It was a genius, almost foolproof plan. Team up with sister magazine Super Street and test our '09 Jetta TDI against an'09 Toyota Prius to see which car was the most miserly in terms of fuel consumption over a pre-planned route. We'd then throw in modified gasoline examples of the VW GTI and Honda Civic for a real-world comparison, measuring the fuel used by all four cars.
And then the house of cards came tumbling down. The best laid plans and all that...
First of all, we must put our hands up. It's entirely our fault. We should never have thought the SS Brain Trust could actually follow our plan! It went wrong right at the start. First their project Civic wouldn't start, being quickly replaced by a stock vehicle. However, the real crusher was when the imports went off-route at the end of the challenge, and the Prius didn't record its final mileage or amount of fuel used!
They're all lovely fellows, but fortunately we had imagined the SSBT might encounter a few hiccups, so the plan made some allowances. As a result, our rainy day spent enduring epic Los Angeles traffic wasn't entirely wasted. In fact, we think you'll be shocked by what we discovered.
The idea for this story arose when VW invited us to drive and subsequently borrow its '09 Jetta TDI - a 140hp 2.0 turbo-diesel (et 1/09). Starting at only $21990 with free servicing for three years and eligibility for a $1300 federal tax credit, the car makes an attractive proposition. Throw in its EPA estimated fuel consumption of 30mpg city and 41mpg highway (manual trans), and you have a no-brainer.
During our first acquaintance, we man-aged to push the overall consumption to 45mpg using a mixture of roads, while some fuel economy experts had covered 9419 miles across the US using an average of 58mpg. So we wanted to see what we could really expect from the TDI on an extended drive over mixed roads.
As a result we chose a route that took us into the congested downtown area of Los Angeles. We'd then drive about 15 miles on traffic-clogged city streets to reach the I-10 freeway. This would be followed by about 35 miles on the freeway before dropping onto Pacific Coast Highway. We cruised out to one of our favorite canyon roads, finally returning home on city streets again.
TDI vs Prius
The brief was to have all four cars drive the same route at a similar pace to directly compare fuel used under these conditions.
The test began with a fill-up for all the cars, ensuring they were to the brim and a note of the odometer mileage was taken. We would then drive to the freeway. The rain was not only causing inevitable traffic jams, but the damp conditions forced us to use our wipers, heater and lights to maintain safe visibility. To be honest, we did occasionally switch off the engine at lights that stayed red longer than 15sec (as recommended), and we previously reverted to the stock wheels (see Our Cars in this issue), but we knew the Toyota Prius hybrid was in its element. By relying on its electric power, only using the gasoline engine to help with acceleration, the hybrid would be using almost no fuel under these conditions.
Our worst fears were realized when we stopped before entering the freeway to compare onboard computers. Admittedly, these aren't the most accurate gauge but, as it turned out, they would be our only indicator.
After fighting through downtown LA, the TDI had burnt an average of 27.2mpg at an average speed of 13mph. Yet the Prius returned 38.3mpg under the same conditions. This clearly shows the advantage of hybrid technology in busy cities.
Once on the freeway, an accident slowed our progress, again helping the Toyota. However, it soon cleared and we were cruising at a relaxed 60mph for more than 10 miles. After the freeway run, the TDI's onboard computer showed a frugal 47mpg, while the Prius recorded 44mpg. Using a light throttle, we expected the diesel to excel here, and given a longer run we could have got the number higher.