TDI mileage has been all over the map in recent years. We pulled together data from all over the diesel world to find out why.
Indirect-injection diesel engines are not known for their fuel economy. Think of those big, clunky old Mercedes diesels, like the 240D and 300D. They were only getting around 25 mpg. The VW diesels achieved decent fuel economy, but they they managed this with low hp and light weight. When VW released the Mk. III TDI, the cars became more efficient and produced more horsepower and torque. TDI mileage has fluctuated over the years, with changes in engine design, car weight, emissions requirements, and VW’s perception of people’s tastes in automobiles (read: engine displacement selection). In recent years, we’ve seen a drop in our beloved fuel economy for TDIs. Why? We dug deep into the numbers and found some interesting answers.
EPA FUEL ECONOMY STATS
The EPA calculates fuel economy for every car on the market in a laboratory by measuring the amount of carbon in the exhaust, which they say is more accurate than using a fuel gauge. However, in the wide majority of cases, EPA lab estimations run lower than reported fuel economy. There’s even a section on fueleconomy.gov for reported fuel economy, and across the board these numbers are higher than the estimates.
At any rate, the chart below details the EPA’s numbers for fuel economy on all the various incarnations of the TDI engine over the years.
| Engine Type | City | Combined | Highway |
| AHU (96-99) | 33 | 37 | 44 |
| ALH (99-03) | 35 | 38 | 44 |
| BEW (04-05) | 31 | 34 | 39 |
| BRM (05-06) | 30 | 33 | 37 |
| CBEA/CJAA (09-10) | 30 | 34 | 41 |
| CJAA (10-14) | 30 | 34 | 42 |
*Representative years for each engine type were chosen. Years were as follows: 1998, 2003, 2005, 2006, 2009, 2014.
**Stats are for a manual Jetta TDI Sedan.
***Changes in the way EPA calculates fuel economy in 2010 lowered MPG #s for all model years.
The numbers here are fairly comparable, but the trend is that with the introduction of the BEW engine, mpg dropped, and city mpg dropped further in the CBEA and CJAA engines.
Given that many people take legitimate issue with the numbers from the EPA, we also went out and checked Fuelly.com to find the average mpg that appears the most for each engine. Here is what we found:
FUELLY STATS
| Engine Type | AHU | ALH | BEW | BRM | CBEA/CJAA | CJAA |
| Mode Average Combined mpg | 46 | 43 &44 | 43 | 41 | 40 | 41 |
*Representative years for each engine type were chosen. Years were as follows: 1998, 2003, 2005, 2006, 2009, 2014.
**Stats are for a manual Jetta TDI Sedan.
As you can see, the mpg on the AHU engine is significantly better than the mileage on the most recent engine, and there’s a clear downward trend for mpg as the years go on. Although it’s not represented in the chart, it’s also interesting to note that in 2014 many vehicles averaged in the 37-41 mpg range (with almost as many people averaging 37 miles per gallon as 41). And, for the cars from 2003, many cars were averaging above 44 mpg, hovering around the 45 and 46 mpg marks.
People have also reported that the higher miles per gallon that were possible with the earlier engines have become very difficult to achieve with the newer engines, and conversely we’ve seen lower miles per gallon numbers from the newer cars than we had ever seen with the older ones. Simply put, the lows are lower with the newer cars, while with the older cars, the highs are higher.
WHAT GIVES?
While mpg have gone down over the years, it’s important to realize that this doesn’t necessarily mean the engines have become less efficient. A number of things have changed over the years that have affected mpg.
With each successive TDI engine, horsepower and torque have increased along with engine displacement. The inverse relationship between hp and fuel economy is well known: with increases in power come decreases in mpg. Bottom line, the greater the engines displacement, the more fuel it uses.
Not only that, but these cars have gotten increasingly heavier. For example, a 2003 Jetta TDI sedan had a curb weight of 3009 lbs., while a 2005 weighed in at 3197 lbs. The 2014 weighed in similarly at 3161 lbs. A 1998 TDI Jetta, however, weighed in at only 2525 lbs. Given the trend, it’s safe to say that these cars won’t be getting lighter anytime soon, and you can expect the increased weight to continue to have an effect on fuel economy.
| Engine Type | HP | Torque (ft./lbs.) | Weight (lbs.) |
| AHU (1998) | 90 | 149 | 2525 |
| ALH (2003) | 90 | 155 | 3009 |
| BEW (2005) | 100 | 177 | 3197 |
| BRM (2006) | 100 | 177 | 3241 |
| CBEA/CJAA (2009) | 140 | 236 | 3230 |
| CJAA (2014) | 140 | 236 | 3161 |
*Representative years for each engine type were chosen. Years were as follows: 1998, 2003, 2005, 2006, 2009, 2014.
**Stats are for a manual Jetta TDI Sedan.
Emissions requirements have also changed, and this has required the cars after 2009 to have updated emissions equipment, which operate at the expense of some power that would otherwise have been used to move the car.
COMMON RAIL DIESEL AND THE FUTURE
So far, we’ve only gone over the ways in which fuel economy has worsened in the newer cars. If fuel economy has worsened, how can we say the engines remain at the same level of efficiency?
Well, given the fact that the new common rail diesels have many things working against their favor—higher horsepower, heavier cars to lug around, fuel-sucking emissions, less efficient fuel—they still manage to get around 39 mpg according to fuelly. One reason for this is they’re more highly pressurized than the PD and rotary pump engines. The common rail engines also have taller gearing, which helps with fuel economy as well. To illustrate, at 2000 rpm in its top gear (the fifth gear), a Jetta with an AHU engine is going around 58 mph, which is obviously a little slower than most people tend to go on the highway. On the other hand, a Jetta with a CJAA engine at 2000 rpm in its top gear (the sixth gear) is going around 74 mph, which is right around the speed most people drive on the highway. This is the reason why the new engines achieve fuel economy that’s so much higher on the highway than in the city.
Compare, for example, these two Golfs sold in the UK. The first, a 2015 Blue Motion 1.6 L TDI, has the new EA288 common rail engine and produces 115 hp. The second, a 2001 Golf TDI, is a rotary pump engine that makes 90 hp. You would expect the second to get better mpg, but compare the numbers:
2001 Golf Rotary pump TDI:
Cold Urban: 41.5
Extra Urban: 65.7
Combined: 54.3
2015 Golf Blue Motion 1.6 TDI
Cold Urban: 74.3
Extra Urban: 94.2
Combined: 88.3
All MPG in imperial gallons.
The new TDI destroys the old one in fuel economy. They’re not even close. With large displacement differences out of the picture, we can see that the newer engines are more efficient.
IS OLDER REALLY BETTER THAN NEW?
After reading all this, you might just be thinking that older TDIs are better than newer TDIs, and why not just go on craigslist and buy on old one. No arguments here, but don’t reach past the TDIs: the Mk. I Jetta got an EPA estimated 38 mpg highway. In 1991, with the Mk. II, the Jetta got slightly less at 36.
Maybe there’s hope in the future. The newest cars (2015 Mk. VII) get an EPA estimated 46 mpg on the highway, which is right up there with the AHUs and ALHs, and is suprising considering the 2.0 liter engine. And there’s always the slim chance that VW reconsiders its market and brings the 1.6 liter TDI they have in Europe over here. At any rate, new TDIs will continue to be well-built, durable, long-lasting cars (even if they drink a tiny bit more fuel).
Sources:
For EPA estimates, fueleconomy.gov
For reported mpg, fuelly.com on 8/5/14
For weights, torque, and HP, edmunds.com car specifications
For fuel economy numbers from the UK, http://www.dft.gov.uk/vca/fcb/carfueldata-tools-redirect-page.asp
