why does e70 get better mpgs than e53
 

can your x5 experts explain to me,

why the new e70 gets nearly 18-20 mpgs, which is a huge improvement over the e53 which usually got around 15-16 mpg...

can anyone shed what is so different between the two models to warrant the better gas mileage figures?

From BMW docs:

Performance & efficiency
• New engines in both models:
---3.0si – N52 6-cylinder engine with Valvetronic, magnesium/aluminum composite construction and other new features; 260 hp/225 lb-ft. torque
---4.8i – N62 V-8 engine, displacement increased from 4.4 to 4.8 liters; retains Valvetronic and aluminum construction; 350 hp/350 lb-ft. torque
---6-speed automatic transmission now standard in both models (previous 3.0i: 5-speed automatic optional, 6-speed manual standard)
---Automatic transmission includes extensive improvements for higher efficiency, quicker response and shifting, and finer adaptation to driving style and conditions
---New-type electronic transmission selector, similar in concept to that of 7 Series but on console instead of steering column
---xDrive fulltime all-wheel drive system, electronically controlled with variable front/rear torque split & traction control

-New technologies enhance fuel efficiency:
• Valvetronic variable valve lift (new to 6-cylinder X5 model)
• Rolling-resistance-optimized tires
• Delivery-regulated fuel pump
• On-demand power steering via electrically controlled bypass valve
• Pentosin power-steering fluid; revised viscosity index reduces power requirement at low fluid temperature
• Improved control of climate-control compressor’s clutch
• New-type rear differential

Forward progress.

In the case of the 3.0si, the E70's N52 engine has newer valve management system (valvetronic), double-VANOS, lower weight-per-hp ratio (lightest 6-cyl in the world).

70 4.8 vs 53 4.4
 

I don't have figures to back this up but having many miles on the 53 and just 18000 on the 70 my seat of the pants says the 53 may be better. They are both about the same on the highway and my 70 is worse around town. Can't say that for all the X's but for my cars thats the way it shakes out!

I don't think that they do get better mileage. The E70 has some more technology, but is heavier. Driving style makes more difference than anything else.

The US government fuel consumption test was changed, so the published numbers are different. You can't compare published figures for different years of vehicles any longer, unless you read a chart that has the different tests listed side by side. In theory, published figures should be worse for 2008, but there are so many adjustment factors included in the published number that it is a bit of a joke.

On long trips I actually did a little better with the E53.

Must be the valvetronic engine. I have an E53 4.8is and get similar gas mileage.

My 2006 E53 did about 18-19MPG. Now I am doing 16-17MPG with the new 2008 E70. I wanted to ask that question for about 4 months.... Why is that?

I thought I was getting better mileage on my '04 E53 than my E70. The added 3rd row makes the car heavier, burn more gas.

My 4.8 with 3rd seat gets close to 14 mpg. Not very green. Not sure what the rest of your 4.8 folks are getting...