59 mpg(computer) for first 50 miles. Well we got a good rain storm today in socal & my avg. dropped to 53 mpg in the next 20 miles driving as similar as I could all city driving.

 

"Good" is relative

Yes, I noticed a considerable loss in mpg last week when I drove 70-75mph (or more) in the rain. I wasn't concerned about mileage, but about getting somewhere on time, but I was still very suprised to see such a drop.

But keeping it all in perspective, even at 40mpg (which I've NEVER seen it drop that low), my Prius gets more than double the mileage that my truck got in the same conditions.

I won't let my stress levels go up over a day of 45mpg.

 

Well, you loose MPG, but other car as well, and maybe even worse/faster than the Prius...