Rain...

More likely, the way there was slightly downhill, accounting for the better mileage, but that also means that the way back is then uphill, accounting for poorer mileage.. My experience driving in the rain is that unless it's a downpour with accompanying high winds, it doesn't make a huge difference to mileage, certainly not 10mpg worth.. Alternatively, if you had the A/C on to defog the windows while driving back, that could account for the additional fuel usage- I find that the Prius is quite bad for fogging up the windows in damp conditions.. I absolutely have to run the A/C to keep the windshield clear, whereas on my old Corolla, opening the outside vent while the ducts were set to defog was generally enough, and if not, opening a window a crack would allow enough outside air in to keep the windows clear.. Not so with the Prius..

My vote is for the road though- even if the road looks flat, there are always some slight inclines and the car is quite sensitive to these.. I drive about 30 km to work every day and the way there, my mileage is always about 2-4 mpg worse than coming back home, even though I'm just travelling the relatively flat terrain of the city..

 

I'd concur, odds are it's not the rain. For the first month or so i owned my prius, I mostly drove it in the rain (walked or rode my bike when it was nice out). Since then, i've started driving it more (moved, don't think i could do the commute on bike and live to tell about it :-p), and haven't noticed much, if any, increase in mileage.

 

The Air Resistance will be greater, but not a huge factor, however Water on the pavement can have a huge impact on Rolling Resistance. The car has to mechanically displace the water. If it's actually kicking up spay, that's a lot of energy.

As my winter mileage can attest, the only thing worse is Slush

 

Had my Prius for two years. Rain makes a huge difference in my mileage. 5-10mpg. It's the resistance. Does the same with other cars, you just can't see it on the screen.

 

<div class='quotetop'>QUOTE(molgrips @ Jul 31 2006, 08:19 AM) [snapback]295025[/snapback]</div>

I'm not sure about other states but here in CA we have slots cut into the concrete freeways to specifically reduce hydroplaning, which I'm assuming will reduce spray as well. And CalTrans is constantly testing to increase drainage, traction, wear, etc.

 

<div class='quotetop'>QUOTE(Dion @ Jul 31 2006, 07:51 AM) [snapback]295080[/snapback]</div>

I've gotten terrible mileage in very heavy rain, like 38-39mpg on the freeway where I'd normally get 45mpg or higher. The drop was so large I initially thought something might be wrong with the car. I thought that higher rolling resistance could be a factor, but did not think it could cause that much of a drop. I didn't have any other explanation though, so it's nice to hear that others have had similar experiences.

 

I've witnessed the higher rolling and 'wind' resistance in rain most vividly when coasting. I'll often coast our Elantra down a long 2 mile hill in neutral and then keep coasting nearly another mile to a stop light. I've seen differences with tire pressures and windows down, but the greatest friction comes from rain, even a drizzle with wet roads. When it's coming down hard, it's not worth trying to coast.

I've found the resistance from wet roads is greatest with worn or less highly inflated tires.

 

Another consideration is the humidy during a rainstorm. Water vapor replaces air in the intake and performance drops off so you push the pedal further when the ICE is running to achieve the same performance..