OK, since things have settled down and we’re in relaxation mode, I can give the vehicle report for the gearheads.   As you’ll recall, we switched from a Nissan Armada to a Mercedes GL320 a couple of months ago.   This trip from Arizona to Vermont was the first big trip with the combination.

The trip was an ideal test for the new vehicle, since it encompassed virtually every condition we expect to tow in routinely:   mountains, windy plains, cities, curvy back roads, and deadly boring Interstate.   It also included a mix of towing and non-towing use.   That’s important because we chose the GL320 partially for its non-towing driveability.   In other words, I wanted all the performance we’d get from a bigger vehicle, but didn’t want to be saddled with an unwieldy truck when not towing.   We use our tow vehicle as our primary transportation for months, when we are on extended trips.

Anyone reading this for advice should first read my initial report on the GL320, since I’m not going to repeat all the things I said there.   There are significant caveats for anyone who might be considering this particular vehicle, or the essentially similar (but smaller) ML320.   I am NOT writing this to convince anyone that they should buy this (or any) vehicle.   I’m only documenting my experience.   The right vehicle for you may be completely different.

Our trip was about 4,000 miles, mostly highway.   The GL320 turned in about 12.5 MPG in the first 2,000 miles while towing, then the fuel economy improved markedly, between 14.0 to 15.2 while towing at 60 MPH.   Going 65 MPH costs us about 1 MPG.   Non-towing fuel economy has been superb for a vehicle of this size: 22 MPG in mixed driving, and 25-27 MPG on the highway at virtually any speed up to 75 MPH.

At this point the odometer shows 5,400 miles total.   We have not had to add oil or AdBlue to date, despite the fact that most of our mileage has been towing and the engine is probably still breaking in.   The AdBlue tank is scheduled to be refilled by the dealer at the 10,000 mile scheduled service interval, and I am interested to see if it gets low before then.   AdBlue consumption is related to fuel consumption and of course we use more fuel when towing.   The Bluetec system is a relatively new technology and there are reasonable questions about how whether the standard AdBlue tank is large enough to accommodate lots of towing.   Mercedes says it is.

Performance has been spectacular.   You would never know that this is a 3.0 liter V-6.   We have more pulling power (torque) than even the big 5.6 liter V-8 in our Armada. Up hills, it blows the Armada away, and despite having 7 gears in the transmission, it needs to shift less on hills because of the impressive torque.   Most of the time we are towing in 7th, with occasional shifts down to 6th and rarely 5th on moderate hills.   That’s with the full 7500# load that the car is rated for.   I am sure the engine is capable of much more.   We have yet to find the top speed (and probably never will), but in west Texas on I-10 where the speed limit is 85 MPH, it felt capable of every bit of that.   I personally never tow over 65 MPH for sustained periods, and usually keep the cruise control set around 60-62 MPH for best economy.

I am very interested to see the high-altitude performance, since that’s where we always struggled with the Armada. The normally aspirated gas engine lost a lot of power at altitude (like in Colorado and Utah), where the turbodiesel should do much better. But the performance of the turbodiesel is apparent even on flat plains because a headwind on the Interstate can be just as tough to deal with as an 8% grade in the mountains.

The brakes are also impressive.   The GL320 has 14 inch vented discs front and rear, again bigger than the Armada’s, and there’s no doubt in my mind that the brakes are up to the task of stopping both truck and trailer if they have to (in the event of trailer brake failure).

I know a lot of people get freaked out by the word “unibody,” but it’s not true that unibodies are always weaker or less capable of towing compared to traditional body-on-frame trucks.   In any frame, there are weak designs and there are strong ones.   The GL320 has an extremely strong unibody structure with lots of high-strength steel.   During the trip we never heard so much as a creak from the body, nor any indication of unusual flexing.   Tire wear has been normal thus far.   Every engineer I talk to says the same thing, “Unibodies are often stronger than ladder frames.”   I’m very comfortable with the vehicle structure but of course the proof will be a few years down the road.

My major beef with the car has been the completely hopeless hitch receiver that Mercedes put on it.   We reinforced ours in Tucson, which probably would have been enough, but then for added long-term durability I let Can-Am RV do their preferred reinforcement on it as well.   It is now very strong and distributes the stress of the hitch weight over much more area.

The Can-Am RV crew also changed the Hensley shank from a 2″ drop to a straight shank.   For our combination the 2″ drop bar was better for keeping the trailer level, but the straight one was recommended for slightly more weight distribution to the front axle.   I don’t if it was the new shank or the stiffer receiver, but when we weighed after the modifications, we had an additional 200 lbs on the front axle (and the same amount less on the rear axle).   This improved the ride slightly, and handling remained about the same (which is to say, very good).

The only problem with going to the straight shank is that the back of the trailer now rides about 1″ lower. We already had problems with the back occasionally scraping the road when we entered gas stations, and this makes it slightly worse.   I may switch back to the 2″ drop this fall if my experience is not good.

Handling-wise, there is still the usual SUV “squishiness” in the tires.   I felt this in the Armada as well.   The recommendation I’ve gotten is to change to a tire that more closely matches the width of the rim.   The rims are 8″ wide and the tires (275mm wide, or about 10.8 inches) overhang them by quite a bit.   I may try this when the stock tires are worn out.

I’m also still unimpressed by the lack of a spare tire.   It may be possible to shoe-horn a spare into the usual trunk space, but in any case I’m carrying a tire plug kit and a CO2 tire inflator.   I love this combination — it will fix 90% of flats and it all fits in hardly any space at all.   If you are interested in buying a CO2 inflator from Power Tank, type “AirstreamLife.com promotion” in the Comments box on their order form and you’ll get a free tire plug kit worth $40 with your purchase.   I’m also doing a review of their product, which will appear in the Fall 2009 Airstream Life Online Edition.

It should be apparent by now that I like the turbodiesel.   As I’ve said, the engine is most of the reason I bought this vehicle. It is astoundingly quiet, well-mannered, and the exhaust is so clean you can only tell it’s there by the warm steamy air.   Can’t smell it, can’t see it.   None of the diesel traits of the bad old days are present.   Most people can’t tell it isn’t a gas engine, until they stomp on the accelerator and it leaps forward with a different (but quiet) sort of engine rumble.

I wish there were more options to get these engines.   In Europe they’re everywhere, but in the US/Canada there are few available. As a result, today’s options for V-6 diesels mostly come from the European manufacturers: Land Rover, Mercedes, Volkswagen, Porsche, Audi, BMW.     (If it bothers you to buy a “foreign car,” keep in mind that the “made in America” thing is a red herring at least in the case of the GL320 because the Mercedes GL-class is built in Alabama. I still get grief about this.   But I know people still believe that a Dodge now owned by Fiat, or a Suburban built in Mexico, is more patriotic than a Mercedes made in Alabama, so that particular issue will probably dog me for years.)   In any case, there should be more of these diesels from the big truck manufacturers, because they offer an excellent compromise between power and economy.   Why should the only option for American diesel truck buyers be a 6.7 liter Cummins engine that makes enough noise to wake the dead, and only on the 2500-series trucks?

The best thing about the GL320 is driving it while it’s not towing.   I’ve never been a luxury car buyer before.   I’m still not, really.   If we had a shorter Airstream, I probably would have bought the VW Touareg 2 (now with 3.0 liter V-6 turbodiesel) instead. But since we needed the extra space and the third row seating, I can admit that the GL320 is a pleasure to drive when not towing.   It is no sports car but just constantly reminds you that it is competent and safe.   It is much nicer to drive and park than the Armada.   Eleanor even likes to drive it, and she always hated driving the Armada.   The safety features are extensive, so much that I can’t even get into them all here, but suffice to say it is in every way a safer vehicle for us to be driving.   Finally, I love the fact that I’m getting nearly 600 miles from a $60 tank of fuel, versus 345 miles in the Armada, while putting out much less exhaust emissions.

The worst thing about driving it is that it is so quiet and competent, highway drives are rather boring.   I have discovered that I tend to get drowsy, which is definitely not a good thing with three and a half tons of RV behind you.   I never had a tendency to fall asleep with the Armada.   The solution has been to play music from the iPod.   I guess in the big scheme of things, that’s not such a bad solution.

I will tell you one last thing.   A big part of my reasoning for buying this non-traditional tow vehicle is that I believe tow vehicles are headed in this technological direction.   Rising CAFE (fuel economy) standards and rising emissions requirements will put huge pressure on traditional tow vehicle designs. Simply making trucks lighter won’t address the challenge — manufacturers have to make their vehicles smarter.   I don’t think diesel is the whole solution either, but I do believe that a combination of technological advances (in body design, electronics, engines, transmissions, emissions controls, etc.) will lead us to the next generation of tow vehicles.   I bought this vehicle partly because I think it represents the first wave of where we are headed as an industry (I’m speaking of the RV industry), and I wanted to get some experience with it to understand the future.     If you wish to do the same, just remember that the leading edge is always sharp, so you need to be smart about your choices and do your research.