mileage (MPG) drop

[John in CT]

Last weekend my 1995 525im traveled 200 miles North to NH and returned 28-29 mpg by the trip computer. After being parked for the weekend the retuurn trip involved three hours of steady driving rain. A few hours into the trip I was playing with the MPG function when I noticed it was low, like 22-23 at best highway conditions remained.

The O2 sensor is pretty new, plugs maybe 15K miles. I'm wondering if the air filter element could have gotten soaked in the driving rain, any other thoughts?

Thanks in advance,

John in CT
1995 525im w/ 91K miles

[Torque]

There's too many factors ... Steady driving at 65 will return different gas milage than steady driving at 60 etc ...

Or maybe you were looking at Avg1 instead of Avg2 or vice versa.

[632 Regal]

wind direction, altitude change IE climbing up rather than down, different fuel etc etc.

[John in CT]

Nope, I zero'ed both averages using s/r button, speed has surprising little effect I've noticed in the past, but I have always been able to hit 28-29 mpg at steady 75-80 mph. I'll investigate more fully tommorrow,

Thanks, John

[John in CT]

nope, home is generally downhill from New Hampshire, tank was not filled except at beginning of trip. Temps were a little cooler, plus the driving rain....

[632 Regal]

OK sorry I thought this was actual not OBC. The humidity and temp could have changed the readings it got

idonno trying to guess here.

[John in CT]

You know, the "low coolant" flashed for a few seconds when i moved the car before coming home, it never relit. Maybe there's something there......now i'm nervous....

Thanks,

John

[uscharalph]

You know, the "low coolant" flashed for a few seconds when i moved the car before coming home, it never relit. Maybe there's something there......now i'm nervous....

Thanks,

John
What year / model is your BMW? You should fill out your information.

[John in CT]

1995 525i 5 speed as stated in first post...

[uscharalph]

1995 525i 5 speed as stated in first post...
Sorry, my bad.

[SRR2]

Sure. It's the rain. A lot of power goes into displacing a gallon or two of water off the road every second and throwing hundreds of gallons of it up into the air.

How much water is displaced... let's say that there's .05" film of water on the road that is completely displaced by the tire tread that contacts the road. Assume that the actual contact patch is 60% of the tire width of 8" = 4.8" At 60 mph you're traveling 88' /sec = 1056"/sec So, you sweep 4.8 x 1056 x .05 x 2 = 506 cubic inches/sec off the road. That's 2.2 gallons of water accelerated from rest to approximately the speed of the car (granted, this is a crude approximation) every second. A gallon of water weighs 8.6 lbs. so the weight you accelerate from rest to 60 mph is ~19 lbs. 19 lbs to 88 ft./sec is 1672 ft-lbs/sec. That's 3 horsepower! Now, it takes only 8-12 hp, give or take, to move the car down the road normally, so this extra 3hp represents 25%-35% increased load on the engine, even more if you factor in drive line inefficiencies. So, it's pretty reasonable to assume that you will lose at least 25% of your ordinary mileage. I'd say that your fuel usage was about what you'd expect it to be.

[mzarifkar]

wow! you are not a physics teacher by any chance are you? Never would have thought there was such a loss involved driving trough the rain.

[SRR2]

Heh heh, no, -- EE, actually -- but I've been called Mr. Physics more than once.

[Bill R.]

through a driving rain... I imagine that the low cd of the body on the e34 still has much more resistance passing through a wall of water at freeway speeds.




Heh heh, no, -- EE, actually -- but I've been called Mr. Physics more than once.

[DanH]

I would take some actual measurements of the mpg by using the (driven mileage / fillup amount) formula just to prove your obc is still accurate. Mine read about 1 mpg low last time i checked.

[SRR2]

I know, and you'd also have to account somehow for the water thrown up and evaporated but the dominant effect is most likely displacement. I was going for an order-of-magnitude approximation only, simply to demonstrate that water, even if it's just laying on the road, has a serious negative impact on fuel usage. Besides, the effects of displacing water are relatively easy to estimate, the others aren't. The funny thing is that the estimate was so close to the observed data.

[uscharalph]

through a driving rain... I imagine that the low cd of the body on the e34 still has much more resistance passing through a wall of water at freeway speeds.
That's exactly what I was thinking!

[John in CT]

Well, thanks for all the thoughtful advice, just wanted you to know it was temperature related, today is warm (the rainy day was cold-ish)and on the highway I have the 28-29 mpg displayed which had given me the warm fuzzy feeling before.

Now, to check display mileage verses actual as was one of the suggestions,

Thanks again,

John
1995 525im