tires are staggered... diameter for rear is measured at 28.68" (72.85cm) installed brand new, fronts are measured at 28.66" (72.80cm) installed brand new... as tires are wear (i am sure, unevenly), the numbers change.
1 mile = 1,609.34m = 1,609,340.00cm
circumference of rear tires = pi x diameter
3.14 x 72.85cm = 228.75cm
3.14 x 72.80cm = 228.60cm

rear 1,609,340.00/228.75 = 7,035.37 rotations
front 1,609,340.00/228.60 = 7,039.98 rotations

difference is 4.61 rotations

without even the tire wear, we somehow have fronts rotating 4.5 revolutions more than the rears... i can only assume that the transmission and the Xdrive somehow compensates for the difference...

however, for this argument, lets say that the tires worn down 1cm in diameter (5mm in radius)

3.14 x 71.85cm = 225.61cm
3.14 x 71.80cm = 225.45cm

rear 1,609,340.00/225.61 = 7,133.28 rotations
front 1,609,340.00/225.45 = 7,138.35 rotations

rear 97.91 extra rotations per mile
front 98.37 extra rotations per mile

say, 100 extra rotations per mile extra. to register a full mile, we need 70 real miles that will be registered as 71 miles. for a typical tankful of gas, which say would drive 350 miles (of course this number varies), the car will register 355 miles for 350 actually travelled...

the difference is 1%... if the diameter of the tire is reduced by 1 cm, the difference in registered travel vs real travel is about 1%

the MPG observed is 19MPG vs 16MPG, which roughly translates to about 1.2%

EDIT: I incorrectly took the 19/16=1.1875 and rounded to 1.2%. the correct number is 18.75% as JCL correctly noted below

so, theoretically, it is possible to have increased mileage due to the reduced diameter of a worn tire... <<<--- ignore this statement

now i need to check to see, how realistic is the 1 cm reduction in the diameter of a used tire vs new tire - i don't have that info...

I was with you up to here (although I think 1 cm of wear in the diameter is high).

By my reckoning, that 3 mpg reduction is 16%, not 1.2 %.

As gas prices increase, I keep checking mileage different ways until I get a number that I think justifies ownership. I inflate it if someone is asking me the mileage in case their intention this is to say the X5 is further proof I am a dipstick.

Anything I do to improve the mileage will go on the bad side of the feel good fun factor meter so it is as likely to happen as me buying a Prius. My guess is there are as many X5 drivers that would drive the way it takes to get the best mileage for more than a day or two as there are leadfoot Prius pilots pushing to keep pace. I am not going to be either.

I like 19mpg. No one would believe 20, 19 is a stretch but I'm not challenging Term--straight up guy but his latest post makes my head hurt---that could happen--time to recalculate to see if I can find a way to get there.

typing and calculating on cell phone is not one of my forte...

Back to the original thread :D

I get 400 mi per tank city and hwy mix drive!

I try not to let my tank run that low for a couple of reasons:

1) fuel cools the pump, so keep it submerged as much as possible,

b) I never want to be low on fuel when the zombie attack. Gotta get away!

my bad - i was looking at wrong spot after decimal... 19/16=1.1875... the difference in fuel consumption is 18.75%

in this case, a single percentage point in the tire wear probably can't easily transform into 19% MPG reading...

and yes, before i saw routinely 350 to 380 miles per tank before i would fill up, now i see routinely 430 to 455 miles before i have to put the same amount of gas...

and i agree, it is better to keep the tank always with some gas in it, and not run it dry - i learned my lesson in Panama, when going from costa Rican border, through David to Panama City without any gas stations in sight... running on fumes!!! lol...
now, since i am back to civilization, i just feel lazy and slipped back to the old ways... :nanana:

I'm only getting about 12-13mpg city on my 3.0 but on the highway I get about 25mpg+ when I drive out to my friends house in jersey, I usually do 130-140 miles round trip and use about a quarter tank

here are some additional numbers from the car's internals -

4282 wheel sensor impulses to register 1 kilometer in odometer
4229 wheel sensor impulses to register 1 kilometer in speedometer

these are the vallues stored in the E53-HP, which is the iS model with 20" standard staggered wheels... these numbers suggest that the rolling values of the mounted tires are different from the nominal values that I got from the tire calculators - 233.53cm as per the 4282 impulses in odometer ("correct" value as opposed to optimistic 4229 impulses for the speedometer value)

these numbers do not affect the calculations of the MPG much, as we used our numbers to calculate the difference in diameter and those numbers were a percentage points of a ratio...

nevertheless, now i want to go actually measure the tire diameters on my car...

How much does rolling friction change over the tire wear life impact the decrease in circumference?