Rain mpg drop!
I just had a reality check yesterday. Mpg dropped from measured 57/58 in summer weather (max speed 65 mph) to indicated 46 mpg (down from indicated 61 mpg) on a 90 mile trip just after a fill up. Moderate to very heavy rain, and the temperatures dropped into the low 60s. Dramatic 25% drop! A surprise, but it shouldn't have been - normal.
So it is well known that wet roads do increase rolling resistance a lot. And I've learned that the optimal outside temperature is 85 degrees. Higher or lower decreases mpg, and I've been able to notice the difference between 70 and 80 degrees myself. And of course winter drops are large in part because of different gas formulations, but more about temperature.
I'm resetting my four season average mpg. I had been hoping to achieve 52 to 54, but now I'm thinking 50 to 52. It all depends on how many miles I put on my car in the summer versus winter. I use my motorcycle more than a car in the summertime because it gets 78 mpg so that skews my car's annual average. I bought the car end of January 2018 and it averaged about 46 mpg in the coldest weather. Summer is pretty consistent (I haven't driven in rain before yesterday) at high 50s. My current measured total mpg after 9,000 miles is 52.01.
I may go on a trip this week to California. That will be interesting. Prevailing winds will (on average) drop my mpg going west, and increase mpg on the way home. As I will be going 100% on interstates, I will also lose out on the mpg bump from street driving on shorter trips around town.
2018 Kia LX HEV Metal Stream with Advanced Tech