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I was looking through some of the archives and didn't see anything specific to driving in rain or with the road reasonably dry right after a rain, except one post which really didn't show the same thing as I am experiencing. I have been experiencing about a 4-5 mpg drop running approx 55mph during these conditions. Today in moderate to light rain on my commute, I got 64 mpg. Usually I get 69-70 mpg trying to run 55. A few days ago it had rained pretty hard during the day, but on my way home that night at about 8PM, it wasn't particularly cold, about 58 degrees and the road surface was not wet enough to kick up any water from the tires, I couldn't get over 65mpg and the lean burn was difficult to get to go over 75. The commute is pretty hilly, and other than the humidity, I don't see much change. Yesterday in the dry, I averaged 70mpg.
Does anyone else see this kind of change?
Thanks,
Robert
 

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I have not done thorough testing of this, but I think that I notice such.

Here in N CA it tends to rain when it is chilly, so that it makes sense that the cold wet air causes it to be even harder than normal for the car to warm up - even with mods.

It may be a warm/cold phenomenon more than humidity (which makes the air carry more heat away) changing combustion etc. Maybe others have some information on how humidity changes combustion in the Insight.

How do people feel who drive where it rains when it is hot?

Where it is super humid and hot, but not raining? Midwest summers vs. Arizona summers?
 

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Robert,
There is a thread about best weather for a insight. The cold is a killer as well as rain however my experience is that high humidity is good. For example on my trip to DC on the 20th (rainy) I had a average in the low 80's through SC. Once I got North of Charlotte the rain stopped but it was overcast and the road was still wet. I average 103 mpg between Charlotte and Richmond (high humidity but no rain). Once I got to Richmond the rain became very heavy and I averaged in the 70's for the rest of the trip. Last year when the huricane came up the coast and Richmond had some flooding I was driving through and had to downshift to 3rd to plow through the water (avg in the 40's). They actually closed the interstate at a few points.
As another point of reference my best tanks seam to come in Aug when it is hot and humid. High humidity levels also lower the air density which could help reduce air resistance although its probably not that significant. With the Snow in DC today I decided to drive the Civic back as I still have my hot air intake that doubles as a snow plow. Have fun, Rick
 

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I notice a subjective drop in mpg during hot rainy humid weather. Some might be due to the running of A/C which one tends to do due to the heat and windows fogging. I seem to recall from my flying days that hot rainy humid weather also makes your airplane perform like a dog.
 

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Well, think about it :) Assume that your average raindrop is falling straight down, thus having zero horizontal velocity. Now along you come and whack the poor little thing with your Insight, accelerating it to 65 mph or so. That takes energy, which has to come from your gas tank.

Repeat for each of the many thousands of drops you hit on an average trip, and you can see why any car gets lower mpg in the rain :)
 

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High temperatures and high humidity should improve fuel efficiency for the Insight.
The higher the temperature and humidity, the lower the partial pressure of oxygen, which will cause the engine computer to reduce the fuel input. There's less power available, but it's typically not required.

The lower density air also reduces the aerodynamic drag, further improving fuel efficiency.

My best mpg numbers have consistently been on hot, humid days despite open windows.

'May be wrong about this, but I believe some of the major reasons that rain negatively impacts fuel efficiency are the additional energy associated with the viscosity and surface tension of the water on the road that the tires must push through.

'Ever notice how cars slow down when you enter a deep puddle on the road?
 

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In the early nineties General Motors did a tremendous amount of research on their electric vehicle prototype, the Impact (which later became the EV-1). They tested the car for range under a variety of conditions, including cold temperatures. The battery packs were kept at uniform temperatures for the experiments, so they were comparing apples to apples.

The greatest mileage losses due to cold temperatures came from the increased air density. The colder the temperature, the greater the density, and therefore the greatest decrease in vehicle range. Keep in mind that the mileage loss was with an electric vehicle and therefore was not at all dependent on air fuel ratios and such.

The second greatest mileage losses due to cold temperatures came from the low rolling resistance tires. General Motors was using tires developed especially for the Impact by Goodyear. The tires were truly exceptional for lowering rolling resistance until they reached a temperature of 32 degrees F. At that point they became worse than a traditional tire. Later on, GM began using Michelin low rolling resistance tires on the production cars, and I am not sure if the Michelins were affected by freezing temperatures or not, but I presume not. Incidentally, tire pressures were adjusted in the above tests to keep a consistant pressure in the tires for the various test temperatures. It would be interesting to know how much, if any, the LRR tires on the Insight are affected by freezing temperatures.

All that being said, differences in air density really affect the mileage of our Insights. The hotter the temperature, the less dense the air, and we get greater mileage. Higher altitudes have a similar effect; the higher the altitude, the less dense the air and we get better mileage.
 
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