& Fuel Economy
Updated March 26, 2013
Why do people buy a diesel light-truck or SUV? Many do for the fuel economy 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 fuel economy and the 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 a few years on down the road. 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 24 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. Saving money on auto parts is another good reason to choose diesel. Since there are no spark plugs in a diesel engine you can invest in other upgrades or replacement auto parts at directfitautoparts.com and other auto parts stores.
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 photo on the left was taken June 6, 2009 here in western Montana. Since at least 1986, when I began driving a diesel pickup, the pump price of diesel fuel has been less than self-serve gasoline most of the time - by as much as $0.20/gallon. However, during the summer of 2008, diesel fuel was 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).
Before buying a late model used Diesel, 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 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. However, GM 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 might 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've been running a fuel economy poll in our online Bulletin Board Forum for quite some time. The above graphs reflect current stats as of 8/4/05, and the numbers in parenthesis indicate the number of truck owners voting in each category. 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 2011 fuel economy in our bulletin board.
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We've learned that fuel economy for vehicles equipped with a 6.2 or 6.5 diesel engine can vary quite a bit depending on vehicle health, transmission options, gearing and driving habits.
The "GM Product Service Training Manual for the 6.2L Diesel Engine" (#16015.05-1D) has this to say about GM diesel fuel mileage:
"The diesel, like any engine, is affected by driving habits. Speed is more critical on a diesel than a gas engine. On the highway, in the 50-75 mph range, the fuel economy will go down about 3 mpg for each 10 mph increase in speed. A gasoline engine will lose about 1-1/2 mpg for each 10 mph increase in speed. This condition is perhaps the most significant factor in obtaining good fuel economy. Fuel economy may vary as much as 5 mpg in a given vehicle with different drivers."
Diesel engines have about half the useable rpm range as compared to a gas engine. A gasoline engine will produce acceptable fuel economy over a fairly wide rpm range. The diesel engine has a relatively narrow rpm range that will produce acceptable fuel economy.
The relationship of gearing to fuel mileage has to be the single most misunderstood aspect of the GM diesel engine. A typical 6.5TD with 3.42 gearing will produce in excess of 20 mpg at 65 mph because the engine rpm at that speed is right at the engine's torque peak of about 1800 rpm. A truck with 4.10 gearing is running at about 2250 rpm at the same speed, which typically produces about 15 mpg. A 500 rpm difference might not sound like much, but in a diesel, that's about half of the useable rpm range.
6.5TD engine rpm (4L80E automatic transmission) and typical fuel economy at 65 mph:
If you do a lot of high speed interstate type driving, the GM diesel will deliver the performance you're after as long as you gear your truck for the speed you want to drive. The a 3.42 gear ratio will allow you to drive at any sane speed. The 3.73 and 4.10 gears are more at home towing at a lower speed.
If you're buying a new diesel truck and want the best possible fuel mileage, get a truck with the highest gear ratio available and an overdrive transmission. Specify a 3.42 gear ratio unless you intend to use the truck to tow. 1998+ axle ratio options for diesel engine equipped C/K trucks include 3.08, 3.42, 3.73, 4.10, and 5.13 gearing is offered in some heavy duty model GMC 6.5TD trucks . The 3.08 ratio is available only for the C1500 series versions (2WD) and the K2500 series Suburbans are only available with the 3.73 or 4.10 axle ratios. The new 4L80-E overdrive automatic or a 5 speed overdrive manual transmissions are available in all diesel trucks.
General differential ratio recommendations for towing with the 6.5TD:
These recommendations are based on many reports from 6.5TD owners who use their truck to tow. Owners of non-turbo GM diesels should use the next lower (higher numerically) axle ratio increment to ensure adequate towing power. Wind drag factors needs to be considered as well. For example, I know a rancher in southern Montana who tows a 4 horse trailer with his 3.42 geared 6.5TD. This trailer weighs close to 6,000 lbs loaded, but since horse trailers have a relatively low wind drag, his 3.42 geared truck does really well. A 6,000 lb RV trailer would tow significantly harder.
For towing trailers that weigh 4,000 pounds or less, a 3.42 differential ratio is probably the best choice in a 6.5TD equipped truck. Your truck will tow heavier trailers up to the maximum trailer weight of 8,000 lbs better with a 3.73 or 4.10 ratio, but your fuel mileage and comfortable cruise speed could suffer when not towing. A 4.10 gear ratio is more useful when used to tow trailers heavier than about 6,000 lbs.
The 4.10 ratio seems to be the most prevalent gear ratio found on 2500 series trucks and Suburbans. I can only assume GM thought the diesel ¾-ton models would be primarily used for towing. Again, if you're mostly interested in the best highway fuel economy and high speed interstate cruising, buy a truck with taller gears. You might not be happy with 4.10 gears otherwise.
Differential gears sets are currently about $150 each. Several suppliers can be found in any one of a half dozen different Off-Road or 4X4 magazines. If you're unhappy with your gearing, but like your truck, installing different gears makes sense. However, the best way to get a 1988 or newer 4x4 geared for better fuel economy and higher road speeds is with a Gear Vendors or US Gear Overdrive. These cost about $2500-3000, but you can remove it when you sell the vehicle, and recoup much of your original expense.
Replacement speedometer translation gears are available for the 1982-91 GM trucks to correct for any speedometer error caused by gearing or tire diameter changes. These translation gears are located in the tailshaft of the transmission/transfer case.
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 6.5L Turbo Diesel volume, and will show you how to modify your current VSSB 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:
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