Fuel Economy - PeachParts Mercedes-Benz Forum

VegiBenz · #1 11-17-2005, 11:01 AM

Hi again. Recently I thought my car was using more fuel than normal but since I was blending vegetable oil and not filling completely, I was not able to check the mileage. As a test, the last 3 tankfulls of straight diesel were only 21.5 MPG. When I first got the car, I got consistant 27-28 MPG on diesel. I did not change my driving habits and nothing else was changed on the car. It runs good and doesn't smoke unless I start cold or really give it a lot of throttle. Its a 1984 300D with 235K. I keep the maintenance up and the tire pressure proper too. Anyone have any thoughts as to the cause of such a drop in mileage? Thanks, Eric.

boneheaddoctor · #2 11-17-2005, 11:03 AM

Winter blend fuel will do it......

New2MB · #3 11-17-2005, 11:12 AM

My mpg drops noticeably for all of my vehicles in the winter, all that extra friction in the fluids I guess, not to mention all the tire spinnin' miles that didn't actually propel the vehicle forward.

Uriah · #4 11-17-2005, 12:51 PM

Mine drops slightly in the winter (less then 1mpg, normally), I can't imagine a 8mpg drop, I'd go nuts.

dlssmith · #5 11-17-2005, 08:17 PM

Mine burns a lot more fuel in cold weather. The winter blend is part of the reason, since it doesn't have the "energy" that summer blend fuel has - as I understand it. But another thing is the cold starts. I think they take a lot of fuel too. Much is unburned since the warm up period is so much longer in the cold north. Man, it was 4 F last night...

J. R. B. · #6 11-17-2005, 08:24 PM

My mileage drops in the winter too with a mix of #1 but I try to counter it with Howe's fuel additive and it adds lubricity to the fuel.

ZackaryMac · #7 11-17-2005, 09:14 PM

#1 fuel has a lower centane rating and less energy in it.
During the warm weather, you probably start and go, vs cold weather start, warm-up to see out/warm the oil/whatever, then go. Any fuel used other than to move the car will lower your MPG figures.

Just as another thought, did you check to see if your brakes are dragging any? They don't have to be dragging hard to lower your MPG, yet not be very noticable while driving.

sailor15015 · #8 11-17-2005, 11:26 PM

#1 could cause a drop in mileage because the stuff that gells your lines in the winter is the same stuff that gives you more power in the summer blends. I'm no expert but it seems highly unlikely that it could cause that significant of a drop. Have you tried plugging your car in at night to help with cold starts? If you have a block heater, that is.

Ralph69220d · #9 11-18-2005, 12:48 PM

Another reason for drop in mileage in cold weather is that the air is denser that the car has to push through.

angst · #10 11-19-2005, 08:11 AM

I have an 84 sd also. I have had it for just over a year. When I first got it I was plagued by mileage wowes.
I went and looked at my log of maintenance and mileage. All the winter mileages I measured were like. 20.3, 21.3, typical November to Feburary. (Tennessee temps.)
Mileage went up 1.5 - 2.5 after a careful valve adj. in late March and measurements starting in May were consistantly between 24 to 26mpg.
Hard to say what effect warmth/fuel had on mileage increase but adj. the valves had a real dramatic effect on my cold starting ability.

ralph wrote

Quote:

Another reason for drop in mileage in cold weather is that the air is denser that the car has to push through.

Thats interesting. I had never considered that. Everyone always talks about the denser cold air in your intake but Ive never considered this being offset by the car moving through denser air.

bullwinkle · #11 11-19-2005, 08:20 AM

I know its an old dead horse, but I'm going to BEAT IT AGAIN!!!-check your tire pressures- they've probably dropped 5 or 6 psi due to the cold. My 300D personally loves nitrogen!!!

Ralph69220d · #12 11-19-2005, 08:43 AM

Quote:

Originally Posted by angst

I have an 84 sd also. I have had it for just over a year. When I first got it I was plagued by mileage wowes.
I went and looked at my log of maintenance and mileage. All the winter mileages I measured were like. 20.3, 21.3, typical November to Feburary. (Tennessee temps.)
Mileage went up 1.5 - 2.5 after a careful valve adj. in late March and measurements starting in May were consistantly between 24 to 26mpg.
Hard to say what effect warmth/fuel had on mileage increase but adj. the valves had a real dramatic effect on my cold starting ability.

ralph wrote
Thats interesting. I had never considered that. Everyone always talks about the denser cold air in your intake but Ive never considered this being offset by the car moving through denser air.

I have to say I don't know if the denser air for combustion outweighs the friction coefficient or not. Another thought came to mind. This applie's to any tire other than a totally inflelxible "tire", such as a steel wheel. When the car is moving with air inflated tires it is essentially always going up hill as far as the tire is concerned as it is continually flexing the tire in those several degrees before the tire meets the pavement. With colder outside temperature's the tire is less flexible, hence the "uphill" phenomena worsens and mpg drops, though I have no idea what percent drop in mpg this would cause. Ive always been curious to know wheel bearing grease temperatures as a function of outside temps. I know there's a forum member here who studie's metallurgy and interactions with lubricants. Hope he see's this post and can address these things better than I. The brief history of the development of synthetic lubricants is: in the war we had in vietnam the Navy was having trouble's with their fighter aircraft having early wheel bearing failure. I am told, but don't know, that the fighter aircraft, for a number of good reasons I won't go into here, used extremely rapid descents from 50,000 foot plus altitudes on to the carrier's. The wheel bearing grease never had time to thaw, hence early failed wheel bearings (taking into account the abuse that carrier landigs put on wheel bearings). They asked Mobil Oil (and maybe other's) to come up with a grease to combat this. Mobil developed a synthetic bearing lubricant, and here we are today.

rg2098 · #13 11-19-2005, 09:14 AM

I've got a 3 mpg drop, which is annoying going from 23-24 down to 19-20. My tires are definetly looking lower and I have regular dino oil in it (change to synethic in about 500 miles / about a week)

VegiBenz · #14 11-19-2005, 10:02 AM

Thank you for all the replies. It seems to make sense about the #1 diesel and tires. I am sure the tires are up but I will check them as its oil change time anyway. I am going to do a veggie oil conversion soon and then I won't have to worry about mileage but I want to know the car is OK before I do. Otherwise it runs great and is a joy to drive. Eric.