I was asking the same question about 3 weeks ago and ended up with a 2006 4.7.
Initially I though "Hey, the v6 is clearly going to be more fuel efficient!" but after doing some research and using some common sense I came to the conclusion that the V6 is not very suitable for such a heavy vehicle and requires more power/gas to get it moving.
The V8 is more fitting because it is a more powerful engine. It takes less strain to get moving...
I checked a popular fuel efficiency website and the average MPG of both models was actually the same (15mpg), so that tells ya something! The site is http://www.fueleconomy.gov
Love my commander though. The price of mpg sucks no matter which way you go, but its worth it for such a kick ass little beast imo.
I don't think you will find many 3.7 owners that would do things differently for the ENGINE.
Probably more so for the 4X system QT I, QTII or QDI.
This coming from a 3.7 owner that has driven his for what is just touching 141,000 miles. Not owned one for 3 weeks.
The statment above that the 3.7 is not very suitable is way off base.
I would have no problem towing anything within the manufactures recomended specifications and along with that comes the maintenace.
If I was towing long distances over 1000 miles I would look into additional trans cooling. The 3.7 only has the electric cooling fan.
The engineers at Mercedes are smart enough to figure out what drivetrain to put next to the 3.7 to get her rolling. I don't run the 1/4 mile with it and I'm not heavy on the gas pedal.
As far as I'm concerned they got it right.
On another note I previously had a Ford F150 4X4 with a 6cyl in it. I retired it with 238,000 miles on it.
Not because of drivetrain issues but the rear gas tank leaked, air bag light came on and it needed tires.
I think the engineers at Ford also got this right.
As far as mileage is concerned they are not all the same! I average 19.6 off highway. My communte to work is 22 miles of 2 lane country roads with speed limits of between 35 and 45 mph only 4 stop lights.
As far as highway mileage is concerned the best I ever got was from Mineapolis to Rapid City SD at 25mpg. Generally it is around 21mpg highway.
Mileage will always depend on how you drive, where you live and road conditions.
Keep the 3.7 around 2100 rpm for the best possible mileage on the highway.
My XK has been to, thru and in 43 of the lower 48 states. July 19th it leaves fully loaded for Az and Nevada. Another 4000 + mile road trip.
Now if I was to buy another XK it would be a different 4x system. Probably QTII which was availble in 06 with the 3.7 as a option but you won't find many of them.
I would do this for the off road capabilities. I have taken my XK off road with the longest adventure 68 miles in Moab. I ended up renting a modified unlimited wrangler while I was there because I didn't want to bust mine up knowing I had a 1,700 mile drive to get home.The only thing I won't do with QTI is MUD.... Commander Chaos an older member, worked his 3.7 QTI hard off road and I'm not remembering him having many problems.
On the 3.7 you can expect to change the water pump at around 80,000 miles.
My advice to you is buy the commander that suits your needs off and on road. Buy one that you can afford and afford to maintan. Doing the reuired maintence is key to a long happy life.