If you stick the numbers in a spreadsheet you can quickly "see" what the savings are per mile, for your daily commute, per X miles, per year, and so on. I didn't buy the 35d purely for the fuel economy, but there are some nice perks compared to the 35i:

• Savings of $200 to $230 per year due to better average economy
• Average of 33 fill-ups instead of 39 fill-ups due to better range
• Difference of only $2.25 to $3.60 more per fill-up for diesel

All figures based on the average economy from fueleconomy.gov and the low and high prices for Colorado from gasbuddy.com. I've been getting 22-23 on average, which jibes with the 22 MPG combined posted on fueleconomy.gov, and I'm assuming the 35i would get the 19 MPG combined in similar driving conditions. I used 10k to 13k miles per year based on my daily 40 mile round-trip commute plus other trips.

What's funny is that the range of the X5 35d seems limited to me. My 2003 GMC Sierra HD diesel "only" gets 16 to 20 MPG, but it has a 34 gallon tank, so the range is typically 550 to 600 or so miles. I've never driven it on a trip without towing, but it might surpass 700 miles unladen. Not that I'd rather be in it over the X5, but not having to fill up often makes long trips nice!