Prices for the X5 start at $47,500 for the xDrive30i; the diesel costs $51,200 and the lineup peaks at the $56,200 xDrive48i. Standard features include faux leather upholstery, power front seats, a panoramic moonroof and a 12-speaker CD stereo. Typical of BMW, there's a smorgasbord of options: heated and ventilated seats, genuine leather, a navigation system, the third-row seat, 16-speaker audio and much more. Check all the boxes, and a fully loaded model tops out above $80,000.

Under the IRS' Advanced Lean Burn Technology Vehicles program, the X5 diesel is eligible for an $1,800 federal tax credit, double what the ML320 Bluetec qualifies for (Q7 TDI credits aren't yet available). Remember that tax credits go straight to your bottom line, as opposed to tax deductions, which simply lower your taxable income. The credit shaves the xDrive35d's premium over the xDrive30i to just $1,900 — a reasonable cost for the diesel's extra oomph.

The other cost to consider is fuel. Due mostly to supply and demand, diesel fuel costs more than gasoline: As of this writing, AAA's national average cost for diesel fuel was $2.21 per gallon, versus $2.13 for premium unleaded. Because of diesel's increased efficiency, however, the xDrive35d still saves about 15 percent in annual fuel costs versus the xDrive30i and 25 percent versus the xDrive48i.