Poor x5 diesel mileage.
 

Picked up an 2009 x35d.
Used out of warranty.

Runs great. Sounds great. But I'm finding people on this forum getting 25 - 30mpg.
Im getting around 17-19 within the city.

I don't have any mods.
Any thoughts what i can change?
Oil/air filter are up to date (well according to the on board computer). Maybe I'll change that and report back.

I just turned 5k miles on my 2013 and I get 16 or less in city. No mods. Just did a big weekend trip with lots of highway and averaged 24.5. On one leg of the trip I did manage 28 mpg.
I was very impressed with the 21.5 I averaged in the mountains tho. I have both sport packages with the big wheels, which no doubt impacts the city mileage. Just my two cents.

Fuel filter.

And driving habits. Gentle acceleration, and plan ahead to reduce on/off the throttle.

We are at an average of 20.8 in mixed driving since January. On the highway- straight 70-75 MPH we are at +-27. I think yours is about right.

I'm at 21.7 average with 9k miles on the clock.

I'd go ahead and change out the air filter, installing a K&N type so you can clean it as needed to save some money. Remember that your engine is pulling a much larger volume of air through it than a gasoline engine does.

Second, run a can or two of Berryman "Diesel Pro" additive through your next two tanks to make sure your injectors are clean. The stuff works wonders. I managed to get 264,000 miles out of a 1981 Pontiac Bonneville diesel - that's right, the infamous Oldsmobile 5.7 liter diesel! But I always kept the fuel treated with Diesel Pro.

2002 X5 3.0 238,000 miles
2004 325i 106,000 miles

In thinking about this more, I would pay attention to how you're driving, it takes a bit of throttle to get the X5d moving as opposed to a gas engine, so maybe you're rabbit starting or staying on the throttle longer?

Have long have you had the X? Have you reset the trip computer info? and have you had to refill the DEF yet? I'm asking about the def because if you're burning through DEF quickly, it's an indication of keeping your foot into it ;) which will lower the MPG.

Fuel mileage on forums is like golfers saying they are shooting in the 80's- after you ask them you discover they sort of play by the rules. The numbers I posted above are real from the display in the dash. You have a couple of others posting similar numbers. Use that as your baseline not the pie in the sky numbers that seem too good to be true.

I would be wary of additional additives- these systems are nothing like a GM 5.7 diesel from the early 80's.

I get fairly consistently 21 in town and I'm not light on the accelerator either. I've found that at freeway speeds I get better mileage at 80mph than say 70 or 72. I get 27 to 28 mpg at 80 and about 24 to 25 at 70-72. I have driven a number of times back home from Portland and also made some jaunts to Seattle and it always does this consistently. I have 20" tires too. My two cents. :)

I find that mileage is better on the highway if you get up to 85 or so and then try to maintain speeds rather than trundle at 70. I think my traffic in the city is much worse than most. Literally stop and go for an hour over 7 miles. When I drove in the mountains at an average 30 mph I got 21.5.