On my first trip of any length (360 miles, vast majority highway) in the 98, I was curious to see what my fuel economy would be. It was a somewhat disappointing 26 mpg. I know winter diesel has an adverse effect. But, since the trip was in almost constant rain, it got me curious as to the effects of rain on fuel economy....increased rolling resistance and other factors. As you might expect, there's no shortage of discussion on the matter in various car forums.

Anyone come to any conclusions on the matter? It would be an added bonus if anyone with a physics/engineering background could chime in.

__________________
14 E250 Bluetec "Sinclair", Palladium Silver on Black, 153k miles
06 E320 CDI "Rutherford", Black on Tan, 171k mi, Stage 1 tune, tuned TCU
91 300D "Otis", Smoke Silver, 142k mi, wastegate conversion

19 Honda CR-V EX 61k mi
Fourteen other MB's owned and sold
1961 Very Tolerant Wife

Yes rain effects energy required to move the car. It depends on how much rain is falling, the speed of the car, and the rain depth on the road. All those collisions with rain drops on the front of the car can add up. Try walking into a water stream from a garden hose or a fire hose. The rolling resistance increases when there is water on the road---more water depth increases the resistance. Try riding a bicycle through a street with two inches of water on the street.

Cold air also increases the resistance to moving the car through the air and requires more fuel. When the air is colder the molecules are closer together and the car is colliding with more air molecules as the car moves through the air.

I've never been able to notice much difference in fuel economy from rain. Wind can make a big difference though and rain is often accompanied by wind.

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

In our Passat we've noticed rain knocks off a MPG or two.

-J

__________________
1991 350SDL. 230,000 miles (new motor @ 150,000). Blown head gasket

Tesla Model 3. 205,000 miles. Been to 48 states!
Past: A fleet of VW TDIs.... including a V10,a Dieselgate Passat, and 2 ECOdiesels.
2014 Cadillac ELR
2013 Fiat 500E.

It takes energy to push the water molecules out from beneath the tires. The more water on the pavement, the more energy required. A very light wet pavement isn't going to reduce fuel economy but an average rain (.25" per hour) is certainly going to reduce it.

Rain effectively increases the mass of the fluid you are traveling through.

__________________
CENSORED due to not family friendly words

Gee, if this holds true then with all the rain wet get in the Pacific NW we should see negative MPG ratings for our cars................Honestly, I have NEVER experienced any noticeable MPG hit riving in rain, and it rains over 200 + days a year around here on average.

__________________
2012 Mercedes ML350 Bluetec 91K (hers)
2005 Corvette 55K (fun car)
2002 VW Jetta TDI 231K (mine)
1998 Volvo S70 T5 Turbo 196K (kids)
1994 Ford F150 4WD 249K (firewood hauler)
1983 Mercedes 300D 376K (diesel commuter)

Around any holiday gas goes up by twenty cents per gallon. That is because more travelers/gallons purchased. Well this will be a new excuse for the filling stations to raise prices by twenty cents per gallon every time it rains. At least diesel has not changed by more than forty cents per gallon in the last five years.

This makes sense. Certinly if you drive thourgh let us say 5 inches of Water you know it slows you down.

To a lesser extent lesser amounts of Water on the Road must do the same thing.

Then it is pretty clear that a Wet Road and Wet Tires do not have the same Traction;any slipping in the direction you want to travel is a waste of Energy.
Driving slower reduces the chance of loss of traction and that may help decrease the loss of mpg.

__________________
84 300D, 82 Volvo 244Gl Diesel

Therre are two components of drag associated with driving in rain. The greatest is "wetting" drag, associated with the depth of the water on the road and the area of the car contacting that water (tires). The second component is "spray" drag, associated with the rain drops impinging on the frontal area of the vehicle.

The spray drag may be expressed as:
D(spray)= pVQw

where: p = density of water (a constant)
V = velocity (vehicle speed)
Qw = volume flux encountered in cubic Ft per second

Qw is a function of several parts; the frontal area of the vehicle, the size of the droplets (fine mist to large drops) and the rate of downpour. A heavy rain is commonly defined as a tropical downpour at 10mm/hr.

Not being willing to measure frontal area of my car I am assuming it approaches that of a barn door (W123). Assuming also large drops at a tropical downpour rate as a worst case, the spray drag is much lower than the wetting drag. It is therefore a very small contributor to overall drag thus has a very small effect on fuel use at all but land speed record velocities which my 300CD seldom approaches.

Thats my story and I'm sticking to it.

Would cooler air temperatures mean denser air; thus more entering the combustion chamber and providing more fuel economy to overcome the drag of the rain water?

__________________
1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
Monark Nozzle Install Video - http://tinyurl.com/ptd2tge

a Diesel operates on a surplus of air anyway... so colder air will only make the slightest of changes in economy by cooling the turbocharged air slightly...

__________________
John HAUL AWAY, OR CRUSHED CARS!!! HELP ME keep the cars out of the crusher! A/C Thread
"as I ride with my a/c on... I have fond memories of sweaty oily saturdays and spewing R12 into the air. THANKS for all you do!

My drivers:
1987 190D 2.5Turbo
1987 190D 2.5Turbo
1987 190D 2.5-5SPEED!!!

1987 300TD
1987 300TD
1994GMC 2500 6.5Turbo truck... I had to put the ladder somewhere!

I always get better mpgs in the rain,gas or diesel.I think the damp air effects the motor like water injection.The wet pavement acts to free up friction.Of course I drive slow and only drive 4 mph over posted speed limits.I swear interstate traffic moves at 87 mph.Oh yeah winter diesel fuel sucks for mileage

__________________
1999 w140, quit voting to old, and to old to fight, a god damned veteran

I guess I would have to take my 240D on a road trip down South to test this...I've only driven it in Washington, rain capitol of the universe.

If this is true, though, it makes me happy to know that I'd get awesome mileage once I'm out of here.

__________________
- 1979 W123 240D 4-speed, 390k miles

What about resistance vs the change in driving style? I'd think that your mileage would actually go up since people tend to drive a bit more conservatively in the rain. I know with my truck I have to be extremely careful in third gear and below or it wants to just spin.