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Re-activateThis is question we're frequently asked, so we decided to keep some of our test cars around and put extra miles on them. The goal: Let's find out if fuel economy changed significantly on the diesel powered vehicles compared to a gasoline powered car as they accumulated more miles.
We chose three models that had already been though our tests: two VW Jettas--one with a four-cylinder, 2.0-liter, 140-hp diesel engine and the other with a 2.5-liter, five-cylinder, 170-hp gas engine--as well as a Mercedes-Benz GL320 BlueTec, which comes with a 210-hp, 3.0-liter V6 diesel engine.
Before we test any car, we make sure they have 2,000 miles or so. For this test, the Jetta diesel had 20,000 miles; both the Mercedes and Jetta gas version had 16,500 miles.
Using our "one-day trip" mixed driving course using several drivers taking turns around a 30-mile loop of public roads that include a highway section, secondary roads, and rural byways. We compared the fuel economy of the diesel and gas vehicles.
Bottom lineAfter running all three through our standard test, we found that the fuel economy results showed no discernable difference in the fuel economy. All came in within 1 mpg from what we had tested originally. Assuming that you perform scheduled maintenance and replace tires as needed your mpg should remain fairly consistent for the life of the car.
For more on saving fuel, see our guide to fuel economy.
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