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Why is my car?s fuel economy such a disaster?

When you notice the gas consumption on your car far exceeds its rating, what steps can you take to fix the problem?

Eric Lai answers readers? auto questions every week for Wheels.

Q: We have a 2009 Toyota Matrix XR. It has 40,000 km on it.

Since it was new, we?ve noticed that gas consumption far exceeds its rating.

The dealer claims that it?s not using too much gasoline. Their test consisted of hooking up a computer to the car to get a digital readout. They didn?t test fuel flow or actual fuel consumption.

We are really struggling with the high cost of fuelling this vehicle. It uses more than 10 litres to travel 100 km on the highway, and up to 14 litres in the city.

Q: I have had a 2004 Impala from new, a 2006 Grand Prix from new, a 2009 Impala LS used 39,000 km, and now, a 2010 Impala LT used 32,000 km from August 2011. All equipped with the 3500 V6 engine (Grand Prix was more horsepower).

The first three cars did about 22-27 mpg city, 35-43 mpg hwy. The 2010 is only doing about 15-18 mpg city, 25-28 mpg hwy with the same drivers and buying fuel at same outlets. No performance issues are apparent. It just uses a lot more fuel.

My service tech says there?s nothing they can do to improve fuel mileage. Is this true?

A: Eli Melnick of Start Auto Electric ( in Toronto replies:

First of all, I?ll presume that you meant U.S. gallons. I did some digging and found some interesting facts. I?ll start with your newest car, the 2010 Impala with the 3.5L V6 engine. The EPA fuel economy figures (which are usually on the optimistic side) for this vehicle are 18 mpg city and 29 highway ? this is exactly what you are experiencing.

The 2004 Impala had a 3.4L engine with an EPA rating of 19 and 29, while the 06 Grand Prix came with a 3.8L and lower EPA estimates of 17 and 26 mpg. The 09 Impala has the same rating as your 2010 model.

So now the question is: how were you able to achieve those unbelievable numbers for the older cars? The fuel consumption figures that you indicate are in Toyota Yaris (1.5L 4 cylinder) territory.

To verify that your 2010 Impala is running efficiently, get a Drive Clean test for $35 and look at the tailpipe gas readings on the report. The Hydrocarbon (HC) and Carbon Monoxide (CO) should be well below the limits. Another clue would require the use of a scanner to look at the on-board computer engine data. Long-term Fuel Trim should be around +/- 5% for a well-running engine.

Eric Lai adds:

Energuide numbers basically allow for a comparison of relative fuel consumption between different vehicles using the same standardized test.

Natural Resources Canada?s Office of Energy Efficiency cautions that ?the actual fuel consumption for your vehicle will vary from its Energuide fuel consumption rating.? Any number of real-world factors, including weather, maintenance, use of power accessories and personal driving habits will affect consumption.

  • Why is my car?s fuel economy such a disaster?

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