6.2L - 6.5L & Duramax Diesel
Economics & Fuel Economy

Updated November 14, 2018

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Why do people buy a diesel light-truck or SUV? Many do for the miles per gallon advantage, some do for the "hard work" reputation of diesels, and some drive a diesel just because they like diesel engines. The economics of buying and owning a diesel powered vehicle have been widely discussed, but much of that discussion leaves out some vital information. I've been driving diesel light-trucks and SUV's since 1986. If you're interested in buying a new or used diesel vehicle, please read on to learn what we here at The Diesel Page know about diesel MPG and the overall cost of ownership.

First, don't make the classic mistake of comparing the economics of gas versus diesel by looking only at the cost differences between the engine options and fuel economy. While the higher initial cost of the diesel engine option keeps some people from buying a diesel powered vehicle, much of the diesel engine option cost will be returned to you when you sell or trade the vehicle at some point in the future. Keep in mind that the diesel engine option will add value to your trade-in. You can compare used gas/diesel vehicle pricing for otherwise identically equipped trucks by visiting the Kelly Blue Book web site. When combining the added trade-in value and the fuel economy advantage, a diesel equipped pickup truck begins to make good economic sense.

Assuming the pump price of gas and diesel were the same, a diesel could save you $1500 or more in fuel costs per 100,000 miles of light duty driving and/or up to $2500 or more when doing a lot of towing or other high power demand driving, when compared to a gasoline powered truck with identical gearing. (These cost comparisons represent the estimated differences in fuel economy between a Duramax 6600 and a 6.0L fuel injected gasoline engine. When compared to a gasoline 8.1L, the diesel will save you even more.)

Leading a hard life of towing or other demanding use shortens the lifespan of any engine, but will impact a gasoline engine more than it will a diesel. When used under identical operating conditions, a diesel engine will likely produce at least twice the engine life of a gas engine. This should be factored into the overall vehicle maintenance costs if you are the type who owns a vehicle past 100,000 miles. Rebuilding or replacing a gasoline engine will likely cost more than the typical diesel fuel injection system maintenance.

New fuel injected gasoline V8s from GM deliver quite a bit better fuel mileage than those engines produced in the 1990s. In light duty use, these gas engines will deliver fuel mileage somewhat close to what you'll get with a finely tuned diesel, but the diesel will still maintain a 10-30% fuel mileage advantage in identically geared trucks. All of the Duramax powered trucks are equipped with 3.73 differential gearing.

Some would argue that the savings in fuel economy are offset by the higher maintenance costs associated with a diesel engine. During my more than 32 years of driving a GM diesel pickup, I've saved many thousands of dollars when compared to a same model-year vehicle equipped with a gas engine, even when factoring in maintenance costs.

When calculating overall fuel cost comparisons, you should take into account the difference in price between gasoline and diesel fuel as well as fuel economy differences. The above photo was taken here in western Montana in 2009. Since at least 1986, when I began driving a diesel pickup, the pump price of diesel fuel had been less than self-serve gasoline most of the time - by as much as $0.20/gallon. However, during the summer of 2008 and beyond, diesel fuel has been as much as $1.00 per gallon higher than even the lowest price gasoline. Some have offered the opinion that the 2007 EPA mandated Ultra Low Sulfur Diesel (ULSD) fuel was a factor in producing a price differential that favored gasoline, but there are likely many factors, among them supply/demand - which is affected by U.S. diesel fuel exports (yes, the U.S. exports a lot of diesel fuel).

The best advice before buying a late model used diesel vehicle: Try it out long enough to get a handle on the fuel economy and general performance. Your GM dealer has access to the warranty repair history of this truck using the GM Vehicle Inquiry System. Your dealer only needs to do a lookup on the truck's VIN to verify whether or not the truck you're looking at has been a problem for its previous owner. The only time you have any real leverage with the dealership or a private seller is before you sign the papers, so make this a condition of sale. Beginning with the 2008 model year, GM returned to a 5-year or 100,000 mile powertrain warranty. Now expired, GM originally extended the Duramax fuel injector warranty for the 2001/02/03/04 models to 7-years or 200,000 miles. In addition to the engine and fuel injection system, the new warranty covers all aspects of the powertrain, including the transmission, transfer case and rear differential. No matter what GM diesel vehicle you buy, being fully informed could improve your satisfaction with your 'new-to-you' GM Diesel.

Anyone... who fears a high-dollar fuel injection service (or any other high-dollar automotive repair), and is contemplating owning or buying a used diesel pickup that is nearing the end of its factory warranty, should buy an extended warranty that covers the drivetrain, which includes the engine and fuel injection system, transmission and differentials. In addition, if you're buying an out of warranty truck or one nearing its warranty expiration, try to negotiate some fuel system maintenance into the final sale price. Odds are that the owner is selling the truck at least partly because of the looming warranty expiration. These trucks are expensive to repair out-of-warranty - somewhat less so if you're able to do most work yourself.

In the June 2000 Issue of Trailer Life magazine, there was a review of a new Isuzu Trooper equipped with a 3.5L V-6 rated at 215 horsepower and 230 lb-ft of torque. The trailer towed for this test was a 21' Coachmen Futura that weighed 4,331 lbs.

Fuel consumption for this rather small vehicle was 15.9 mpg solo and 9.7 mpg towing. Towing conditions were 60 mph, predominantly flat highway. Trucks equipped with either the Duramax 6600 or 6.5L turbodiesels can tow twice that weight at 65-70 mph while producing fuel economy in the 10-13 mpg range. Both Duramax and 6.5L TD fuel economy while running empty could range from 15-20 mpg depending on gearing and driving habits.

A Nisson Xterra was tested in the June 2000 issue of Four Wheeler magazine. Over 8,285 miles, this small SUV produced an average of 16.52 mpg. Crash test dummies (people like you and I) say you're much safer in a 6,000-7,000 lb truck or Suburban than you are in a lightweight import SUV, especially if that truck or Suburban gets as good or better fuel economy as the much smaller vehicle while running solo, and has twice or more the towing capacity.

In the July 2000 issue of Four Wheeler magazine, they tested a 2000 GMC Yukon XL 2500, equipped with the new 6.0L gas V-8. To quote Four Wheeler, "During unladen city/highway blasts, we averaged nearly 12.5 miles per gallon with a best tankful of 13.05 - quite good compared to the last big-block we tested, which averaged 10.8 mpg." A diesel engine is the best powerplant for a large vehicle, and this explains why. Our 2001 GMC 2500HD Duramax/Allison equipped 4x4 crew cab delivers 17-18 mpg during local driving and 20 mpg on the Interstate at 75-mph.

We had been running a fuel economy poll in our online Bulletin Board Forum for quite some time. The above graphs reflect what truck owners voting in each category were seeing for fuel economy. Those voting in this poll were asked to round their fuel economy results to the nearest whole number. The above images are self explanatory, but demonstrate the fuel economy currently being experienced by hundreds of Duramax 6600 owners. It is clear by these data that the earlier 2001-04 LB7 Duramax diesels produce on average about 2 mpg better fuel economy than the 2004.5 or newer LLY/LBZ/LMM Duramax. The increasing EPA diesel emissions requirements for the newer models may play a part in this analysis. The newest 2011 Duramax was promoted as being approximately 10% more fuel efficient than the 2010 model it replaced. You can read more about fuel economy being delivered by the newest Duramax engines in our bulletin board.

The 1992-00 GM trucks utilize what is known as a VSSB (Vehicle Speed Sensor Buffer aka Digital Ratio Adapter) to relay vehicle speed to the computer. Located beneath the glovebox, these VSSBs are specific to the gearing and tire diameter, and need to be changed (or re-programmed) to one that matches the new gearing or tire height if you make any changes to the effective final gearing. Replacement VSSB modules are re-programmable by the truck owner to compensate for any gearing changes. We produced articles in 1999 and 2000 dealing with the "Vehicle Speed Sensor Buffer". These articles are now available as part of the updated for 2018 6.5L Turbo Diesel Volume I, and will show you how to modify your current VSSB (Vehicle Speed Sensor Buffer) module for changes in gearing or tire diameter without spending any money on hardware.

Aside from the need to know how fast you're really driving, the computer also needs to know the vehicle speed so it can manage the powertrain more effectively.

Any GM diesel that is equipped with the 700R4 (4L60-E) or 4L80-E Overdrive Automatic transmission and is getting poor fuel mileage should have the transmission and related systems checked for proper operation.

If you're getting lower than expected fuel mileage, test the complete operation of the transmission to ensure that the transmission goes into OverDrive and the torque converter locks up when it's supposed to. You should be able to feel the transmission shift into overdrive and torque converter lockup. A malfunctioning transmission OD and TC lock-up system could reduce your fuel mileage by 1-5 miles per gallon.

The THM700R4 can be modified to eliminate the electrical systems that control the overdrive and lock-up torque converter. These modified transmissions use fluid pressure, shaft speed and throttle position from the "kick-down" T.V. cable as inputs that enables the transmission to make gear changing decisions and allow torque converter lockup.

All automatics should have an auxiliary ATF cooler, especially if the truck is used for towing. Never tow with a 700R4 in overdrive, and we recommend you shift down on hills to maintain 2500 RPM or above. This will ensure a sufficient flow of ATF cycling through the transmission to help prevent it from overheating. The engine could and should be run at a lower rpm when you're not working the truck as hard.

There are a series of GM's technical service bulletins that attempt to explain the factors that can affect diesel fuel ecomomy. These are rather generic and pertain to all GM products.

According to GM, factors (not in any particular order) affecting fuel economy are:

1. Final gear ratio.
   
2. GVW (Gross Vehicle Weight).
   
3. Wheel alignment and tire pressure.
   
4. Misadjusted or dragging brakes.
   
5. Accessories that increase wind drag.
   
6. Driving 65mph will lower fuel mileage up to 15% as compared to driving 55.
   
7. Frequent stops, speed variations or otherwise poor driving habits.
   
8. OD automatics not shifting into OD, or the lockup torque convertor not locking up.
   
9. Fuel system or other engine related problems.