2009 X5 35d MPG (graph)

[XXX555]

Unless you were stop and go downhill I can't image the mpg not dropping. I left my dealer on delivery with the mpg stat at 12 from all of the idleing while showing/setting up the features...

[Thunder22]

How are you getting exact readings on fuel levels when you fill up? The mileage is easy, but if your remaining fuel calc is off, your MPG calcs are going to be off. (I guess ignoring the #'s after the decimal point is a solution)

[FunfDreisig]

Originally Posted by Thunder22

How are you getting exact readings on fuel levels when you fill up? The mileage is easy, but if your remaining fuel calc is off, your MPG calcs are going to be off. (I guess ignoring the #'s after the decimal point is a solution)

Yes I just noticed that I had let Excel label the mpg to 2 decimal places - DUH. I'll fix that.

Obviously you need an accurate measure of the miles driven and the gallons of fuel consumed. The primary source of error in hand calculated mileage is that each time you fill up you must fill the tank to precisely the same level to get very accurate mpg calculations.

Note: you don't have to completely fill the tank; but you must fill it to the same level as you did the last time. The easy way to do this is to fill your tank using the same pump, set to the same flow rate and let the pump cut off automatically. This reduces the human 'topping it up' variable.

The more gals per fill up the more accurate the mpg. A .1 gal difference on 10 gal produces a roughly .25 mpg error assuming an average of 25 mpg. But at nearly a full tank (e.g. 20 gals), the same .1 gal error only produces a .125 mpg error. BTW .1 gal is the roughly the volume of can of Coke or a single click of most diesel pump handles. So I don't get too fussed about a few tenths of mpg on a 'per tankful calculation.

But the real beauty of the fuel log mpg calc. and graph, is not in the mpg diamonds; its in the mpglong line. The lifetime mpg actually gets more accurate with each fill up. Because over longer distances, the 'fill up' errors reduce to the difference between only the very first and very last fill up. IOW they become negligible

Funf Dreisig

[DSE]

I don't know about BMW diesel engines, but my 3.0 liter gasoline engine got progressively better mileage after several thousand if not more than 10,000 miles. It will be interesting to see how long your your trend line continues.

[FunfDreisig]

Some of you may be interested in the data points for the 35d mpg while towing. The truth is that several of the higher mpg tankfuls include some towing of building materials, etc. for the 3 bay garage I'm building (see photo). But the lowest mpg tankfuls are almost exclusively towing.

All of our towing has been with our heavy duty 5x9 utility trailer w/ electric brakes rated at 5,000 lbs. It weighs about 1,500 lbs empty. A typical tow run includes hauling it empty to the quarry (OBC reads 20-21mpg) and hauling back a couple of tons of sand, gravel, road base etc. (OBC reads 15-18 mpg). Elevation en route ranges between 800-1,100 ft every few miles. I set the CC on 55 or 60mph (which ever is the speed limit). The 35d simply keeps on pulling regardless of the steepness on the hills. A typical weigh-in at the quarry is 6,540lb and weigh-out at 10,860 (net 2.16 tons). A 'towing tankful" includes some lower milage times like leaving the engine running to keep the A/C on in 105 degree heat during loading, hooking up the trailer, etc. And some better mpg segments like to/from the fuel station. So the overall average when towing has been somewhere between 18-21 mpg.

There are two mpg tankfuls that are pure towing (between 3,000-3,500 miles on the graph). We made a 300+mile trip towing our heavy duty 5x9 utility trailer. The mileage during that trip ran a little over 20 MPG. We filled it up when the left, when we got there and when we got home. Ironically the higher of the two MPG/Tanks is the trip back home where we were going up in elevation (sea level->1,100ft) and had a cast iron tub in the trailer.

Funf Dreisig

p.s. for scale the spherical water tank in the trailer holds 550 gal and is roughly 5' in diameter.

[FunfDreisig]

Originally Posted by XXX555

Unless you were stop and go downhill I can't image the mpg not dropping....

Obviously if we had not been moving at all the mileage would have gone down. But we were moving very slowly with the engine rarely above 1,200 RPM. FWIW Based on the mileage not going down during the stop & go segment I believe the 35d gets pretty good mileage when it is running between 500-1,200 RPM. IOW before the small turbo "kicks in". Of course the 26.2 mpg had a history of over 60 miles and the stop and go segment was only about 10 miles. So the overall mpg average is strongly weighted to the better 30 mpg during the 50 miles into town.

Funf Dreisig