MPG at various fuel levels

[bnzpwr]

Because i dont commute tooo far every day, i would normally pump about $20 at a time when it would get down to 1/4 tank. This would get me about 100 miles. The other day I was getting to total empty and i found diesel for the bargain basement price of 4.45/gal. Saved about .50/gal since its about 4.99- 5.10 in the LA area.

Anyway I pumped up to the 3/4 tank mark. I reset my trip and today finally the needle went down to the 1/2 mark. I looked at my trip and it read an astonishing 150 miles. If this holds true, that means i should be getting aproxx 600 miles/tank.

It seems like the gas mileage was better going from 3/4 a tank to 1/2 than it was going from 1/2 to 1/4. Is this normal?? I have been told its better to keep fuel tank full but for the sake of less weight/ less pain at the pump i dont normally fill up that far.

Is it normal for mpg to go down with less fuel in the tank?

[rcounts]

I think that it is more likely that your gauge isn't reading linearly.

In other words the first "half" of the tank (according to the gauge) is 10 or 11 gallons and the second "half (again, according to the gauge) is 7 or 8 gallons.

BTW, how can you figure that when you've driven 150 miles and used a half a tank you're going to get 600 miles out of that tank? If the other half is the same as the first you'll only get another 150 miles and 150 + 150 = 300 (not 600)

Unless you meant that you drove 150 miles and only used 1/4 tank...

BTW, mine seems to get more miles out of that first 1/4 tank than any of the other three quarters. I seem to get around 140 miles before it gets down to the 3/4 mark - but only around 250 to get down to the 1/2 mark, and 340 to get down to 1/4 tank and by the time I get to around 420 she's gettin' down close to the R (reserve) mark..

[bnzpwr]

My initial post read that i was able to get 150 mils going from 3/4 tank full to 1/2 tank full. I was thinking it may not be reading correctly. Most cars have a straight forward tank reading gauge. Meaning, if the entire guage spans say- 1 inch, it is marked at each 1/4" mark for each quarter tank.

The 300SD is not mark evenly. So, even though it is not marked evenly, is each mark actually supposed to represent 1/4 tank levels?? I dont see why on earth they would have done it like this.

[Dana B.]

The guage for each tank of every car I've owned does not drop in a linear, or even, manner. In my MB, I can get 350 miles at the 1/2, but that doesn't mean I'm going to get 700 miles to the tank.

I run my tanks down to the reserve on the theory that the less fuel is in the tank, the lighter the car will be for better ultimate economy.

Don't forget that the filler neck holds almost a 1/2 a gallon and there is several inches above and below the level sender's measurement range.

[bnzpwr]

i have always thought that 1/4 is 1/4 there even if its the first 1/4 or the last in the tank. Now that my guage is exactly on 1/2 Im going to try to pump say- 5 gallons, and see how far it gets me. Then ill pump 5 gallons when i have 1/4 tank and see how far that gets me. I will be very surprised if the two numbers are significantly different. According to yall, there should be no difference and none of you have experienced any difference either correct? i was thinking fuel pressure might have something to do with it??

[JimFreeh]

Ever noticed how most non Mercedes cars will divide the gas, err, fuel gage into equal quadrants?

Notice how MB uses a nonlinear scale.

I've always figured that the tic marks were much more relevant to the actual content on Benz.

They follow pretty close on my 83 300D, around 110 - 120 miles per quarter.

Jim

[WD8CDH]

The MB is closer to linear than many other cars but it still is not linear. Fuel pressure from a more full tank will not change fuel consumption but lighter vehicle weight from a less full tank will.

By the way, since there is some drag in the sensor, the gauge will read differently going down as it will going up.