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There's been a lot of discussion on how driving too fast lowers gas mileage, and everyone knows a cold engine is inefficient. I'm discovering another parameter than lowers gas mileage dramatically.

Rain.

The other day I was driving along my regular route home. The engine was getting warm and I happily watched the mpg creeping up. A cloud burst hit and to my dismay my mileage started to drop. I'm at the point where I drive about 5 mph slower than the speed limit normally, but when the rain fell I dropped another 5 mph.

Why should rain be such a problem? I think the biggest factor is rain actually increases resistance to the tires. Think about how when you hit a water puddle how your car slows. If there's enough water on the road (and there was) you never entirely get out of the puddle. I don't know how much resistance hitting falling rain drops is going to make, but cold, wet air is denser, too.

Looking at tomorrow's forecast I see rain predicted again and out through the weekend. Groan. I'll take cold, dry weather over heavy rain any day.

There are two main things going on to increase your rolling resistance on a wet road. Your tires are displacing (moving) a mass of water aside as the treads hit the road. Then when the treads lift off the road, the water film keeps the air from efficiently flowing into the space between the lifting tread and the road, creating a partial vacuum until the water film breaks. This creates that “zinging” sound that tires make on a wet road. Whenever you hear that wet road tire noise you are losing mileage.

Best thing that can be done is to slow down and try to stay out of any ruts or grooves in your lane where water pools... if the road is flat, try to stay in the tracks of the person in front of you as they may have less water in them (this may depend on how heavy the rain is and how far you are behind them, and you should be very far behind them at this point...)

In heavy rain, MPG pretty much goes out the window, and staying on the road and driving safely prevails... pulling off and waiting it out with the engine off (if you can get off the main roadway) is an option, but that isn't always feasible.

Try shifting over to the right part of the lane - still within the lane but out of the ruts. You can reduce the amount of water your tires hit that way.

Also, as you did, slow down even more.

Then, just grit your teeth and deal with it. :(